Chapter 12: Hydroponic Gardening

Marijuana Horticulture: The Indoor / Outdoor Medical Grower’s Bible

by Jorge Cervantes

Hydroponics is the science of growing plants without soil, most often in a soilless mix. In fact, many growers are already cultivating hydroponically. Cultivating clones in rockwool, peat moss, and coconut fiber is growing hydroponically. Growing mature plants in soilless Sunshine Mix or Terra-Lite, even when watered by hand, is hydroponic gardening. With hydroponics, nutrient uptake and grow medium oxygen content can be controlled easily. Manage these two factors, along with a few other requirements, to grow a bumper crop of buds with every harvest.

The inert soilless hydroponic medium contains essentially no nutrients. All the nutrients are supplied via the nutrient solution – fertilizer diluted in water. This solution passes over or floods around roots at regular intervals, later draining off. The extra oxygen trapped in the soilless medium and around the roots speeds nutrient uptake by tiny root hairs. Cannabis grows fast hydroponically, because it is able to take in food as fast as it can be used. In soil, as in hydroponics, the roots absorb nutrients and water. Even the best soil rarely has as much oxygen in it as a soilless hydroponic medium.

Contrary to popular belief, hydroponic gardens often require more care than soil gardens. If growing hydroponically, expect to spend more time in the garden. Extra maintenance is necessary because plants grow faster, there are more things to check, and more can go wrong. In fact, some growers do not like hydroponic gardening, because it requires too much additional care.

Hydroponic gardening is productive, but exacting – not as forgiving as soil gardening. Soil works as a buffer for nutrients and holds them longer than inert hydroponic growing mediums. In fact, advanced hydroponic growing mediums do not use a soilless mix; they use nothing at all!

In hydroponics, the nutrient solution can be controlled, so plants grow less leafy foliage and more dense flower buds. The stepped-up nutrient control makes plants flower faster and be ready for harvest a few days earlier than soil-grown cannabis.

Small flowering plants grow well in small hydroponic containers and horizontal tubes. Mother plants grow longer and are best suited to a large bucket system, which allows room for root development. The mother plant’s root system is easily contained in the bucket, and she is able to produce thousands of clones during her lifetime. Mother plants must have a huge root system to take in lots of nutrients to keep up with the heavy growth and clone production schedule.

Most grow rooms have two limiting factors: the number of plants in the garden and the electrical consumption expressed in watts. For example, if growing 12 large plants in a five gallon bucket hydroponic system, you will need about ten clones and one mother plant. The flower room could be illuminated with two 600-watt HP sodium lamps. A 40-watt fluorescent fixture could be used to root cones, and a 175-watt metal halide will keep the mother and vegetative plants growing. This is a total of 1415 watts that cost about $35 to $60 monthly. That’s a bargain, considering the garden will yield at least a pound of beautiful hydroponic buds every month!

If flowering is induced when clones are six to eight inches tall, they will be two to three feet tall when they finish flowering. You can pack short plants tightly together in a sea of green or a screen of green to maximize yield. It is easy to grow 60, four inch rockwool cubes on a flood and drain table or in three gallon grow bags full of soilless medium. To get the maximum yield, a plant or two is harvested every day or two. When a ripe plant is harvested, two small clones take its place. The weaker clone is culled out after two weeks.

Hydroponic Systems

hydroponic systems are distinguished by the way the nutrient solution is applied. The first distinction is whether the nutrient solution is applied in an “active” or “passive” manner.

Passive systems rely on capillary action to transfer the nutrient solution from the reservoir to the growing medium. Nutrient solution is passively absorbed by the wick or growing medium and transported to the roots. Absorbent growing mediums such as vermiculite, sawdust, peat moss, etc, are ideal for passive systems. The growing medium can stay very wet in passive systems, and substrate selection is important. Soggy substrates hold less air and deprive roots of rapid nutrient uptake. Although passive gardens are not considered “high performance”, the Dutch have managed to perfect them and achieve amazing results. Wick systems have no moving parts. Seldom does anything break or malfunction. low initial cost and low maintenance enhance wick systems’ popularity.

Dutch growers line the floor of a room with heavy plastic or pond liner. They fill three gallon pots with an absorbent soilless mix that holds plenty of air. They flood the garden with two to three inches of nutrient solution. Roots absorb the nutrient solution in two to five days. No nutrient solution is drained off; it is all absorbed by plants!

One Spanish grower uses passive irrigation to water his garden. He drives a delivery truck and is away from home five days a week. He keeps his indoor garden under a 400-watt HPS lamp. The plants are in rich potting soil, and the pots are in a large tray with four inch sides. Every Monday morning he fills the tray with mild nutrient solution. When he returns on Friday, the plants are strong and happy!

Active hydroponic systems “actively” move the nutrient solution. Examples of active systems are: flood and drain, and top feed. Cannabis is a fast growing plant and very well suited to active hydroponic systems.

Active hydroponic gardens are considered a “recovery” system if the nutrient solution is recovered and reused after irrigation. A “non-recovery” system applies the nutrient solution once, then it runs to waste. The solution is not reused. Non-recovery systems have few complications but are not practical for most cannabis hydroponic gardens. The commercial growers “run-to-waste” systems are avoided, because they pollute ground water with high levels of nitrates, phosphates, and other elements. Indoor growers seldom use non-recovery systems, because they require disposing of so much nutrient solution into the local sewer system.

Active recovery hydroponic systems such as the flood and drain (ebb and flow), top feed, and nutrient film technique (NFT) are the most popular and productive available today. All three systems cycle reused nutrient solution into contact with roots. Recovering and reusing the nutrient solution makes management more complex, but with the proper nutrient solution, schedule, and a little experience, it is easy to manage. Active recovery systems use growing mediums that drain rapidly and hold plenty of air, including: expanded clay, pea gravel, pumice rock, crushed brick, rockwool, and coconut coir.

Ebb and Flow Gardens

Ebb and flow (flood and drain) hydroponic systems are popular because they have a proven track record as low maintenance, easy to use gardens. Ebb and flow systems are versatile, simple by design, and very efficient. Individual plants in pots or rockwool cubes are set on a special table. The table is a growing bed that can hold one to four inches of nutrient solution. Nutrient solution is pumped into the table or growing bed. The rockwool blocks or containers are flooded from the bottom, which pushes the oxygen poor air out. Once the nutrient solution reaches a set level, an overflow pipe drains the excess to the reservoir. When the pump is turned off and the growing medium drains, it draws new oxygen rich air into contact with the roots. A maze of drainage gullies in the bottom of the table directs runoff solution back to the catchment tank reservoir. This cycle is repeated several times a day. Ebb and flow systems are ideal for growing many short plants in a Sea of Green garden.

Flood the table to half to three quarters the height of the container to ensure even nutrient solution distribution. Avoid lightweight mediums such as perlite that may cause containers to float and fall over.

A large volume of water is necessary to fill the entire table. Make sure the reservoir has enough solution to flood the reservoir and still retain a minimum of 25 percent extra to allow for daily evaporation. Replenish reservoir daily if necessary. Do not let nutrient solution stand in the table for more than a half hour. Submerge roots down in the depleted oxygen environment.

Flood the table when the medium is about half-full of moisture. Remember, rockwool holds a lot of moisture. Remember, rockwool holds a lot of moisture. Irrigation regimens will need to change substantially when temperatures cool and light is lacking.

Ebb and flow tables or growing beds are designed to let excess water flow freely away from the growing medium and roots. When flooded with an inch or more of nutrient solution, the growing medium wicks up the solution into the freshly aerated medium.

Air Table

Air tables are simple, easy to use hydroponic gardens. Seasoned growers and novices love their simplicity and low maintenance. The unique operating principle is simple, effective, and nearly fail-safe. The nutrient solution is forced up to the growing bed with air pressure generated by an external air pump. The pump can run on ordinary household electrical current or a solar powered 12-volt system. Once flooded, the nutrient solution stays in the growing bed for a few minutes before it drains beck to the reservoir. Constant air pressure during flooding also aerates the growing medium. The sealed, airtight reservoir limits evaporation, which in turn prevents algae growth and keeps the nutrient fresh. The external pump reduces the overall cost of the system and helps prevent electrical accidents. You can use rockwool, coco coir, peat, or a composite growing medium with excellent results.

Deep Water Culture (DWC)

Growing in deep water culture (DWC) is simple, easy, and productive. If growing outdoors in a DWC garden, a simple overflow drainage hole can be cut in the side of the reservoir to prevent rainwater from causing it to overflow.

Seedling and clones are held in net pots full of expanded clay pellets, rockwool or other growing medium. The net pots are nestled in holes in a lid that covers the reservoir. The roots of seedlings and cuttings dangle down into the nutrient solution. A submersible pump lifts nutrient solution to the top of a discharge tube where it splahes in the access lid. Nutrient solution cascades down, wetting roots and splashing into the self-contained reservoir below, which in turn increases dissolved oxygen in the solution. Roots easily absorb nutrients and water from the solution in the oxygenated environment. Many gardens also keep an air stone bubbling new air into the reservoir to supply more oxygen.

These gardens are simple by design and require no timer, because the pumps are on 24 hours a day. This low maintenance garden is perfect for casual gardeners as well as hydroponic enthusiasts.

Top-Feed Systems

Top-feed hydroponic systems are very productive, easy to control, precise, easy to maintain, and efficient. The nutrient solution is metered out in specific doses and delivered via spaghetti tubing of an emitter placed at the base of individual plants. Aerated nutrient solution flowing into the growing medium and is taken up by the roots. The runoff nutrient solution is directed back to the reservoir as soon as it drains from the growing medium. Rockwool, gravel, coconut coir, and expanded clay are the most common growing mediums found in top-feed systems. Versatile top-feed systems can be used with individual containers or slabs in individual beds or lined on tables.

Top-feed systems come in many configurations. Systems with several gallons of growing medium are best for growing large plants that may require support. Small containers are perfect for smaller plants.

Top-Feed Buckets

Self contained top-feed buckets consist of a growing container nested inside a reservoir containing a pump. Individual buckets make culling out and replacing sick plants quick and easy. Self contained top feed bucket systems are perfect for growing large mother plants. The container can be moved anywhere easily. Some containers have a net pot suspended in the lid of a five gallon bucket / reservoir. An air stone in the bottom of the reservoir aerates the nutrient solution. A separate pump cycles the irrigation to the container. Other self contained top feed buckets use a large growing container filled with expanded clay pellets. A pump constantly cycles nutrient solution in the system, aerating the solution and irrigating the plant. Roots grow down into the nutrient solution to form a mass on the bottom. Irrigation from the top circulates aerated nutrient solution and flushes out the old oxygen poor solution. Some systems contain a one inch pipe to draw air directly down to the root zone. There are many different variations of this system, and they all work. Some systems contain a one inch pipe to draw air directly down to the root zone. There are many different variations of this system, and they all work!

Other top-feed bucket systems employ multiple buckets that are connected to a main reservoir. A flexible drain hose is attached near the bottom of the bucket / reservoir. The hose is connected to a drainage manifold that shuttles runoff nutrient solution back to a central reservoir.

Each reservoir below the growing container holds an inch or two of water. It is important to regularly cycle irrigation in these gardens, so the solution in the bottom of the buckets does not stagnate.

Top-feed buckets can also be lined up on a drainage table. Square containers make most efficient use of space. Plants are fed with irrigation tubing attached to a manifold. Once delivered, the nutrient solution flows and percolates through the growing medium. Roots take in the aerated nutrient solution before it drains onto the tray and back to the reservoir.

Individual containers in top-feed bucket systems are easy to arrange to fit into the allotted garden space. Plants can also be transplanted or removed from pots and cared for individually.

Top-Feed Slabs

Top-feed slab systems are popular among small and large indoor and greenhouse growers. Rockwool r coco slabs covered in plastic serve as growing containers. The nutrient solution is delivered via spaghetti tubes from the top of the slab. An emitted attached to the spaghetti tube doses a specific measure of nutrient solution to each plant. The nutrient solution is aerated as it is applied, before being absorbed by the growing medium and draining back to the reservoir.

A simple nutrient solution delivery manifold consists of emitters connected to spaghetti delivery tubes. The tubes are attached to a short manifold that is fed by a pump submerged in a reservoir.

Emitters are designed to be anchored in the growing medium and to emit a measured dose of nutrient solution.

Some systems use individual trays to contain slabs. Nutrient solution is pumped from the reservoir and delivered to plants via spaghetti tubes attached to emitters. Individual trays are easy to configure for different sized gardens.

Tables of Slabs

You can also setup a drainage table and place slabs on top. The nutrient solution is pumped from the reservoir below the table and delivered to individual plants via spaghetti tubes attached to emitters. The solution flows into the growing medium where it comes into contact with roots. Excess nutrient solution drains from pots onto the table and is carried back to the reservoir. Make sure the table is set up on an incline so it drains evenly. Pockets of standing water on the table contain less oxygen and promote rot.

Individual Blocks

Individual blocks in this rockwool system allow gardeners the possibility of removing or changing plants if necessary. Nutrient solution is pumped via spaghetti tubes from the reservoir below and distributed via emitters into rockwool cubes.

Vertical Top-Feed Systems

Vertical gardens can increase overall yield more than ten-fold over a flat garden. Substrate bags, tubes, or slabs are positioned vertically around an HID. Short plants are placed in the medium and fed gradually with a drip emitter. The runoff drains through the growing medium and back to the reservoir. The solution is re-circulated once it returns to the reservoir.

Use rockwool or coco mixed with lightweight vermiculite as a growing medium to lessen weight when substrate is wet. Irrigate constantly to keep roots supplied with water and nutrients from a well-aerated solution.

Vertical hydroponic systems save space, but require more maintenance. These systems can also be tricky to fine tune so they operate at peak capacity.

Nutrient Film Technique (NFT)

Nutrient Film Technique (NFT) hydroponic systems are high performance gardens that perform well when fine tuned. This relatively new form of hydroponics supplies aerated nutrient solution to roots located in gullies. Seedling ir cuttings with a strong root system are placed on capillary matting located on the bottom in a covered channel. The capillary matting stabilizes nutrient solution flow and holds roots in place. Constantly aerated nutrient solution flows down the channel, or gully, over and around the rots, and back to the reservoir. Irrigation is most often constant, 24 hours a day. Rots receive plenty of oxygen and are able to absorb a maximum of nutrient solution. Proper gully incline, volume, and flow of nutrient solution are key elements in NFT gardens.

Gullies or channels are covered to keep humidity high in the root zone and light from shining on roots. Root hairs responsible for most water and nutrient uptake cover the growing tips of advancing roots. These roots are submerge in turbulently flowing nutrient solution and the tops are intermittently in humid air. The nutrient solution is constantly aerated as it flows down the inclined gulley. the slope of the gulley is adequate to prevent water from stagnating. Often a filter is necessary to prevent debris from blocking gullies and pump.

Although high performance, NFT systems offer practically no buffering ability. In the absence of a growing medium, roots must be kept perfectly moist by the nutrient solution at all times. If a pump fails, roots dry and die. If the system dries out for a day or longer, small feeder roots will die and grave consequences will result. The system is very easy t clean and lay out after each crop. Only growers with several years of experience should try an NFT system if working alone. With help, they are easier to master.

Double reinforced bottom makes gullies durable and rigid when supporting large plants, root systems, and large volumes of nutrient solution. Some NFT gullies have ribs below to provide support and prevent warping and movements. The ribs also function as drainage channels and direct nutrient solution evenly along the bottom of the gully.

Many NFT systems are hybrids. For example, the nutrient solution in some hybrid NFT systems is delivered via spaghetti tubing to each plant. More irrigation sites help each plant receive proper irrigation. The nutrient solution flows through a small basket of growing medium before it runs down the gulley, over the roots, and back to the reservoir. Yet another NFT system employs spray nozzles inside the gulley. The nozzles spray nutrient solution on and around roots to keep the root zone environment at 100 percent humidity. The nutrient solution flows down a PVC pipe, over roots, and back to the reservoir.

Too often, these hybrid systems are poorly planned and designed. Many times they are constructed from white four inch PVC pipe. The thin white walls of the PVC pipe allow enough light to illuminate roots that they turn green or rot more easily. I have also seen systems with nozzles inside the PVC pipe. If a nozzle plugged inside a pipe, there was no easy way to access the nozzle for maintenance.

Nutrient solution is pumped from the reservoir into gullies via a manifold and tubing at the upper end. The table is setup on an incline so the nutrient solution flows quickly over roots t create an environment packed with air and available nutrients. A catchment drain directs the nutrients back into the nutrient reservoir.


Aeroponic systems use no growing medium and offer the highest performance possible. Rots are suspended in a dark growth chamber without growing medium where they are misted with oxygen rich nutrient solution at regular intervals. The humidity in the chamber remains at or near 100 percent 24 hours a day. Roots have the maximum potential to absorb nutrient in the presence f air. Humid air and nutrient solution are all that fill the growth chamber. Plants are most often grown in net pots full of growing medium and suspended from the top of the system.

Aeroponic systems require greater attention to details. There is no growing medium to act as a water / nutrient bank, which makes the system delicate and touchy to use. If the pump fails, roots soon dry, and plants suffer. Systems that use delicate spray nozzles must be kept free of debris. Imbalanced nutrient solution and pH can also cause problems quickly. This is why it is important to purchase quality components or a ready-made system from a qualified supplier.

The RainForest is very popular. Nutrient solution is actually atomized into the air creating 100 percent humidity. Nutrient solution is dripped onto a pinning plate. The solution atomizes, mixing with the air as it spins off the plate. The spinning plate is located above the water in the reservoir.

Growing Mediums

Soilless growing mediums provide support for the root system, as well as hold and make available oxygen, water, and nutrients. Three factors contribute to cannabis’ roots ability to grow in a substrate: texture, pH, and nutrient content, which is measured in EC, electrical conductivity.

The texture of any substrate is governed by the size and physical structure of the particles that constitute it. Proper texture promotes strong rot penetration, oxygen retention, nutrient uptake, and drainage. Growing mediums that consist of large particles permit good aeration and drainage. Increased irrigation frequency is necessary to compensate for low water retention. Water and air holding ability and root penetration are a function of texture. The smaller the particles, the closer they pack together and the slower they drain. Larger particles drain faster and retain more air.

Irregular shaped substrates such as perlite and some expanded clays have more surface area and hold more water than round soilless mediums. Avoid crushed gravel with sharp edges that cut into roots if the plant falls or is jostled around. Round pea gravel; smooth, washed gravel; and lava rocks are excellent mediums to grow marijuana in an active recovery system. Thoroughly wash clay and rock growing mediums to get out all the dust that will turn to sediment in your system.

Fibrous materials like vermiculite, peat moss, rockwool, and coconut coir retain large amounts of moisture within their cells. Such substrates are ideal for passive hydroponic systems that operate via capillary action.

Mineral growing mediums are inert and do not react with living organisms or chemicals to change the integrity of the nutrient solution. Coconut cir and peat mosses are also inert.

Non-inert growing mediums cause unforeseen problems. For example, gravel form a limestone quarry is full of calcium carbonate, and old concrete is full of lime. When mixed with water, calcium carbonate will raise the pH, and it is very difficult to make it go down. Growing mediums made from reconstituted concrete bleed out so much lime, they son kill the garden.

Avoid substrates found within a few miles of the ocean or large bodies of salt water. Most likely, such mediums are packed with toxic salts. Rather than washing and leaching salts from the medium, it is easier and more economical to find another source of substrate.

Air is a great medium when it is filled with 100 percent humidity 24 hours a day.

Coconut fiber is an excellent hydroponic medium.

Expanded clay, also called hydroclay, or hydrocorn, is made by many different manufacturers. The clay pellets are coked at high temperatures in a kiln until they expand. Many little catacomb like pockets form inside each pellet that hold air and nutrient solution. It is an excellent medium to mix with Peat-Lite and to grow mothers in large containers. I like the way it drains so well and still retains nutrient solution while holding lots of oxygen. Examples of expanded cay include commercially available Hydroton, Leca, Grorox, Geolite.

Expanded cay can be reused again and again. Once used, pour expanded clay pellets into a container and soak in a sterilizing solution of ten milliliters hydrogen peroxide per four liters of water. Soak for 20-30 minutes. Remove expanded clay and place on a screen of hardware cloth. Wash and separate clay pellets from dead roots and dust. Let dry and reuse.

Expanded mica is similar to expanded clay.

Foam is somewhat popular. It lasts a long time, lends itself to easy sterilization, and holds a lot of water and air.

Gravel is one of the original hydroponic mediums. Although heavy, gravel is inert, holds plenty of air, drains well, and is inexpensive. Still popular today, gravel is difficult to overwater. It holds moisture, nutrient, and oxygen on its outer surfaces. Use pea gravel or washed river gravel with round edges that do not cut roots when jostled about. Grave should be 0.125-0.375 inches in diameter, with more than half the medium about 0.25 inch across. Crushed rock can be packed with many salts. Pre-soak and adjust its pH before use. Gravel has ow water retention and low buffering ability.

Pumice is a naturally occurring, porous, lightweight, volcanic rock that holds moisture and air in catacomb like surfaces. Light and easy to work with, some lava rock is so light it floats. Be careful that sharp edges on the rocks do not damage roots. Lava rock is still a good medium and acts similarly to expanded clay.

Peat moss, is partially decomposed vegetation. Decomposition has been slow in the northern regions where it is found in bogs. There are three common kinds of peat moss – Sphagnum, Hypnum, and Reed / Sedge. Sphagnum peat is about 75 percent fiber with a pH of 3 to 4. Hypnum peat is about 50 percent fiber with a pH of about 6. Reed / sedge peat is about 35 percent fiber with a pH of six or more.

Perlite drains fast, but it’s very light and tends to float when flooded with water. Perlite has no buffering capacity and is best used to aerate soil or soilless mix.

Rockwool is an exceptional growing medium and a favorite of many rowers. It is an inert, sterile, porous, non-degradable growing medium that provides firm root support. Rockwool has the ability to hold water and air for the roots. The roots are able to draw in most of the water stored in the rockwool, but it has no buffering capacity and a high pH. Rockwool is probably the most popular hydroponic growing medium in the world. Popular brand names include Grodan, HydroGro and Vacrok.

Sand is heavy and has no buffering ability. Some sand has a high pH. Make sure to use sharp river sand. Do not use ocean or salty beach sand. Sand drains quickly but still retains moisture. Sand is best used as a soil amendment in volumes of less than ten percent.

Sawdust holds too much water for marijuana growth and is usually too acidic. Be wary of soils with too much wood matter. Such mediums use available nitrogen to decompose the leglin in the wood.

Vermiculite holds a lot of water and is best suited for rooting cuttings when it is mixed with sand or perlite. With excellent buffering qualities, vermiculite holds lots of water and has traces of magnesium (Mg), phosphorus (P), aluminum (A), and Silicon (Si). Do not use construction grade vermiculite which is treated with phytotoxic chemicals.

Water alone is a poor medium, because it cannot hold enough oxygen to support plant life. When aerated, water becomes a good growing medium.


To reuse a growing medium, it must be sterilized to remove destructive pests and diseases. Sterilizing is less expensive and often easier than replacing the growing medium. Sterilizing works best on rigid growing mediums that do not lose their shape such as gravel, expanded clay, and mica. Avoid sterilizing and reusing substrates that compact and lose structure such as rockwool, coconut coir, peat moss, perlite, and vermiculite. Avoid problems caused by compaction and dead roots by replacing used growing mediums. Once sterilized, the medium is free of harmful microorganisms including bacteria and fungi, plus pests and their eggs.

Remove roots from the growing medium before sterilizing. A three to four month old marijuana plant has a root mass about the size of an old desk telephone. Separate the medium by shaking and pulling roots away. Bounce the medium on a screen so roots come to the top. Scoop up and remove roots by hand. Fewer decaying roots cause fewer pest and disease problems, and decrease incidence of clogged feeder tubes.

Substrate can also be washed in a large container such as a barrel or bathtub. Washing works best with lighter substrates such as expanded clay or mica. Roots float to the top and are readily skimmed off with a screen or by hand.

Once roots are removed, soak the substrate in a sterilant such as a five percent laundry bleach (calcium or sodium hypochlorite) solution for at least an hour. Or mix hydrochloric acid, the kind used in hot tubs and swimming pools. Pour, drain, or pump off the sterilant, and flush the medium with plenty of fresh water. A bathtub and a shower nozzle on a hose are perfect for washing substrate. Place the substrate in the bathtub, set a screen over the drain, and use the shower head or a hose to wash down the medium. It may be necessary to fill the tub with fresh water and drain it a couple of times to rinse any residual sterilants from the substrate.

If you decide to use rockwool or coco a second time, you may have problems with pests and diseases. In general, I recommend reusing a medium only if it does not deteriorate or compact. Examples include: pea gravel, expanded clay, lava rock, sand, etc. Once used indoors, reuse rockwool and biodegradable coco in the outdoor garden.

To sterilize a hydroponic garden, remove the nutrient solution from the reservoir. Pump the solution into the outdoor garden. Avoid pumping it down household drains, and definitely do not pump it int a septic tank. The nutrients will disrupt the chemistry!

Flood the growing medium with the sterilizing solution for at least one-half hour, let drain, and flush again. Pump the bleach solution out of the system and down the drain. Do not dump the sterilants outdoors; they will defoliate plants where they are dumped. Use lots of fresh water to leach and flush the entire system including beds, connecting hoses, drains, and reservoir. Make sure all residues are gone by flushing entire system twice for a half hour. Remove all solution from the tank, and scrub rockwool outside on a sheet of black plastic, and cover with black plastic. Let the sun bake the layer of slabs, or flock, for several days. The temperatures in the rockwool will climb to 140F or more, enough to sterilize for most all harmful diseases and pests.


The pH of a nutrient solution controls the availability of ions that cannabis needs to assimilate. Marijuana grows well hydroponically within a pH range of 5.5 – 6.5, with 5.8 – 6.0 being ideal. The pH in hydroponic gardens requires a somewhat vigilant eye. In hydroponics, the nutrients are in solution and more readily available than when in soil. The pH of the solution can fluctuate a half point and not cause any problems.

Roots take in nutrients at different rates, which causes the ratios of nutrients in solution to change the pH. When pH is above 7 or below 5.5, some nutrients are not absorbed as fast as possible. Check the pH every day or two to make sure it is at or near the perfect level.

Deviations in pH levels often affect element solubility. Values change slightly with different plants and grow mediums. Follow manufacturer’s guidelines for pH eve, and correct the pH using the manufacturer’s suggested chemicals, because they will react best with their fertilizer.

The pH can easily fluctuate up and down one full point in hydroponic systems and cause little or no problem with nutrient uptake.

Follow the directions on the container, and remember to mix adjusters into the reservoir slowly and completely. Fertilizers are normally acidic and lower the pH of the nutrient solution. But nutrient solution is still taken by plants, and water transpires and evaporates into the air, which causes the pH to climb. Stabilize pH f the water before adding fertilizer. Make a correction if readings vary on half point.

EC, TDS, DS, CF, PPM, EC Meters

Pure distilled water had no resistance and conducts little electrical current. When impurities are added to pure distilled water in the form of fertilizer salts, it conducts more electricity. A water analysis will indicate the impurities or dissolved solids fund in household tap water. These impurities conduct electricity.

Nutrient (salt) concentrations are measured by their ability to conduct electricity through a solution. Dissolved ionic salts create electrical current in the solution. The main constituent of hydroponic solutions is ionic salts. Several scales are currently used to measure how much electricity is conducted by nutrients including: Electrical Conductivity (EC), Conductivity Factor (CF), Parts Per Million (ppm), Total Dissolved Solid (TDS), and dissolved Solids (DS). Most American growers use ppm to measure overall fertilizer concentration. European, Australian, and New Zealand growers use EC, however they still use CF in parts of Australia and New Zealand. Parts per million is not as accurate r consistent as EC to measure nutrient solution strength.

The difference between EC, CF, ppm, TDS, and DS is more complex than originally meets the eye. Different measurement systems all use the same base, but interpret the information differently. Let’s start with the EC, the most accurate and consistent scale.

Electrical conductivity is measured in milliSiemens per centimeter or microSiemens per centimeter. One microSiemen/cm = 1000 milliSiemens/cm.

Parts per million testers actually measure in EC and convert to ppm. Unfortunately, the two scales (EC and ppm_) are not directly related. Each nutrient r salt gives a different electronic discharge reading. To overcome this obstacle, an arbitrary standard was implemented which assumes “a specific EC equates to a specific amount of nutrient solution.” Consequently, the ppm reading is not precise; it is only an approximation, a ball park figure! It gets more complex! Nutrient tester manufacturers use different standards to convert from EC to the ppm reading.

Every salt in a multi-element solution has a different conductivity factor. Pure water will not conduct electrical current, but when elemental salts / metals are added, electrical conductivity increases proportionately. Simple electronic meters measure this value and interpret it as total dissolved solid (TDS). Nutrient solutions used to grow marijuana generally range between 5000 to 1200 ppm. If the solution concentration is too high, the internal osmotic systems can reverse and actually dehydrate the plant. In general, try to maintain a moderate value of approximately 800 to 1200 ppm. If the solution concentration is too high, the internal osmotic systems can reverse and actually dehydrate the plant. In general, try to maintain a moderate value of approximately 800 to 1200 ppm.

Nutrient solution concentration levels are affected by nutrient absorption by roots and by water evaporation. The solution weakens as plants use nutrients, but water also evaporates from the solution, which increases the nutrient concentration. Adjust the concentration of the solution by adding fertilizer or diluting with more water.

Many factors can alter the EC balance of a solution. For example, if under-watered or allowed to dry completely, the EC reading will rise. In fact, the EC may increase to two or three times as high as the input solution when too little water is applied to rockwool. This increase in slab EC causes some nutrients to build up faster than others. When the EC doubles, the amount of sodium can increase as much as four to six fold under the right conditions! There should not be any sodium present in your garden unless it is in the water supply, and it should not be in excess of 50 ppm.

Let 10-20 percent of the nutrient solution drain from the growing medium after each irrigation cycle to hep maintain EC stability. The runoff carries away any excess fertilizer salt buildup in the growing medium.

If the EC level of a solution is too high, increase the amount of runoff you create with each flush. Instead of 10-20 percent runoff, flush s 20-30 percent of the solution runs off. To raise the EC, add more fertilizer to the solution, or change the nutrient solution.

A dissolved solids (DS) measurement indicates how may parts per million (ppm) of dissolved solids exist in a solution. A reading of 1800 ppm means there are 1800 parts of nutrient in one million parts solution, or 1800/1,000,000.

An EC meter measures the overall volume or strength of elements in water or solution. A digital LCD screen current flowering between the two electrodes. Pure rainwater has an EC close too zero. Check the pH and EC of rainwater t find out if it is acidic (acid rain) before using it.

Distilled bottled water from the grocery store often registers a small amount of electrical resistance, because it is not perfectly pure. Pure water with no resistance is very difficult to obtain and not necessary for a hydroponic nutrient solution.

Electrical conductivity measurement is temperature sensitive and must be factored into the EC reading to retain accuracy. High quality meters have automatic and manual temperature adjustments. Calibrating an EC meter is similar to calibrating a pH meter. Simply follow manufacturer’s instructions. For an accurate reading, make sure your nutrient solution and stock solution are the same temperature.

Inexpensive meters last for about a year, and expensive meters can last for many years. However, the life of most EC meters, regardless of cost, is contingent upon regular maintenance. The probes must be kept mist and clean at all times. This is the most important part of keeping the meter in good repair. Read instructions on care and maintenance. Watch for corrosion buildup on the probes of your meter. When the probes are corroded, readings will not be accurate.

To check the EC of the nutrient solution, collect samples from both the reservoir and the growing medium. Save time and effort; collect EC and pH samples simultaneously. Collect samples with a syringe or baster at east two inches deep int the rockwool or coco. Collect separate sample from the reservoir. Place each sample in a clean jar. Use calibrated EC meter to measure the samples. Under normal conditions, the EC in the slab should be a little higher than the nutrient solution in the reservoir. If the EC of the solution drawn from the growing medium is substantially higher than the one from the reservoir, there is a salt buildup in the substrate. Correct the imbalance by flushing substrate thoroughly with diluted nutrient solution, and replace with new solution. Regularly check the EC f your water, slab, and runoff.

Hydroponic Nutrients

High quality hydroponic formulations are soluble, contain all the necessary nutrients, and leave no impure residues in the bottom of the reservoir. Always use the best hydroponic fertilizer you can find. There are many excellent commercial hydroponic fertilizers that contain all the nutrients in a balanced form to grow great cannabis. Quality hydroponic fertilizers packed in one, two, or three part formulas have all the necessary macro and micronutrients for rapid nutrient assimilation and growth. Cheap fertilizers contain low quality, impure components that leave residue and sediments. These impurities build up readily and cause extra maintenance. high quality, soluble hydroponic nutrients properly applied are immediately available for uptake. Exact control is much easier when using pure high grade nutrients. Many companies use food-grade nutrients to manufacture their fertilizer formulations.

Nutrients are necessary for marijuana to grow and flourish. These nutrients must be broken down chemically within the pant, regardless of origin. The nutrients could be derived from a natural organic base which was not heated or processed to change form, or they could be simple chemical elements and compounds. When properly applied, each type of fertilizer, organic or chemical, theoretically produces the same results.

Nutrient Solutions

To avoid nutrient problems, change the nutrient solution in the reservoir every week. Change nutrient solution every two weeks in systems with a large reservoir. Change the nutrient solution more often when plants are in the last stages of flowering, because they use more nutrients. You can wait to change the nutrient solution, but imbalances are more common. Plants absorb nutrients at different rates, and some of them run out before others, which can cause more complex problems. The best form of preventive maintenance is to change the solution often. Skimping on fertilizer can cause stunted growth. Nutrient imbalances also cause the pH to fluctuate, usually to drop. Nutrients used at different rates create an imbalanced formulation. Avert problems by using pure nutrients and thoroughly flushing the soilless medium with fresh tepid water between nutrient solutions.

hydroponics gives the means to supply the maximum amount of nutrient a plant needs, but it can also starve them to death or rapidly overfertilize them. Remember, hydroponic systems are designed for high performance. If one thing malfunctions, say the electricity goes off, the pump breaks, the drain gets clogged with roots, or there is rapid fluctuation in the pH, major problems could result in the garden. A mistake could kill or stunt plants so badly that they do not have time to recover before harvest.

Solution Maintenance

Plants use so much water that nutrient solutions need to be replenished regularly. Water is used at a much faster rate than nutrients. Casually topping off a reservoir with pH-balanced water will keep the solution relatively balanced for a week or two. Some growers top off the nutrient solution with 500-700 ppm-strength nutrient solution every two to three days. Never let the nutrient solution go for more than fur weeks before draining it and adding fresh solution. Smart growers leach the entire system with weak nutrient solution for an hour or more between changing the reservoir. Wipe down the entire system with the solution.

Do not flush with plain water. Flushing with mild (quarter-strength) nutrient solution will actually remove more excess fertilizer than plain water. Check EC of reservoir, growing medium, and runoff nutrient solution at the same time every day.

Use an electric EC pen to monitor the level of dissolved solids in the solution. Occasionally you will need to add more fertilizer concentrate to maintain the EC level in the reservoir during the topping off times. Keep the reservoir full at all times. The smaller the reservoir, the more rapid the depletion and the more critical it is to keep it full. Smaller reservoirs should be refilled daily.


Hydro Organic is a means of growing cannabis in an inert soilless medium and feeding with a soluble organic nutrient solution. Organic fertilizers are most often defined as containing substances with a carbon molecule or a natural unaltered substance such as ground up rocks.

Dedicated growers spend the time and trouble it takes to grow hydro-organically, because the natural nutrients bring out a sweet organic taste in buds. Indoor and outdoor crops grown in less than 90 days do not have time to wait for organic nutrients to be broken down. Organic nutrients must be soluble and readily available for short cannabis crops to benefit.

An exact balance of organic nutrients can be achieved with constant experimentation and attention to details. Even when you buy ready-mixed commercial fertilizers like BioCanna, Earth Juice, or Fox Farm, you will need to try different feeding amounts and schedules to get the exact combination to grow top quality bud.

Taking an accurate EC reading or mixing the exact amount of a specific nutrient is very difficult in organic hydroponics. Chemical fertilizers are easy to measure out and apply. It is easy to give plants the specific amount of fertilizer they need in each stage of growth.

Organic nutrients have complex structure, and measuring content is difficult. Organics are difficult to keep stable, too. Some manufacturers, including BioCanna, Earth Juice, and Fox Farm, have managed to stabilize their fertilizers. When buying organic nutrients, always buy from the same supplier, and find out as much as possible about the source from which the fertilizers were derived.

Combine premixed soluble organic fertilizers with other organic ingredients to make your own blend. Growers experiment to find the perfect mix for their system and the varieties they are growing. Adding too much fertilizer can toxify soil and bind up nutrients, making them unavailable. Foliage and roots burn when the condition is severe.

Soluble organic fertilizers are fairy easy to flush from the growing medium. Like chemical fertilizers, organic fertilizers build to toxic levels easily. Look for the same symptoms as in soil burned leaf tips, discolored misshapen leaves, brittle leaves, etc. Organic nutrients require heavier flushing. Rinse medium with three gallons of water for every gallon of medium. Some growers flush with plain water the last two weeks f flowering to get all fertilizer taste out of buds.

Mix seaweed with macronutrients and secondary nutrients to make a hydro-organic fertilizer. The amount of primary and secondary nutrients is not as important as the menagerie of trace elements that are in an available form in the seaweed. Major nutrients can be applied via soluble fish emulsion for nitrogen; phosphrous and potassium are supplied by bat guano, bne meal, and manures. More and more organic growers are adding growth stimulators such as humic acid, trichoderma bacteria, and hormones.


Nutrient solution reservoirs should be as big as possible and have a lid to lessen evaporation. Gardens use from 5-25 percent of the nutrient solution every day. A big volume of nutrient solution will minimize nutrient imbalances. When the water is used, the concentration of elements in the solution increases; there is less water in the solution and nearly the same amount of nutrients. Add water as soon as the solution level drops. The reservoir should contain at east 25 percent more nutrient solution than it takes to fill the beds to compensate for daily use and evaporation. The greater the volume of the nutrient solution, the more forgiving the system and the easier it is to control. Forgetting to replenish the water supply and/or nutrient solution could result in crop failure.

Check the level of the reservoir daily, and replenish if necessary. A reservoir that loses more than 20 percent of its volume daily can be topped off with pure of low (500ppm) EC water. Sophisticated systems have a float valve that controls the level of water in the reservoir.

If you reservoir does not have graduated measurements t denote liquid volume, use an indelible marker to make a full line and the number f gallons or liters contained at that point on the inside of the reservoir tank. Use this volume measure when mixing nutrients.

The pump should be setup to lift the solution out of the reservoir. Set reservoirs high enough so spent nutrient solution can be siphoned or gravity-flow into a drain or the outdoor garden.

Reservoir Temperature

The temperature of the nutrient solution should stay between 60-75F. However, nutrient solution will hold much more oxygen at 60F than it will at 75F. never let the nutrient solution temperature climb above 85F. Above 85F, the solution holds little oxygen. Roots are easily damaged by temperatures 85F and above. Heat damaged roots are very susceptible to rot, wilts, and fungus gnat attacks.

To save energy and money, heat the nutrient solution instead of the air in the room. Heat the nutrient solution with a submersible aquarium heater r grounded propagation heating cables. The heaters might take a day or longer to raise the temperature of a large volume of solution. Do not leave heaters in an empty reservoir. They will soon overheat and burn out. Aquarium heaters seldom have ground wires, a seemingly obvious oversight. But I have yet to learn of an electrocution by an aquarium heater. Avoid submersible heaters that give off harmful residues.

When air is cooler than water, moisture rapidly evaporates into the air; the greater the temperature differential, the higher the relative humidity. It will also promote the uptake of nutrients.

An air pump submerged in the reservoir not only aerates the solution, it will help level out the temperature differential between ambient air and reservoir.


Irrigation is a science unto itself. Irrigation cycles depend on plant size, climate conditions, and the type of medium used. large, round, smooth particles of substrate drain rapidly and need to be irrigated more often – four to twelve times daily for five to thirty minute cycles. Fibrous mediums with irregular surfaces, such as vermiculite, drain slowly and require less frequent watering, often just once per day. The water comes to within one half inch of the top of the gravel and should completely drain out of the medium after each watering. Top-feed systems cycle for about five minutes or longer and should be irrigated at least three times daily. Often growers cycle the nutrient solution 24 hours a day, especially when growing in fast-draining expanded clay or similar mediums.

In fast draining mediums, overhead irrigation is continual. Drip irrigation in coco coir is four to five times daily. Flood and drain irrigation cycles are five to ten times daily.

During and soon after irrigation, the nutrient content of the bed and the reservoir as the same concentration. As time passes between irrigations, the EC and the pH gradually change. If enough time passes between irrigations, the nutrient concentration might change so much that the plant is not able to draw it in.

Nutrient Disorders

When the hydroponic garden is on a regular maintenance schedule, and the grower knows the crop well, nutrient problems are usually averted. If nutrient deficiency or excess affects more than a few plants, check the irrigation fittings to ensure nutrient challenged plants are receiving a full dose of nutrient solution. Next, check the substrate around affected plants to ensure nutrient solution is penetrating entire medium and all roots are wet. Check the root zone to ensure roots have not plugged drainage conduits and are not standing in stagnant solution.

Change the nutrient solution if there is a good flow of nutrient solution through the root zone, but plants still appear sickly. Make sure the pH of the water is within the acceptable 5.5-6.5 range before adding new nutrients.

If changing solution does not solve the problem, changing to a new brand of fertilizer may do the trick. Leach growing medium with dilute nutrient solution to solve simple overdose nutrient problems.

Hydroponic gardens have no soil to buffer the uptake of nutrients. This causes nutrient disorders to manifest as discolored foliage, slow growth, spotting, etc, at a rapid rate. Novice gardeners must learn how to recognize nutrient problems in their earlier stages to avoid serious problems that cost valuable time for plants to recoup. Treatment for a nutrient deficiency or excess must be rapid and certain. But once treated, plants take several days to respond to the remedy. For a fast fix, foliar feed plants.

Nutrient deficiency or excess diagnosis becomes difficult when two or more elements are deficient or in excess at the same time. Symptoms might not point directly to the cause. Solve mind bending unknown nutrient deficiency syndromes by changing the nutrient solution. Plants do not always need an accurate diagnosis when the nutrient solution is changed.

Over-fertilization, once diagnosed, is easy to remedy. Drain the nutrient solution. Flush the system at least twice with fresh dilute (5-10 percent) nutrient solution to remove any lingering sediment and salt buildup in the reservoir. Replace with properly mixed solution.

Nutrient disorders most often affect a strain at the same time when it is receiving the same nutrient solution. Different varieties often react differently to the same nutrient solution. Do not confuse other problems – wind burn, lack of light, temperature stress, fungi and pest damage – with nutrient deficiencies. Such problems usually appear on individual plants that are most affected. For example, foliage next to a heat vent might show signs of heat scorch, while the rest of the garden looks healthy. Or a plant on the edge of the garden would be small and leggy because it receives less light.

Marijuana Horticulture

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