Climate Control

Marijuana Horticulture

by Jorge Cervantes

Even the best greenhouses will lose heat through radiation, conducting, convection through glass, walls, and floor (or soil), and also through vents, doors, and cracks. To counteract external variables, the internal structure of the greenhouse is, in some ways, more complex than the selection of framing and covering materials.

All greenhouses need ventilation and most need fans. Look for an extraction fan with the capacity to change the air once every minute. Capacity refers to the amount of power needed to circulate the air volume of your structure.

Calculate the volume by multiplying the square footage of your greenhouse by the height. Multiply the volume by sixty air changes per hour to get the cubic feet per minute capacity of the greenhouse.

The combination of louvers and fan will force the hottest, most humid air out while protecting the plants from draft. Vents control temperatures in all seasons and improve growing conditions. Hand-operated roof vents will require frequent checks, or you may install automatic vents with with an electric motor and thermostat that will respond to conditions around the clock. Venting is important with a cold frame, too. The high end models have wax-filled vents that operate automatically, opening when the heat rises in the frame and contracting as the temperature cools.

Heating systems are important to keep plants healthy during cold nights. Cannabis grows well with night temperatures of 60-65F, but colder nights will require an additional heat source for sustained growth.

You can turn a cold frame into a hot frame by insulating it with manure or heating it with steam, hot water pipes, or electricity. To make the most efficient use of electricity, purchase soil-heating tape or cable, with a thermostat that will automatically control the temperature. Lay the cable on the soil at the bottom of the bed or on a bed of sand or vermiculite and cover with about two inches of sand. You will need to provide 10-15 watts of electric heat for every square foot of growing area. Heat cables are also useful in greenhouses for warming seedlings, clones, or flowering plants without the cost of heating the entire structure.

Small greenhouses can be heated relatively economically with an electric space heater, or more effectively with thermostatically controlled forced air using ducts or plastic tubing to distribute heat. Larger units may be heated with forced air or by a coal or natural hot-water or steam system. Steam can also be used t sterilize growing beds and potting soils. Then there is the low tech method of greenhouse warming: compost. A grower in Portland, Oregon, stacks organic matter on the sides of the greenhouse to a height of about five feet inside and out. As compost decomposes, it gives off heat keeping the structure warm at a very low cost.

Evaporative cooling eliminates excess heat and add humidity, reducing water needs. Moist air circulates through the structure while warm air is expelled through roof vents or exhaust fans. Properly installed, a cooler can reduce the interior temperature by as much as 30-40F in hot, dry climates, less in wetter areas. As with fans, the size f the cooler is determined by the size the greenhouse. A general guideline is to find a cooler equal to the cubic space of the structure plus 50%. To provide both cooling and humidifying effects, the cooler must be installed on the outside of the greenhouse; otherwise, it simply humidifies without dropping the temperature.

Misting and watering are also important components of greenhouse gardening. Extended periods of growing and higher sustained temperatures make adequate water essential. Again, there are methods to suit every temperament from low tech to automatic.

Most companies offer watering and misting systems by component, which can be mixed and matched to suit grower’s needs. Automatic systems will have a timer that triggers the mist or water at preset intervals. You may want a toggle switch that allows you to rotate between manual and automatic watering.

A lower tech method of mist and watering control consists of a series f screens that tilt downward with the weight of the water shutting off the flow then raising to restart the cycle as the screens dry. It is fully automated by the weight of water or lack thereof. Of course, there is also hand-watering which is very effective and requires no mechanical intervention. Automatic systems, both high and low tech, are alternatives to hand watering which can be most helpful during a gardener’s absence.

heating and watering devices depend on that other cost of greenhouse keeping: how much time the grower has t spend tending plants. You can keep equipment costs to a minimum if you plan to spend a lot of time in the greenhouse. For growers who are away from the structure for long periods, automatic systems are a good investment.

In addition to shelter, heat, water, and ventilation, plants need light. This section will offer a brief treatment of lighting. Fluorescent light offers higher efficiency with low heat and is the most widely used. Incandescent light – 60-500 watts – may be used t extend day length. High intensity discharge (HID) offers long life, and the sodium lamps emit the best light to be combined with natural sunlight. Regardless of light source selected, you may want t purchase a light meter. It will be very useful in setting the light lever in your greenhouse for maximum efficiency.

Carbon Dioxide (CO2) is another important aspect of the greenhouse environment that will be only touched upon in this section. Closed greenhouses often have too little CO2 during the day for plants to be able to use light effectively. Enhancing the levels of CO2 will accelerate plant growth; methods for doing s range from expensive CO2 equipment with infrared sensors to block dry ice kept in a pressure bottle until needed.

Security is always a concern when growing cannabis. There are several ways t camouflage the greenhouse such as growing other plants with the cannabis. Paint the walls with sun blocking paint so light still enters but prying eyes do not. Go to the discount store and purchase artificial flowers. Put the artificial flowers on and around the cannabis plants so it looks like they are growing from the cannabis. Remember to pay attention to other plants that could be blooming during this time and follow suit.

Planting in the earth floor of the greenhouse allows you to use organic methods. The plants cannot be moved easily, but they grow bigger and require less maintenance than container-grown plants. Without containers, plants also retain a lower profile. Growing in Mother Earth is always better than planting in pots!. Most all the principles that apply to outdoor growing apply to growing in a greenhouse, too.

Greenhouses can be darkened to induce flowering during mid summer. This practice will allow you to harvest up to three crops a year! Cannabis plants flower when nights are long (12 hours) and days are short (12 hours). Darken the greenhouse so that plants receive 12 hours of uninterrupted darkness every day to induce flowering. When the greenhouse is darkened daily so that plants receive 12 hours of darkness, a crop of clones planted May 1st can be harvested by the middle of July.

Automatic darkening machinery is available for large commercial greenhouses. Smaller greenhouses are normally covered with black plastic to “black out” the interior for 12 hours.

When combined with natural sunlight, artificial light is optimally used during non-daylight hours. Greenhouse growers turn the HID lights on when sunlight diminishes (30 minutes before sunset) and off when sunlight strengthens (30 minutes after sunrise). Turn on the HID when the daylight intensity is less than two times the intensity of the HID. Measure this point with a light meter. Turn off the HID when the daylight intensity is less than two times the intensity of the HID. A simple photocell that measures light intensity can be used to turn the lights on and off automatically.

Supplementary lighting has greatest effect when applied to the youngest plants. It is least expensive to light plants when they are small.

Many different types of covering are available for greenhouses and cold frames. The best greenhouse films are UV (ultraviolet) resistant and still transmit plenty of light. Lexan is rigid and full of thermo-storing channels. It is one of the best greenhouse plastics available. Lexan lasts for years and transmits almost as much light as glass while retaining greenhouse heat. The only problem with Lexan is that it is clear. Some growers disguise greenhouse cannabis by wiring ornamental plastic flowers to the branches visible to passers by, which is advisable where neighbors are not curious and laws lax.

Regulating heat in a greenhouse is much more difficult than in an enclosed grow room. Greenhouses heat up quickly on sunny days, and cool equally fast when the sun ducks behind a cloud or drops below the horizon. This fluctuation in heat is difficult and expensive to control. Hot and cold dips also affect the ratio of nutrient to water plants plants need to use, which makes growing in a greenhouse more demanding than growing indoors.