Greenhouses and Cold Frames

Marijuana Horticulture

by Jorge Cervantes

This simple overview of greenhouses and cold frames will give you a feeling of what to look for and how to plan your project and reap a heavy harvest.

Greenhouses, cold frames, and hot frames are all useful in extending the growing season and / or protecting new plants and seedlings. Which type of structure you select depends on the size and location of your growing area, how much money you have to spend, how much time you have to grow, and security issues, Simple cold frames and hot frames can be assembled from common materials like old framed window panes and hay bales. Greenhouses are generally larger and more com0plex. They can be expensive t build and maintain but offer more flexibility for growing time and building use.

When deciding on a growing structure, first carefully analyze the project on paper. Consider how much space you have for the footprint and how many plants you can grow safely. Cold frames are small and can be as simple as glass or plastic frame set on the ground with no artificial heat source. Their basic function is to protect young plants and seedlings from wind and cold in the early spring, but they can also be blacked out to induce early flowering and harvest. Hot frames are similar in size and structure but provide heat through manure, electricity, steam, or a hot water pipe). You may use a hot frame to raise early seedlings and clones, after which the structure can be converted into a cold frame. Both frames share the advantages of economy, simplicity, small size, and portability.

Both large and small greenhouses cost more money, time, and space. With the exception of the lightweight “hoop” house or miniature greenhouse, they are also more permanent. The type of greenhouse selected will be determined by the planned use of the space and where it will be located. A lean-to or attached greenhouse will probably be smaller and less expensive to build than a freestanding structure.

Total area of the greenhouse is determined by the number f plants you intend to grow. Allow one square yard per mature plant. Do not forget to allow about six inches space fr air circulation between benches and side walls. Add space for walkways – standing room only or room for a wheelbarrow – and possibly a center bench. Glass, plastic panels, and sheeting all come in standard widths, and it is easier t build in size compatible with these units rather than have to cut panels down.

Budget, building skills, and security will weigh heavily in the decision making process. The least expensive structure per square foot is an even span 16-foot wide that will house two side beds or benches, two walks, and a wide center bed or bench. An 8 to 123 foot wide lean-t with wide beds or benches and a central walk is the least expensive option overall. Whichever option you choose, building it yourself can be cheaper and more secure than hiring a contractor. You can purchase much f the plumbing and electricity installations in kits or pre-assembled to avoid compromising security.

Climate will play a role in choosing your greenhouse. For example, a cold frame in the mild Pacific Northwest can give you a six week jump on the growing season. This would not work in a colder region like the upper Midwest. Likewise, a hot or tropical area will require more shade and water. While the large cold frame is the most economical of structures, it will not function a s a cold-climate garden. Location and exposure will depend on climate, but in general, you will want the greenhouse to be sheltered from curious eyes and strong winds and to be away from any areas where failing limbs or other debris might be a problem.

There are a number of external design options. Cold frames can be as simple as a window sash laid over a rectangle of straw bales or a piece of plastic stretched over a metal or PVC pipe frame and held in place with camps. Duct tape also works wonders to keep plastic in place. The advantage to plastic sheeting is that it can be removed during the day t take advantage of fresh air and the sun’s warmth and then replaced at night to protect plants from cold air. The cold frame can be easily converted to a hot frame by installing electric heat and a watering/misting system.

Greenhouses can be attached (lean-to, window-mount, even-span) or free standing. A lean-to uses an existing structure for one or more sides and is limited to a single or double row plant benches with a total width of seven to twelve feet and length up to that of the building. Without considering security, the advantages of a lean-to are its proximity to electricity, water, and heat, but on the downside are its limited size, light, ventilation, and temperature control.

A window-mount replaces an existing window providing a relatively low cost way to grow short plants, small seedlings, or clones. It can be installed fairly simply with common household tools. The disadvantages are its small size and possibly public view.

Low profile greenhouses are perfect from crops of short plants. It is easy to setup a low profile hoop house or a greenhouse alongside a building that gets full sun. The short greenhouse or cold frame is simple to darken during full summer and lets you reap the benefits f the harvest early!

Small greenhouses and cold frames also work well on patios, balconies, and rooftops. They protect plants from wind and prying eyes of neighbors.

An even span can be an attractive option. Like the window mount or lean-to, the even-span is attached to the house and bears similar limitations of size, light, ventilation, and temperature. Unlike the lean-to or window mount, the even-span can be larger and can open into the house – providing heat and humidity – or even function as a conservatory, an attractive place to relax. It is, however, more expensive to heat and maintain. Such greenhouses are most popular where security is a minimum concern.

The freestanding greenhouse offers the most flexibility in size and location. It can be built to take full advantage of the sun, but it does not retain heat well and can be expensive to keep warm. Many frame types and coverings are available in kits or raw materials.

Framing can be in wood or metal. You may select a panel frame which is more expensive to build but has the advantage of quick installation and breakdown for storage. If portability is an issue, there are miniature greenhouses and hoop houses which can be purchased as a kit for under $300. These structures, because they can be picked up and moved, are usually considered temporary by municipalities, and often do not require permits.

Plastic is much cheaper than glass, can be heated as effectively as glass, and is equal to glass in producing quality plants and buds. Polyethylene (PE) is low cost, lightweight, provides ample light, and can withstand fall, winter, and spring weather. It does not tolerate summer UV levels, however, and must be replaced annually. Ultra-violet inhibited PE lasts longer, but both types lose heat more quickly than glass. During the day, this can help keep plants cooler, but at night the heat loss requires the use of artificial heat source. Poly Weave is a plastic fabric made of 8-mil polyethylene reinforced with nylon mesh. it transmits up to 90% sunlight, can be sewn or taped, and has a lifespan of up to five years.

Polyvinyl chloride (PVC) is two to five times more expensive than PE but can last five years or longer. PVC is pliable, transparent, or translucent, and comes in four to six foot widths which can be sealed together to provide a super wide piece. Ultraviolet inhibited corrugated plastic panels provide another option. The panels can be used in cold frames, propagation houses, and greenhouses to provide excellent wind and snow protection and optimal solar heat collection.

Corrugated fiberglass is lightweight, strong, and comes in eight to twelve foot panels. Poor grades will discolor, reducing light penetration, but a good grade of clear fiberglass can cost as much or more as glass. Its lower weight has an advantage, and it is more difficult to see through!

Framework and covering are only the beginning. Growing plants in a greenhouse is often more demanding than growing plants indoors. Air temperature, humidity, light, and air quality must all be controlled in relation to a constantly changing greenhouse climate.