Driven by ever expanding technological innovations, the complexity and sophistication of greenhouse cultivation operations are practically boundless.
However, it is safe to assume that increased automation is jointly tied to increased expenses and decreased labor. The less money one is willing to spend on a wintertime greenhouse crop, the more human attention it will require. With these notions in mind, here are a few guidelines covering the basics of wintertime greenhouse gardening:
Climates and Greenhouse Frames
The primary factor in deciding how to construct, control, and cultivate a prospective winter greenhouse operation is which sort of climate is being engaged. Correspondingly, the colder a geographical location is, the less options there are for growing a successful crop. Along this line of thought, for both cold and temperate climates in North America, ‘hot frame’ greenhouses are the only option for wintertime cultivation.
Hot frame greenhouses are generally permanent structures that can hold up to the intense demands of wintertime gardening, providing the platform and infrastructure for effective climate control.
Greenhouse frames can be custom built by professional carpenters and builders, but the majority of horticulturalists prefer the simplicity of greenhouse kits. In conjunction, wintertime gardening requires well-insulated, thick greenhouse walls. Depending on one’s budget, these walls can be constructed with fiberglass, glass, Sollex, and polycarbonate.
"The more money one is willing to spend on automation, the more effectively one can maintain an ideal atmospheric equilibrium."
Heat Sources and Supplemental Lighting
When planning a wintertime cultivation operation, it’s crucial to understand that controlling a constant greenhouse temperature is extremely difficult. Again, the more money one is willing to spend on automation, the more effectively one can maintain an ideal atmospheric equilibrium. With a well-insulated greenhouse, wintertime cultivators must balance climactic requirements with both heating methods and supplemental lighting.
In the design of a greenhouse heating system, gardeners need to weigh both ambient temperature and root-zone temperature against the demands of their desired crop. In geographical locations where temperatures are cold enough to freeze the ground, keeping root zones warm is an essential practice.
Root zones can be protected by growing in raised beds off the ground, using heating mats beneath pots, and running hot water in tubing beneath pots.
Finally, ambient temperature in a greenhouse can be controlled with a variety of methods which are dependent upon greenhouse size, outdoor climate, and budget. These can range from a simple electrical space heater for your novice operation to thermostat controlled, industrial propane heat sources for a large-scale operation.
For cultivators interested in growing light-intensive crops in the midst of winter, supplemental lighting is a must. Generally speaking, HPS (high pressure sodium) and DE (double ended) lights are utilized in wintertime greenhouse gardens to supplement sunlight on cloudy days and to extend growth periods on short winter days.
Supplemental greenhouse lighting has two serious implications for the wintertime gardener. To begin with, HPS and DE lights emit a large amount of heat—this heat must be balanced with the ambient temperature in the greenhouse during both day and night.
Secondly, these lights are expensive to operate and should be utilized under consideration of a cost/benefit ratio. For growers in northern states, this ratio makes it obvious that it’s less practical to grow light-intensive plants in greenhouses than indoors during the winter.
For horticulturalists in warm and temperate climates, however, wintertime greenhouses are economically viable. To further illustrate the challenges and benefits of wintertime growing in a temperate climate, I asked Matt Johnson, a northern California greenhouse expert, for some detailed advice. Here is our short Q and A:
Maximum Yield: Is expensive automation a necessity in wintertime greenhouse cultivation?
Matt: That’s totally dependent upon your goals and lifestyle. If you have an industrial size grow operation, it’s a must. Otherwise, it’s going to be a really hands-on experience 24/7.
MY: How do you deal with the extreme temperature fluctuations of greenhouse growing?
Matt: I have an industrial propane greenhouse heating system which is run by a thermostat. This is used in conjunction with industrial dehumidifiers so that we exhaust the greenhouse as little as possible. In this way, the exhaust system doesn’t pull in cold air from outside and push out the warm air from the heating system.
MY: How much do your supplemental lights really have to work?
Matt: We have 20 DE lights in a 30ft x 96ft greenhouse that we use during both the vegetative and flowering cycles. During the veg phase, we run the lights from 4 p.m. until 1 a.m. to provide 18 hours of light during the short winter days (currently the sun rises at 7 a.m. and sets at 4:45 p.m.).
Once we reach flower, we use our lights only on cloudy days. But, when it’s sunny, like it is most days in California, we don’t need to turn the lights on at all because the short days trigger the 12/12 photoperiod. So I’d say, during flower, we use the lights on one-third (33 per cent) of the days.
MY: What are your estimates on power savings when compared to traditional indoor growing?
Matt: You have to consider the size of your canopy against what you are paying in electric bills. It’s tough to give an exact figure because the amount of time we use our lights (during flower) is totally dependent upon the weather.
However, our canopy is 1,720 square feet (86x20) with 20,000 watts in electricity. When you compare this figure against an indoor operation canopy at 400 square feet (20x20) and 16,000W of electricity, the savings per square foot are already quite evident, even during vegetative growth.
When it comes time to flower and we only run the lights one-third of the days, the savings are astronomical.