How To Grow Marijuana In A Greenhouse During The Winter

I’ve maintained a small greenhouse for most of my adult life—a necessity because I’m a food and garden writer who’s constantly experimenting with new plants other than marijuana. I need to germinate chile and tomato seeds in the spring. I also love tropical and subtropical plants like bananas, hibiscus, mangos, and Meyer lemons, so I need a greenhouse to winter them over. But a winter greenhouse is not the ideal place to grow marijuana. Winter light is not the best light for growing marijuana. Not only are the days shorter, which provides less total light for the plant, but also the intensity of light in the northern hemisphere is greatly reduced because of the low angle of the sun. Some sources indicate that winter light intensity is often less than one-tenth of that of typical summer daylight, which greatly inhibits plant growth because it reduces photosynthesis. In simple terms, plants that require intense light cannot produce enough food for themselves during the low light of winter, causing leaf yellowing and dropping. Essentially they are dropping inefficient leaves and making new ones closer to the light source at the top of plants.

And the leaves are usually narrower and more delicate, further reducing their effectiveness and causing the plants to become “leggy.” Growing marijuana in a greenhouse like this one—where Mario Dadomo grows chile peppers at the Azienda Agraria Sperimentale Stuard in Parma, Italy—would be overkill for a medical marijuana operation. But one can always fantasize.

Marijuana is demanding of full sunlight, and if it doesn’t get it, its quality diminishes. A high level of THC requires intense light, and the low light in a winter greenhouse will make a high level difficult, if not impossible, to achieve. In other words, the potency of marijuana grown under winter low-light conditions is significantly reduced. Adding lights to correct this requires a waterproof greenhouse roof. In addition, insect pests are always a problem in the winter greenhouse.

Some growers attempt to use a sunroom for marijuana cultivation, but I don’t think that they’re efficient at all. In the summer, the sun is too high in the sky, so the angle is wrong for most sunrooms and the plants will receive diffused light rather than direct light. In winter, the sun is lower, so the plants will receive direct light, but it’s not strong enough for proper growth, just as in the greenhouse. I say forget about using a greenhouse or sunroom and simply grow the plants under HID lamps. More about marijuana in greenhouses during the winter here.