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Cannabis grown in a greenhouse using light deprivation
Cannabis grown in a greenhouse using light deprivation

Summer Greenhouse Setup

The sealed winter greenhouse should give way to a more open and naturally ventilated greenhouse during the hot summer months. To allow natural ventilation, take the end wall coverings down or reattach the sidewall covers a few feet off the ground to improve airflow.

What Type of Greenhouse Cover Do You Need in Summer?

Let’s face it — we all know what its like to be in a car with the windows rolled up on a hot summer day and you don’t want to do that to your plants. Lots of greenhouse growers are also beginning to install diffused greenhouse coverings.

Diffused light covers have a positive influence on production, especially during the summer, because it changes the light penetration to the crop and increases capacity for photosynthesis. A crop like cucumber can utilize diffused light better than direct light. In addition, diffused roof material result in a lower crop temperature — especially higher in the crop canopy — which likely leads to more optimal conditions for photosynthesis.

Polyethylene/polyweave covers are stronger and last longer than normal film. Greenhouse film is usually only four to six millimeters thick and very easy to puncture—we see it in landfills far more often than the polyweave material. The film is quite a bit cheaper, but its lack of longevity and the problems you’ll face using it in the summer just aren’t worth the savings.

Think Glass is Great for Greenhouses? It’s Not.

Most people think glass is the ultimate greenhouse covering. If you’re in the Netherlands or somewhere that never experiences hot summers, then glass is great. Glass does not insulate, however, so it can turn out to be an expensive way to cover your greenhouse while unfortunately making it less efficient at the same time. It’s also twice the cost of polyweaves and polycarbs. Glass in the higher elevations of your greenhouse also tends to magnify light, which creates hot spots on leaf surfaces.

As the greenhouse warms up, your plants begin to transpire by bringing water from the roots to the leaves, the natural way plants cool or protect themselves. Once the water transfers to the leaves, we begin to see the humidity increase and the plants begin to sweat. Check out this 15-second video of how the transpiration process works.

At first the transpiration process can be protective to the plant, but as the process continues we see more problems begin to develop. The humidity creates water droplets or condensation on the inside of the greenhouse roof. Rain on the inside! These droplets also act as little magnifying glasses all over the roof, which creates hot spots and the potential for mold. Add a power outage in the middle of the day—rendering exhaust fans and circulation fans useless—and you’ve got a recipe for disaster.

A simple diffused polyweave or diffused polycarb would have helped this situation and is half the cost of glass. A twin-wall polycarb is also a better insulator than glass, so it is much more efficient. But aren’t I getting more lumens with glass? you might ask.  That is somewhat true — the sun is so intense, however, that in the heat of the summer you will actually gain more light by diffusing it rather than by allowing direct sunlight. By diffusing the light you are taking a light particle and breaking it into more light particles, which can then actually penetrate deeper into the canopy and provide more light to the lower branches. Learn more about light diffusion in this 15-second video.

What an Optimal Greenhouse Looks Like in Summer

An ideal summer greenhouse setup can be anything with a diffused cover and good ventilation—preferably, something with roll-up side walls and doors at both ends. That way you can avoid solar gain, cool your plants and the soil (even if the air temperatures are in the 100°F+ range) and keep the air fresh. What you don’t want is stagnant air that continues to heat up. A roof vent is also an excellent option because it allows you to release the hot air that gathers in the peak of your greenhouse while keeping the cooler air that stays lower in the structure.

On a hot summer day, a greenhouse with a roof vent, roll-up sidewalls and doors on both ends will allow you to open up, avoid solar gain and in turn cool both the plants and the soil. This will result in decreased plant stress and allow photosynthesis to continue even when outdoor plants are suffering — and you won’t even need to run the big exhaust fans or evaporative cooling systems.

In fact, some small horizontal airflow (HAF) fans might be all you need throughout the heat of the day. But that is it’s own major topic of discussion! More on that soon.

Most companies outfit greenhouses for cold winter use only — it never seems to occur to them you might want to use it in the summertime. Sometimes just switching out your roof covers to match the season will do the trick. In the summer, a polyweave cover will protect you from rain and the sun’s intensity, while switching this cover out to a double-inflated poly film could add to the efficiency of your greenhouse come wintertime.

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