6 Tools for the Perfect Greenhouse Temperature

Too much heat can kill a greenhouse full of plants just as too much cold can. Here are ways to maintain a balance.

To make a garden is to create an artificial environment: Some garden plants need extra water or more protection against pests than plants growing in nature. To garden in a greenhouse is to work with an even more artificial environment. When set up and managed correctly, it can keep the lethal hands of winter at bay. But a lapse in management can kill a greenhouse full of plants with too much heat as easily as with too much cold.

Here are a few tools that can help keep your greenhouse—and the plants inside it—at that perfect temperature.

1. Thermometer

The first thing any greenhouse grower needs is a minimum/maximum thermometer. These are also valuable to growers using hoop houses, cold frames or just starting seeds in the basement.

A glance at one of these thermometers tells you the highest and the lowest temperatures over the past 24 hours. Check it in the evening, and you’ll know how hot things got in the day. Check in the morning, and you’ll know how cold the night was.

2. Heat Mats

One of the first hard lessons in keeping a greenhouse is learning how much energy it takes to keep the air warm at night. But by using heat mats under your flats, you can keep your plants growing while dialing down your fuel expenses. Heat mats run on electricity, but they’re designed to be waterproof, so no one gets a shock. But the steady stream of diffuse heat stimulates the growth of the roots while the cooler air minimizes diseases and reduces transpiration.

3. Barrels

Another way to reduce fuel costs on chilly nights is to line a wall of your greenhouse with plastic or steel barrels (usually 55-gallon) painted black and full of water. As long as they are exposed to sunlight during the day, the mass of water in these barrels will slowly warm up. After the sun sets, they will slowly give off that heat in the greenhouse. If it ever becomes so cold that the water freezes, at that point they give off a last gasp of heat that benefits the plants for a while. One caveat: Such barrels can help keep temperatures just above freezing for growing things such as greens, but they can’t keep it warm enough for overwintering tropical plants such as bananas or orchids without supplemental heaters.

4. Automatic Vent Opener

Depending on your climate, cold days might be mixed up with mild or even warm days. A closed greenhouse in those conditions can bake some plants like a pizza oven. An automatic vent opener is a nonelectric piston that responds to rising temperatures by expanding and pushing open the vent on which it’s mounted. When temperatures drop again, the piston contracts and closes the vent. These also work well for cold frames.

5. Fan

When the temperature is mild and the air is still, diseases can spread through a greenhouse. Moving the air with fans provides good disease control. Plus, it can move cool air into a hot greenhouse during the day.

6. Shade Cloth

It’s always good to have nonelectric means for moderating temperatures in a greenhouse. Shade cloth draped over the outside of the glass or plastic will block a lot of the heat. Shade cloth is available in densities of 30 to 90 percent. That indicates the amount of light reflected away: the larger the number, the darker the fabric and the cooler the greenhouse.

This story originally appeared in the January/February 2018 issue of Hobby Farms.