Although the temperatures are dropping outside, they don’t have to drop inside your greenhouse. Gardeners have been using a variety of methods to heat their greenhouses for centuries, and many, particularly those with tropical plants, heat their greenhouses in the winter to temperatures in which their plants can thrive. Gardeners may also seek to heat their greenhouses later in the winter, when they begin to start seeds for spring planting. Thanks to modern technology, you can even control the temperature of your greenhouse electrically if you wanted to. Still, there are ways to keep the greenhouse warm that won’t increase your monthly utility bill.
Using a Heater in Your Greenhouse
One of the most common types of greenhouse heaters is the electric fan heater. These heaters move heat and air throughout the greenhouse and can hook up to the main power of your home. Position it in a central spot at one end of your greenhouse and ensure it’s not touching other objects, water, or plants. With electric heaters, you can set the thermostat to the best temperature for your plants and program it to only turn on when you need it, like on nights when the temperatures dip below freezing. Although these heaters are already intended to circulate warmth, you can always set up an additional fan or box fan in the greenhouse to help them out.
The downside to these heaters is that they can increase your electric bill during the winter months. To save on utility fees, measure your greenhouse ahead of time and purchase the heater meant to heat a space of that size. You can also group your plants together according to their minimum temperature requirements and heat them accordingly.
Paraffin heaters are another good, albeit less popular, option. They’re cheaper to run than electric heaters, but it may be difficult to find one in your area. You also can’t regulate the temperature on these or schedule them to turn on and off when you want them to. Since they don’t have fans, the hot air they produce isn’t able to circulate well.
Regardless of what option you choose, be sure to ventilate your greenhouse to control humidity. Open the vents in the morning and close them before dark in the afternoon.
Even if you rely on technology to heat your greenhouse, you may still be able to use some of these non-electrical options to help reduce your heating costs.
Horticultural bubble wrap is designed for outdoor use, unlike the stuff you use to wrap up valuables when you move. The larger bubbles let in more sunlight while insulating the greenhouse's glass panes. Run it along the glass of your greenhouse, and if you have temperature-sensitive plants, wrap it around their pots, too.
Horticultural fleece is like a blanket for your plants, and it serves to provide insulation and increase the temperature by several degrees during freezing cold evenings. You may also be able to purchase fleece cloches for the plants you've planted in your greenhouse. In either case, be sure to remove the fleece during the day so your plants can get air.
Tap into thermal mass by using objects to absorb the heat from the sun during the day and release it back into the greenhouse in the evening. Rocks and bricks provide a popular and inexpensive way to store heat, especially if they're darker in color. Place gravel or large black stones (you can even paint them black if you wish) on the floor of the greenhouse. You can also paint cinder blocks or ceramic pots black and place them in the part of the greenhouse that gets the most sunlight. Place the blocks or rocks near your plants, and the heat will radiate out from them and keep the greenhouse warm.
If you don’t have rocks or bricks on hand, you can also use drums of water (painted black) to absorb heat and radiate it throughout the greenhouse. You can also use gallon jugs of water (or empty milk jugs filled with water) — just place black trash bags over them during the day to help them absorb more heat. Then place your heat-loving plants around them. If a cold snap is in the forecast, fill the jugs with hot water and place your plants around them.
These are all tried and true ways to heat your greenhouse. However, the downside of using them is the inability to control the exact temperature of the area.
Protect Seedlings and Starts From the Cold
Many seeds can be started indoors several weeks before the last frost of the season. If you don’t have the space indoors and/or intend to start plants in your greenhouse during the winter, consider getting a heated seed mat. This black rubber mat uses electricity to provide heat to your seeds and encourages propagation. Since it's electric, you'll even be able to control the exact temperature. Keep in mind that you may need to water your seedlings more often when using a seed mat, as the soil is more likely to dry out.