Winter Growing: Heating Greenhouses

Winter GreenhouseKeep your winter greenhouse productive with these heating and heat-conserving ideas.

Greenhouses are wonderful places, especially in the spring when benches are filled with brilliant green starts, and in the summer, its doors and roof vents propped open, with cucumbers trailing from the ceiling and tomatoes ready for picking. But in winter? Not so much. Overwintering herbs and potted plants cluster together for warmth. A few brown, leafless cucumber vines hang from an overhead trellis. Kale and spinach are over-picked and the seeds you planted have yet to sprout.

It’s a winter-time fact in most parts of the country: greenhouses, even those that might be attached to the house or garage, need some kind of heat source (of course, supplying appropriate light is equally important). My first, only partially realized, greenhouse up in the rain forests of Washington’s Olympic Peninsula needed heating just to keep the humidity down. But in most parts of the country, cold is the problem. You may have built your greenhouse with visions of supplying your family fresh, year-’round greens. But winter growth and germination are difficult when soil temperatures seldom climb out of the low 50 degree level. Sure you can use a heating mat to encourage germination. But even the hardiest green grows slowly — very slowly — when nighttime air temperatures plunge.

The germination mat is one kind of way to bring the temperatures you need to your greenhouse. There are as many ways of heating your greenhouse as there are greenhouses, and some of the new energy-conscious heating techniques (fuel is expensive!) are promising if not proven.

One fuel-avoiding, sustainable-friendly method is to build a trench down the center of your greenhouse and, after covering it with palettes or some cobbled walk-way, make compost in it. This might be limited to a small hole in the center of a hobby-sized greenhouse. Even at that, the compost will help moderate temperatures in the greenhouse and you’ll always have a ready supply of garden gold. And the daytime temperatures in the greenhouse should encourage your compost to heat up.

Now. . . full disclosure . . . we haven’t tried this. Our greenhouse was attached to the south facing side of the house in an attempt to get what little sun we had to give us some solar-generated temperature gain. And it worked, a bit, even on some cloudy days, the concrete grow box and stone walkway serving as heat sinks, giving off warmth to the bathroom and the kitchen porch at night. But an open pile of compost situated (technically) inside the house wasn’t considered to be a good idea.

Another way of creating heat sinks that will absorb energy during the daylight hours and give it up slowly in the cold dark, is to place 55 gallon barrels (or whatever is available and convenient) in corners and other practical locations in the greenhouse. They should be painted black for maximum solar gain. Even buckets of water in a hobby-sized greenhouse, will moderate temperatures just enough to make a degree or two difference, a difference that might be critical. We know a guy who wrapped black garbage bags around packaged tube sand and laid those out in his small house. The bags slowly disappeared as he and his son needed more weight in the back of their pickups.

Electric room heaters are the easiest and probably most popular way to heat a winter greenhouse overnight. Be sure that you follow all safety instructions and makes sure your heater is stable and away from any flammable material. Also take care if you’re running an extension cord out to your greenhouse. Make sure all connections, especially those inside the house, are snug.

Heat circulation is important when using an electric heater. Moving the warm air around will prevent hot spots (and their contrasting cold spots) as well in reducing condensation that heating will encourage. Some heaters have built-in fans, some need additional circulation.

We’ve been surprised at the number of greeenhouse operators using wood heat to warm their buildings. With propane becoming more and more expensive, wood and pellet stoves, again with provided circulation, are operating effectively and more cheaply than other fuels. Large, commercial-sized greenhouses are finding wood a viable alternative to expensive gas and petroleum products. When installing a wood stove in the green house be sure to follow all your local code requirements. Stand alone pellet stoves are especially easy to load and operate and most come with some kind of temperature control. Some have blowers to circulate heat.

I don’t think I would put a wood stove in a plastic covered greenhouse. Stovepipes can get very hot and the risk of melting or igniting Visqueen and other plastic covers seems great short a stove vented out through a masonary foundation or some other such careful planning. I had hoped to put a small, slow-burning wood stove in my attached glass greenhouse, both to heat the growing things and the rooms on that side of the house; one of the unrealized dreams in my green house.

What about a plastic covered greenhouse? Insulation is a great way to conserve heat without expending fuels. I’ve seen it recommended that those with Visqueen covered houses line the inside of their plastic with bubble wrap. Now,again, I can’t say we’ve tried this but it makes perfect sense that the bubble wrap, with its large bubbles of air, would preserve indoor heat while letting sunlight through. Hmmm…anybody else try this or have other ideas on heating greenhouses? You know we’d love to hear about it. In the meantime, here’s a guide to heating greenhouses from those resourceful, greenhouse-happy British. Notice they put the bubble wrap idea first. If you don’t have a green house and the idea seems impractical for whatever reason . . . how about a hotbox cold frame?

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