Heating Your Greenhouse: Keeping Your Plants Cozy This Holiday Season and Beyond

Posted on December 4, 2020 | Categories: Donna Balzer, greenhouse, Heating your greenhouse
greenhouse for snow

Snow settled on my greenhouse is a good hint it is not losing heat to outdoors

Imagine my surprise when a small hole we drilled through our greenhouse foundation became a runway for mice. We pulled an extension cord into my greenhouse through that hole and the tiny gap became a neon sign for rodents.

The grandchildren, fascinated by mom and her naked babies, watched carefully as the whole mouse family was carefully moved from a greenhouse pot, where they were living, into a big cardboard box in our utility trailer. “We are taking the whole family to the country,” I lied.

Well, the dump is in the country so I guess it was more of a fib than a lie. But lesson learned.

When I moved my greenhouse, I changed things up a bit.  We dug a trench from our house, ran wiring underground and hard wired a plug inside the greenhouse. We haven’t seen a mouse since.

greenhouse tomatoes

Inside the greenhouse even the hardy crops need an extra layer on cold days. Here Agribon is draped over small plants

The plug inside my greenhouse is now my secret to winter plant survival in the greenhouse. Hardy plants don’t need much help but in spring, especially, tender seedlings like tomatoes need help on cool evenings and fans on warm days.  So, I insist all my friends add electricity to their greenhouse. And it’s not just about mice, it is about keeping plants cozy in the cold season and well aerated year-round.

greenhouse vegetables

Winter broccoli like red spear is great in the winter greenhouse because it tolerates heavy frost


If you want to keep plants thriving, I always suggest the simple electric plug.

Here are my five more tips for keeping your greenhouse cozy this winter:

  1. Don’t wear your bathing suit to the ski hill. To keep heating costs down, grow only cold-tolerant plants in your greenhouse over winter. Wear your bathing suit to the tropics or hot tub and keep your heat-loving tropical plants, like Basil, orchids, coffee, African violets and most houseplants inside your home for the winter. Temperate plants, like Meyer lemons, hardy cactus and olives tolerate light frost. Hardy vegetables, like spinach, mustard greens, arugula, kale, beets, carrots, and swiss chard also thrive in a cool greenhouse and can freeze completely and spring back to life when conditions warm up. I try to keep my greenhouse at or around zero C (32 F) in winter to keep temperate plants happy.
growing food in greenhouse

Spinach can freeze solid and still come back so it is perfect as a winter crop in the greenhouse

2. Add a scarf: Providing heat just where you need it is easily accomplished with old-school Christmas lights or soil warming cables. Scrounge around your grandpa’s attic or visit the local thrift store for old lights. They actually give off heat when turned on so can keep small areas warm. I wrap my lemons in lights to add just a bit of heat when temperatures drop quickly. I lay the water-proof soil warming cables (also sold as gutter de-icing cables) on the soil and set potted plants over the heat for happier winter plants.

hot house greenhouse

Kishu mandarin is normally a heat lover but I put it under Agribon with a heater connected to a thermostat

3. Consider a few hand warmers: A Thermostat with a plug-in for your lights as well as a space heater will keep the chill off the plants when the temperatures really dip low while you are tied up in your office or fast asleep late at night. If you simply depend on a heater, you’ll get home from work on a sunny winter day and find out you have been heating the outside air all day while your vents are wide open to cool off the space. Get yourself a thermostat, plug in anything that provides heat and only use electricity when the plants really need it. You set the temperature and keep it just warm enough in winter You are not trying to re-create Belize in May. You just want to keep your hardy plants out of the deep freeze. A thermostat plugged into the heater, will turn on the heat as needed and then turn it off when the solar effect of your greenhouse kicks in during the day.

citrus in greenhouse

My baby grapefruit tree was planted this spring and is seasonally protected with Agribon inside my greenhouse

4. Layer up. Your BC Greenhouse has an air space built into the polycarbonate cover but as mom always says “wear an extra layer” to keep warm. The same thinking applies to your greenhouse. Adding a layer of Agribon adds 2-7 degrees of frost protection, depending on the weight of Agribon you use. Higher numbers like Agribon 30 give more frost protection than lower numbers like Agribon-19. And this low-tech solution to growing in winter just got better.

cold weather greenhouse

Helpful husband built a frame inside my greenhouse to protect the tender citrus (orange and grapefruits)












This year Helpful husband built a mini-greenhouse inside my big greenhouse for seasonal protection of my tender oranges. Unlike lemons, oranges and grapefruits need it at least 5 degrees Celcius so we cover the whole warm citrus area with Agribon and plug a small heater to our Thermostat. This is different from our lemons. They have Christmas lights and a quick wrap of Agribon. I also cut lengths of Agribon and lay it over the rows of plants growing in pots or in the ground. I roll it up as the weather improves but I don’t put it away completely until all danger of cool weather is past. Then I wash it, fold it and put it away for the summer.

heating your greenhouse

Tomatoes are tender crops so they are best planted in later winter/early spring and covered with Agribon














5.  Don’t forget to breathe. Moving air is less likely to freeze than air trapped in cool corners or pockets of the greenhouse. Running a fan continuously in your greenhouse, winter and summer, keeps the air moving and prevents cold zones from affecting areas and parts of plants in winter. In summer, it keeps things cooler. Air blowing over plants also keeps them stronger and sturdier.

greenhouse vegetables

Hardy winter salad from the greenhouse includes spinach, chives and purple cabbage

For more great tips from Donna, visit www.donnabalzer.com.

You can also read Donna’s gardening books: No Guff Vegetable Gardening with Steven Biggs and her just released Gardener’s Gratitude Journal:  Part Diary, Part Personal Growing Guide.

donna balzer