Keeping the Greenhouse from Freezing

Earlier this fall, I set up my pop-up greenhouse out on the patio, as I do every year.  At first, I move in only the most tender plants, and just zip the doors shut at night.  As the weather continues to cool off, I keep the doors closed all of the time.  Then, before the first hard frost, I add a second insulating layer of plastic, by hanging a layer of bubble wrap inside the walls and roof of the greenhouse.  I also put in plastic containers of water, painted in dark colors to absorb the sun's heat during the day and re-radiate it at night.  Real solar greenhouses have very large containers to hold a lot of heat; I just have 1-2 gallon containers, such as milk jugs.


These heat-saving mechanisms have been enough in past years to help keep the hardier plants like parsley, mint, strawberries and petunias green for a couple of months longer than outside.  Some container plants that might otherwise die overwinter just go dormant, and come back in the spring.  I have also been able to grow lettuce off and on during most of the winter months.

This year I am trying something new to help add a bit of warmth to my greenhouse.  Every winter I put a heated dog dish out to provide water to not only my dog, but also for birds and other small animals that visit my yard.  This dish keeps the water just warm enough so that it doesn't freeze. I found a second heated dog dish on sale late last winter.  Last week I put this second heated dish in the greenhouse, only the water in it is in a large zip-lock bag to keep it from just evaporating.  The heating elements only uses 25 watts of electricity, so it won't cost much to run. Last night when I opened the greenhouse to water the plants, the bag of water was warm to my touch.  This won't be adding a lot of heat to the greenhouse, but I hope it will be enough to help the plants survive the cold front coming in later this week.