Winterizing a Hobby Greenhouse

A small hobby greenhouse is an ideal way to extend the growing season as much as three weeks on each end. Winterizing a small hobby greenhouse for continued winter use can present a challenge in areas such as ours. Our average minimum winter temperature is -9ºC (15.8ºF) for January. At these temperatures I found it was not feasible to keep the greenhouse going year round even with the winterizing I did and a small electric heater.

One problem with hobby greenhouses is they are leaky. To partially solve this problem I used clean marine grade silicone to seal along all seams inside the greenhouse. This solution was pratical and cheap. It did improve both heat and moisture retention. This is important because once a hard frost threatens water hoses should be disconnected. Two warnings, do not put any plants in the greenhouse until the silicone is cured! The fumes will kill plants hence the second warning. Keep the door and vent open during application. A fan will help move the fumes out quicker.

The hobby greenhouses generally have one roof vent that is difficult to seal and since it may be needed for temperature contol during the day cannot be sealed shut for winter. The best solution to this is to use a piece of bubble wrap or quilted material attached with velcro. The worst air leak problem is the door. When I bought mine, the door was the sliding design. It is virutally impossible to get a tight seal. I see where they have now changed the design of the door so perhaps the new hinge style is a bit better.

Heat Sinks

Every greenhouse needs one or more heat sinks. My greenhouse has 4" gravel base that absorbes sun then reflects it back into the greenhouse. In summer it acts in reverse when hosed down cooling the inside temperatures. However, for extending the growing season and winterizing, additional heat sinks are needed. One problem that will become immediately clear is in an 6' x 8' greenhouse there is not a lot of room for large barrel style heat sinks although I am still contemplating trying a smaller barrel.

For this project, I used recycled margarine tubs, plastic food pails, matte black spray paint and water. I got margarine tubs and food pails from two local restaurants that were happy to let me cart them away for free. The spray paint cost about $5 for store brand. I washed the containers then set in the sun to dry. Then I put them in the shade upside down on carboard and gave them two coats of spray paint. Once dried I moved the containers into the greenhouse, filled each with water leaving 1" headspace and snapped the lids on. The containers were distributed throughout the greenhouse to absorb the sun during the day.

I found these small heat sinks worked relatively well. They went through the first winter well and held up well during the following summer. The next winter, the paint started flaking off likely from age, expansion and contraction. I left them for this winter as I simply shut down the greenhouse. This spring I plan to clean them up and re-spray. The only thing I plan to differently is add an anti-freeze agent to the water.

Bubble Wrap North Wall

The north wall was sealed with clear marine grade silicone then insulated using bubble wrap and clear draft sealing tape. I used regular packaging bubble wrap that comes in a roll and draft stop clear tape. Total cost for bubble wrap and tape was about $This insulation remains on that wall year round. It is going into it's third season and has held up very nicely. I've done nothing to it since installation.

Installing the bubble wrap wasn't really difficult just tedious at times. I wiped the trim down with rubbing alcohol and let that dry before installing the strips. Strips were installed horizontally. The trick I used was to use a small piece of tape to hold the strip in place then secure all around the strip. The lower section took two full width strips. The upper section took one full width strip and a triangle about 8" high. Each strip slightly overlapped the previous and the joint was sealed with draft tape. As the strips went up so to did the temperature and humidity level in the greenhouse.

Inside View

After the installation was complete, the plants were loaded into the greenhouse. These were mainly plants I wanted to take clippings of along with seedlings I hoped to grow during the winter. This picture taken at night shows the amount of condensation than in itself provides low temperature protection. All in all I was pleased with the results of the bubble wrap but the true test was how it would last the winter and if it would allow me to keep the greenhouse open year round. As already mentioned the bubble wrap did not provide enough insulation to keep the greenhouse open year round but it has proved to be very effective, durable and low cost.

Roll-up Door Insulation

I created a bubble wrap roll-up blind for the door. The sliding door is almost impossible to seal so this was the next best thing since I had left over bubble wrap. On cloudy days and at night, I simply unrolled the blind and secured with two rocks then shut the door and put a rock in front to hold the door tight.

The first year the door would actually fall off in high winds something that I couldn't have with plants in the greenhouse. I took to using a rock until my husband devised a modification to secure the door.

Other ways of effective insulation include styrofoam or straw bales around the base and putting a blanket on the roof at night. Once the snow falls, my husband certainly would not be too keen at the blanket idea. The straw bales make a surprising amount of mess on garden paths and since mine are gravelled that was a consideration. The biggest problem with straw is it provides rodents a place to nest for the winter. Then there is the availability issue. Securing the styrofoam is a problem but I'm sure one could be overcome if you are so inclined.

A word on heating: There are many ways to provide supplemental heating for your greenhouse. My philosophy is to always start with the easiest then move up to other methods if needed. I started with a 150 watt grow light. It threw off enough heat so even heavier frosts were well tolerated. A long burning pillar candle an be added if the light is not enough heat. Be sure the candle is in a tip proof container secured by wet sand. I do not recommend this method overnight! Here the next level that is easiest and cheaper than other methods is surprisingly a small electric heater. These work well but only to a point with the smaller greenhouses and if you are anything like me the idea of the electric meter spinning because of the greenhouse heating is not a comforting thought. Other heating sources recommended but not tried sterno (cost), campstyle catelytic heater (cost), woodstove (definitely not inside for a 6' x 8'), hot water heating powed by a woodstove (ok if you have a shed near the greenhouse that will house the woodstove) and natural gas (expense of running lines). So that's as far as I am on heating the greenhouse.

Happy Gardening,

Garden Gnome