Overwintering and growing winter hardy plants and vegetables can be done in permanent or portable greenhouses with proper preparation and planning for plant protection. Many of our customers use their greenhouses during the winter to protect their plants from freezing temperatures, but, depending on your climate, a greenhouse by itself may not provide adequate protection. Greenhouses trap solar heat from the day, however, you will likely need supplemental heat sources for below freezing temperatures.
Eagle Peak has a wide variety of portable and semi-permanent greenhouses to fit your climate and fill all of your gardening needs. You can see our complete line of greenhouses and greenhouse accessories here.
Greenhouses collect solar energy during the day warming the ground, soil in planters and beds, rocks and any other items with mass. A greenhouse will also trap humidity and moisture that evaporates, and moisture in the greenhouse can lead to frost on your plants if temperatures fall to below freezing. Be sure to open windows for ventilation during the warmer day time hours to reduce the amount of moisture within the greenhouse before closing them at night to help retain the collected solar heat.
One of the major purposes of the greenhouse is to help trap solar energy during the day to keep plants warmer at night. Make sure you plan the location of your greenhouse to receive maximum amount of sunlight during winter months along the longest side of the greenhouse. Orienting the shorter sides of the greenhouse (if rectangular) east to west will position the longer side with southern exposure. Ideally the location is shielded from wind and has a minimal shadowing from other trees, shrubs or buildings.
The use of outdoor rated greenhouse heaters is recommended for colder climates. Electric heaters can help heat the interior greenhouse space once temperatures drop. Electric heaters need to be designed for greenhouse use, which means that they are IPX4 rated for water resistance for maximum safety. They should also be plugged into a GFCI (ground fault circuit interrupter) outlet using a properly sized, outdoor-rated extension cord.
You’ll need to multiply the square footage of your greenhouse space by an estimated U-factor of 1.15 btu/hpft for a PE cover/top. Multiply that number by 70. Examples for our most popular sizes of greenhouses are as follows:
GH48 6x8ft: (6*8)*1.15*70 = 5040btu
GH96 8x12ft: (8*12)*1.15*70 = 7728btu
GHT70 6.6x10ft: (6.6*10)*1.15*70 = 5313btu
Heater output is usually listed as either btu or watts. You can use this calculator to convert btu to watt output needs.
We recommend floor style heaters such as the ones listed below for maximum safety in portable greenhouses with PE covers. Use of hanging heaters should be reserved for greenhouses with glass or polycarbonate window panels.
A lot of heat is lost rapidly through a soil floor. Brick pavers, rocks, and large clay or terracotta planters in your greenhouse will absorb solar energy through the day and help regulate the temperature inside the greenhouse at night. A full greenhouse with a lot of heat absorbing mass will fare much better than a sparsely filled greenhouse. You can also help heat retention by lining the outside and inside of the lower wall with additional straw, bricks, rock, or 4x4 lumber to help seal the lower wall of the greenhouse cover and prevent winds and cold air blowing underneath.
Another suitable material for capturing heat is water. We use plastic jugs filled with water placed within the greenhouse. Water stores thermal energy much more efficiently than air, so save your detergent bottles, milk bottles, etc and fill them ¾ full with water and place them in the greenhouse. We prefer closed containers to reduce the possibility of evaporation and to minimize the humidity level in the greenhouse.
Some gardeners also use the Candle and Clay Pot Hack. We do not recommend using open flame heat sources within the confined space of a greenhouse. To use this method, place a candle in the center of a terracotta saucer. Light the candle and place a suitably sized terracotta planter upside down over the candle. The heat from the candle flame will warm the terracotta which will radiate the heat.
Composting discarded natural waste is a great way of generating natural heat. Heat is generated as the natural compostable materials break down. In fact, the center of a compost pile can reach 100-160F. While you may not want to put a compost pile inside your greenhouse, you can always put one close by the outside of your greenhouse. You can build a closed loop system with water filled tubing to make use of the heat. Make a coil of tubing under the compost pile and run it into the greenhouse. A small recirculating pump is used to make the water flow from the warmer area under the compost pile and into the tubing loop inside the greenhouse.
Portable greenhouses use a single wall Polyethylene or PVC cover. While this is a perfectly suitable material, it is a single layer of insulation, and there are ways to improve the performance with additional insulation materials. Insulating the northern wall of a greenhouse is highly recommended to prevent heat loss, as this side does not receive any benefit of the warming rays of the sun.
Packaging bubble wrap’s flexible sheeting and bubble texture can be hung from the inside framework of the greenhouse frame. This material is translucent and lets light and solar energy in, but also helps build an insulating air layer between it and the greenhouse cover. Bubble wrap can also be used around individual planters and flowerpots and secured around the stem of the plant. Agriculture fleece is a different type of polypropylene insulative material which can be hung from the greenhouse frame or even lightly draped over your plants.
The above materials are translucent and allow sunlight and warming to occur from the east, south and western greenhouse walls. It is also advisable to use a reflective insulating sheet on the northern roof and wall sides. Reflective mylar bubble sheeting helps to reflect the daytime solar energy back into the greenhouse from the typically cooler northern wall.
Even the most basic greenhouses are said to improve the USDA hardiness rating by one zone. The average temperature inside the greenhouse will be, on average, 8 degrees warmer than outside without any supplemental heating, insulation or heat retention techniques. By orienting the greenhouse properly, and employing the insulation techniques above, the average temperature inside the greenhouse can be an average of 20-30 degrees warmer than the outside air temperature.
We hope this information helps you to have a successful winter gardening season.