Whether your greenhouse will be filled with plants during the coldest months of the year or soon after, there are things you can do to better prepare your greenhouse for the cold months.
Check the greenhouse film for torn or loosened areas. Tears should be repaired with greenhouse tape, and loose sections secured. If the film is past its guarantee and is tearing in several places, it may need replacing. When the greenhouse will be used through the winter, make sure the film is strong enough to withstand winter storms and early spring winds. If an empty greenhouse will be filled early in the spring, the film can be checked a few weeks before the greenhouse will be used and replaced then, if necessary.
At least once a year, the greenhouse should be thoroughly cleaned and sanitized. Doing it as a part of the winterizing process, is the easiest for me. When the greenhouse is empty in January, cleaning can be done just before planting begins. I remove everything from the greenhouse, sweep or vacuum the ground cover, and spray the entire greenhouse with a sanitizing solution.
When I plant a winter crop the cleaning is done in August or September, just before planting begins. Because I still have plants growing in sections of the greenhouse, the cleaning needs to be done section by section, but it is still important to clean and sanitize each area of the greenhouse at least once a year. Since I replace winter crops with spring crops as they are harvested, sometimes plant-by-plant, there is no chance for me to sanitize at that time.
Check the equipment; provide regular maintenance, and stock parts that may be needed for routine service through the winter. Do you have a back up system for heat, in case the main system stops working? Kerosene heaters work well as a back up to other small greenhouse heating systems. I use kerosene heaters for my regular heating system. They need to be cleaned and extra wicks stocked, which need to be replaced occasionally during the heating season. I also have an extra heater in case one malfunctions or the electricity goes off and I need to use one temporarily in my electrically heated germination chamber.
If you plant directly in the ground, you can install an insulating barrier under the wall of the greenhouse when it is built or add it later. That will allow the ground to stay warmer in the winter providing more heat for plants at night. That is especially important if you want to germinate seeds directly in the soil or want plants in the soil to continue to grow through the coldest part of the winter.
In my area the ground freezes to a depth of about 24 inches, maximum. Two-foot wide pieces of Dow Blue rigid foam can be used as a barrier against the cold. It comes in ½”, ¾”, 1″, 1½”, and 2″ thicknesses and will last indefinitely. Its R-value ranges from R-3 to R-10. If that brand is not available, be sure you use a closed-cell type, not the cellular/pebble or Styrofoam-type, which breaks up easily.
A dark ground cover used around the outside of the greenhouse for weed control or landscaping will also absorb the sun’s heat warming the ground underneath. The ground rarely freezes under my ground covers, so the soil next to the greenhouse stays warmer. A minimum width of 3 feet should be used. It is less expensive and easier to install than the rigid foam, but is less permanent and less effective unless wider widths are used.
My wooden owner-built greenhouse has small spaces between some of the bottom boards and the ground. I always need to check for cracks and fill them to prevent cold air from blowing into the greenhouse. It is also important to fill cracks around doors and air vents to prevent loss of heat.
Double and Triple Covers
By covering plants with additional layers of greenhouse film or insulating blankets, cold hardy plants can be raised, or at least harvested in most of the continental United States, throughout the winter, without the use of additional heat. Supports can be constructed over sections of the greenhouse floor using wire, wood, or other materials you have available. Supports should be sturdy enough to hold covers and withstand the covering and uncovering of plants/beds on a regular basis. These covers should be ready to use when the first cold night arrives.
Wire supports should be constructed of good quality wire, 12 gauge or stronger. Galvanized is the least expensive of these, but aluminized, aluminum clad, and stainless steel wire is also available. You can buy hoops, but buying the wire allows you to make any size you need and saves you money. At Kencove.com you will find a large selection of the more rust-resistant wire types. Since high-tensile wire is usually sold in longer lengths than you may need, you may be able to buy a smaller amount from a local animal farmer.
Since the closed greenhouse will get hot during sunny winter days, covers made from greenhouse film, plastic, or other nonporous materials, will need to be removed every sunny day to prevent plants from overheating. Covers that do not allow enough light through for adequate growth will also need to be removed.
Insulating blankets that allow excess heat to escape can be left on during the day, but they restrict light and slow growth to varying degrees. Atmore Industries makes a UV-stabilized row cover, Gro-Guard UV, that increases soil and air temperature during daylight hours and slows heat loss at night. It can be used inside the greenhouse to provide additional protection during the coldest months of the year and then used again outside to provide crops earlier in the spring. The heaviest can protect plants to temperatures as low as 18º F, but it transmits only 40% light and should be removed for maximum growth to continue. Gro-Guard can be purchased in four weights and several sizes.
To grow warm weather plants during the winter, double and triple covers will save on heating bills. In my larger greenhouse, I section off a smaller part to conserve heat. I use greenhouse film, polycarbonate sheets, or insulating blankets to enclose one corner of the greenhouse. Small temporary structures, used outside for early spring crops, also can be set up inside the greenhouse to save heat required for tomatoes, beans and other warm season plants.
Gini Coover is the author of The Natural Greenhouse, Growing Plants and Food for Profit. She has grown greenhouse plants and vegetables for twenty-seven years, selling retail and wholesale from her greenhouse and Farmers Markets.