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Charley's Greenhouse
17979 State Route 536
Mount Vernon, WA 98273


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Energy Conservation for Winter Greenhouses
Energy Conservation for Winter Greenhouses


Taking time to winterize your greenhouse now will help your plants (and your pocketbook) survive even the worst that Mother Nature sends your way! One of the first steps in winterizing your greenhouse is heat conservation. Now is the time to replace broken glass, mend torn poly or damaged polycarbonate. Caulk as necessary, especially around the foundation. Apply foam tape to doors and vents to ensure a tight seal. For large gaps use a foam sealant to stop heat from escaping and insects from getting in.
North walls and solid walls can be lined with one of several different energy saving materials for maximum insulation and light reflection. If you frequently have freezing weather, line all but the south wall with reflective materials such as "Reflectix" or foil insulation. For sub-zero climates, place this material across the south glazing at night.

In greenhouses glazed with glass, bubble insulation can provide significant heat savings. Simply apply to the inside of the walls and roof. Installed inside your greenhouse glazing, this translucent bubble plastic can reduce your heating costs up to 45% making it just as effective as insulated glass. It transmits evenly diffused light through the greenhouse. When the weather warms up, remove it, roll up and store until needed again. It has a 3-4 year life span and works well with all types of greenhouse glazing.

One of the easiest and least expensive ways you can provide heat is by using thermal storage. By using a material with a dense thermal-mass, such as water, brick or rock, you store incoming solar heat. The heat is then released back into the greenhouse as temperatures drop. Winter shield materials, usually made of tough, durable poly, act as winter heat blankets to hold radiant heat in the greenhouse and reduce heating costs during a cold snap.
What you can grow successfully depends on the temperature you choose to maintain for your winter greenhouse.

"Cold Greenhouse" (Night temp. 35-45 degrees F)
GROWTH is limited. Some vegetables (such as carrots and Cole crops) mature in the fall months and can be held over for winter harvest. Simply provide enough heat to keep the greenhouse from freezing.

"Cool Greenhouse" (Night temp. 45-50 degrees F)
Actively grow many leafy and root crops; e.g., lettuce, cabbage, Brussels sprouts, beets and carrots. Citrus fruits and grapes can be wintered over at these temperatures. Automatic heating with thermostatic control is recommended.

"Moderate Greenhouse" (Night temp. 55-60 degrees F)
Many options are available, and some plants will propagate. Possibilities include cucumbers, beans, Bibb lettuce, herbs, strawberries, mushrooms, cool variety tomatoes, dwarf fruit trees, figs and grapes. (For propagation, use a heat mat to warm potting mix to 70 degrees for starting seeds and rooting cuttings. The entire greenhouse environment does not need to be this warm.) A reliable heating system is necessary and a back up emergency heater is recommended for protection against a cold snap or power outage.

"Warm Greenhouse" (Night temp. 65-70 degrees F) While more expensive to heat (we recommend insulated glazing), there are many unusual and tropical plants you can grow; e.g., lemons, bananas, melons, dwarf corn, peppers, eggplant and a wide variety of tomatoes. (Additional light may be necessary in winter.)

It is a good idea to make a checklist of what to do if severe weather strikes or your greenhouse heater fails. A battery-operated temperature alarm is essential if the heat goes off in the middle of the night. Keep a backup heater ready for immediate use. Emergency gas heaters can save your plants in case of a power outage. These usually require some ventilation. Another emergency step that can be taken is to drain the household water heater and place buckets of hot water in the greenhouse. (Water has an added benefit over other types of thermal mass. If the greenhouse starts to freeze, the water will freeze first and as it does, it will release heat. This will give the plants an additional buffer before they start to freeze.) If freezing is unavoidable: BE SURE TO DRAIN ALL WATER LINES! Bring rare and sensitive plants into the house if at all possible. Cover remaining plants with newspapers, or set high-back chairs around nested plants and cover with blankets.

Jack Frost is planning to visit many greenhouses this winter. With good equipment and thoughtful planning you can send him on his way, and enjoy your indoor garden all winter long!