Dig It! Green Houses and Cold Frames
by Sage and Snow Garden Club
August 7, 2007
In our area the purpose of a greenhouse is not to keep plants through the winter but to get an early start on your spring garden. You can start your perennials in early March in a greenhouse with a propane heater. You also will be able to keep plants as late as Oct.
A greenhouse can be simple or as elaborate as you want. When deciding on the kind of a greenhouse you want ask yourself these questions:
1. What purpose will it serve?
2. Will it be a purely functional building hidden from plain view (so the appearance is unimportant) or will you integrate it into the landscape?
3. Would you like it to also serve as a space were you could sit and relax?
There are two basic greenhouse styles: freestanding and attached. Freestanding allows you to choose the best site. Your greenhouse should receive full winter sun and its longest side should face south. Because freestanding greenhouses are more exposed to the elements they can cost more to operate (e.g., heating). With the attached type, you might have to compromise on sun exposure. For example, if you place it on the north side you will have to supplement lighting or grow plants that prefer shade.
The frame can be wood, metal or polyvinylchloride (PVC). Wood is the easiest to use, but is heaviest and costs more to ship. You have to treat the wood to prevent rotting. Galvanized steel and aluminum are easy to work with, but they conduct heat and cold. PVC frames are only appropriate for lightweight, film-covered structures.
The glazing can be glass, polycarbonates, acrylic, fiberglass or plastic film. One misperception is that transparency means better light transmission, but many translucent materials transmit as much light as clear glass. Tempered glass is the longest lasting and easiest to take care of. Polycarbonate and acrylic are almost as permanent and transparent as glass, however they are more expensive than fiberglass and require a stable foundation. Fiberglass is relatively inexpensive and long lasting, but you canít see through it and its ability to transmit light tends to fade over time. Plastic film ranges from transparent to translucent. Some with ultraviolet protection can last 5 years, but others must be replaced yearly.
A cold frame is nothing more than four walls to trap heat and shelter plants and a transparent lid that admits light. The lidís size will determine the size of the cold frame. The cold frame should be 2-3 feet x 4-6 feet. Build the back 4 to 6 inches higher than the front to maximize the amount of light that reaches the plants and allow water or melting snow to drain off the top. Cold frames are simple - you can stack bricks or bales of straw in a square and cover with glass or you can make a box and cover with an old storm window.
The best site for the cold fame is a south-facing, sunny spot with good drainage and some protection from the wind. Orient it east to west so it gets full sun from mid-morning to mid-afternoon.
The key to using a cold frame is to pay attention to the temperature - the trick is in keeping it cool rather than warm. The temperature inside should stay below 75 degrees for summer plants and below 60 degrees for plants that normally grow in the spring and fall. A good rule of thumb is when outdoor temperatures are above 40 degrees, prop open the lid 6 inches. When the outdoor temperature clears 50 degrees, remove the lid or open it all the way if it is attached. Be sure to close the lid in late afternoon to trap the heat inside for the cool night.
To keep plants from freezing on really cold nights, pile insulation (e.g., old blankets, straw or newspaper) on top as most heat escapes through the glass. Snow insulates well, too, but be sure to brush heavy snow off the glass so it doesnít break.
Cold frames are used to warm soil, grow plants or to protect plants. It is a good way to hold flats or seedlings or plants that arrived too early to plant. Cold frames can be as warm as a green house by locating it near a wall of a heated building or by insulating it. You can use it as a soil warmer by placing the cold frame in the garden 7 - 10 days before you want to plant. With the cold frame still in place, plant your plants directly into the cold frame. Remove the cold frame when the weather stabilizes.
Cold frame tips:
1. Make the frame lower on the south side so it collects more sun.
2. Paint the inside wood of the cold frame white to help reflect sunlight.
3. Vent the cold frame to keep it from getting too hot and to reduce dampness.
4. Keep watch for air leaks which can let warm air seep out.
5. Use water that is at least as warm as the soil as cold water can cool the soil, reduce the effectiveness of the cold frame, and chill the seedlings.
6. Place small votive candles inside the frame (one candle for every 2 square feet of space). The candle will burn for hours and protect plants until morning. Be sure to place candles away from leaves and from combustible materials such as straw, sawdust or the walls.
There are many sources for plans for greenhouses and cold frames. Contact the Sage and Snow Garden Club if you want a plan for a PVC greenhouse. The Garden Club meets at noon on the second Tuesday of each month (except July and August) in the Pinedale Library. Contact the Garden Club at firstname.lastname@example.org or Box 2280, Pinedale, WY 82941.