CORVALLIS, Ore. - Do surprise frosts or cool temperatures foil your plans to ripen peppers or tomatoes? Do you want to keep lettuce or other greens growing over the winter?
If so, try building a simple coldframe or a cloche, which is like a mini-greenhouse with an arched roof.
Coldframes can prolong the growing season in the fall and be used to start flower and vegetable plants before normal outdoor planting dates in the spring. Young plants are protected from frosts, pummeling rains, icy sleet and wind. The sun enters the clear top of the coldframe by day, heating the soil. At night, the coldframe slows the loss of heat.
Built with wood or metal sides, coldframes can have a hinged or removable clear top so the cover can be raised on sunny days and then lowered during cool nights. Side walls can be as high as needed, but 8 to 12 inches are the usual height. The north wall of the frame box is usually built higher than the south for better sunlight exposure. A top with a layer of fiberglass works well; glass or plastic do not hold in heat well overnight. For maximum cold protection you can put an insulating material over it at night or use a layer of clear fiberglass on each side of the roofing frame.
For an easy, inexpensive cloche, Pat Patterson, a Master Gardener trained by the Oregon State University Extension Service, recommends using concrete reinforcing wire.
"I have used these wire mesh frames for cloches for years and they nest well for storage," said Patterson.
She suggests buying a roll of wire mesh and cutting it to form "Quonset hut" arches to cover an area you want to protect. Concrete reinforcing wire comes in 5- and 7-foot rolls, so plan accordingly.
Cover the wire with two layers of clear plastic, a sheet of fiberglass, or one layer of row cover plus one layer of plastic.
"Even old bed sheets will work if you just need nighttime protection from frost," she added.
Another way to build a cloche is by stretching plastic sheeting over PVC hoops. For instructions on that technique, see the OSU Extension guide called How to Build Your own Raised-bed Cloche.