Using cold frames is a good way to extend the growing season for gardeners that live in cold climates. Cold frames are very effective and easy and cheap to build. Many northern gardeners have been using them for a long time.
What Are Cold Frames?
Although there are several types of cold frames, most of them are insulated box-like structures with windows on top. The frame lays directly on the surface of the ground, or is partially submerged, but it has no bottom. Plants are planted in the ground within the cold frame or enclosed in pots or seed trays.
A cold frame has a slanted top positioned toward the south to allow for the angel of the winter sun. The cold frame creates a microclimate that allows plants to grow or be maintained in the cold winter.
Types of Cold Frame Structures
A sunken cold frame is insulated by the earth and is very effective at keeping plants safe during the cold winter. The cold frame box is usually about 12 to 14 inches high in the back and 8 to 9 in the front. The frame is often made from 2-inch planks, concrete blocks or straw bales dug into the ground about 1-inch less than the front of the cold frame. Planks need to be untreated wood and nailed together with aluminum nails, so they won’t rust.
Cinder or concrete blocks should be stabilized by rebar or other metal stakes hammered into the ground going through the holes. It is a good idea to fill the holes of cinder blocks with sand or soil to further insulate them.
Hay bales are the least insulated of all, but will work in a pinch. The top of the cold frame is usually made from windows, storm doors, shower doors or any glass door.
They are normally attached with hinges to wood structures and can slide off concrete blocks or hay bales. Sometimes the top is made with heavy plastic, but windows are much better. The tops must be able to be propped up a little bit to vent so that heat does not build and burn the plants inside on sunny days.
A portable cold frame is made in the same way, but it sits on top of the ground. It may need a little more insulation, and this can be achieved by leaving room in each corner for a kitty litter plastic container painted black and filled with water. During the day, the water will heat up and radiate its heat when there is no sun.
It is also possible to line the inside of the cold frame with foil to reflect the heat and retain it. Placing a portable cold frame near the foundation of a house can also insulate it as the heat coming from the wall of the building can transfer into the cold frame even if it is just a little bit.
How to Use a Cold Frame
Cold frames come in handy for several applications. Planting seeds in early spring is one of them. Maintaining tender and tropical plants during the winter is another. Growing winter crops is another use for a cold frame and the final use is hardening off new plants before putting them in a garden. Because there are so many uses for a cold frame, it is worth having one or two.
Seeds can be planted directly in the ground in a cold frame or placed in seed trays that lay on the ground. Have the cold frame cleaned out, tilled and ready about 2 weeks prior to planting in very early spring. By the time it is warm enough to plant in the garden, your plants should have several leaves and be able to start the hardening off process to go into the garden.
New plants need a time to get used to being outdoors when coming from a greenhouse or from indoors under grow lights. This usually happens after the last frost is expected in the area. Just place the plants in the cold frame and leave them there overnight for a few weeks prior to planting in the garden.
Growing in the Winter
It is possible to grow vegetables all through the winter in a cold frame. Plants most suitable are the ones that grow during the cool season in the summer. They would be below ground crops of carrots, beets, radishes, potatoes and above ground crops of lettuce, broccoli, cabbages, chard, green onion, kale, and spinach. Start seeds of leafy vegetables in the end of July and underground crops in September.
Maintaining Tender and Tropicals
Maintaining tender plants can be a challenge, even in a cold frame, where the winter is extremely cold. Cut back the plant as far as possible and make sure the cold frame is well insulated. You must hope for the best in this case. Do not over-water plants in the cold frame because the water does not evaporate much, and the plants can rot.
Venting and Protection
Because the top of the cold frame is a window, the temperature can get very warm while the sun is out during the day. This means the cold frame must be vented on sunny days especially when the outside temperatures exceed 40 degrees F. The inside temperature can get so hot, it can actually burn the plants or make them gasp for water and air. Inside temperatures should not go above 60 degrees F.
Just prop up the windows to allow outside air to get in. It is a good idea to install a thermometer inside the cold frame so you can see what the inside temperature is reading and vent appropriately.
At night the lid comes down so that any heat in the cold frame is maintained when there is no sun. Another way to keep the sun from heating up the cold frame too much is to install white plastic over the windows. This still lets some of the sun’s rays in but shades the cold frame.
Extra protection might be required in the dead of winter. A thick blanket can be thrown over a cold frame when the temps go below zero at night. Remember that snow is a natural insulator.
As long as the snow is light, leave it alone to insulate during a winter storm. If the snow is heavy and there is more than 6 inches, it could break the glass. Remove snow when it is heavy or when the sun comes back out by using a broom to brush it off.
Plants in the cold frame still need a little water during the winter. Keep seeds evenly moist and then water only when they seem to dry out. Avoid over watering since it is hard for the water to evaporate and it might freeze during the night and kill the plants or rot the roots of the plants. It is fine to use a watering can, but better to use a spray bottle to water.
Try getting your cold frame materials from a resale store. They often have windows, shower doors, cinder blocks and even wood planks for sale and they are much cheaper than buying them new. You can get away with building your own cold frame for about $25 or less.
If you want a premium quality cold frame for gardening then you should check out this one by Exaco.