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By River Stillwood
With the days lengthening and the weather warming, it’s time to start hardening off your seedlings and starts and prepare them for the garden. To do that, you’ll want to provide them with a protected environment in which they can safely adjust to outside temperatures. The best environment is a cold frame.
A cold frame is simply a rectangular box with a clear glass or plastic top used to keep tender plants warm and safe from wind while getting them accustomed to the outside. Cold frames are easy to build, can be temporary or permanent, and can be built on a raised bed, on the ground’s surface, or, as is most commonly done, below ground level. The three conditions that matter most in the choice of location is that it is south-facing, gets full sunshine, and provides good drainage.
A cold frame can be built out of wood, metal, stone, brick, or cement blocks. If you use wood, make sure that it has not been treated with chemicals that are toxic to plants. Redwood and cedar are ideal. Next best is untreated wood painted with white greenhouse or latex. If you use metal, insulate it well with soil thickly packed against its outer walls to prevent heat loss. If you use stone, bricks, or blocks, be sure the spaces between the pieces are completely filled. This will keep in heat and keep out insects.
The design of a cold frame is simple. The back, or north wall, should be about 18-inches high. The front, or south wall, should be lower. The sides should be sloped downward from back to front. Ideally, the slope is 1-inch lower per each foot length of the sides. So, if the back wall is 18-inches high and the sides are two feet long, the front should be 16-inches high. (18-2=16). The size of the frame is dependent upon the size of the cover you use.
The cover can be made of glass or plastic. The most commonly used materials are old window sashes. Single or double paned, they’re sturdy, usually inexpensive, and easy to find. One drawback to window sashes: they don’t usually contain safety glass. If you use a window sash with standard glass, protect it from running kids and animals. You don’t want anyone getting hurt.
.4 to .6 millimeter plastic sheeting also works well. It’s inexpensive, easy to use, and can be custom fit on any size of frame. Its drawback is that it must be replaced every year. Because plastic loses heat quickly, it’s more efficient when doubled. Wrapping it twice around a frame and stapling it in place creates a much warmer double-pane effect than a single layer. It’s more durable that way, too.
Whether made from an old window sash or plastic over a frame, the cover should be hinged on the front side of the cold frame so that it can be opened from the back. This prevents strong winds from raising the cover and creates better temperature control.
The best location for a cold frame is against a south facing building or wall. If no wall is available, protect it from icy north-blowing winds and weather by creating a strong windbreak. Straw bales, metal, wood, even shrubbery work well.
Once you choose the location, you need to decide how you want to it to sit. While the cold frame can be set atop the soil level, it will provide much more protection to your seedlings and starts if you put it partially below grown. To do this, dig a trench the size of the frame and about 20” deep. To ensure good drainage, fill the trench 6-inches deep with coarse gravel. Then, put in your frame so that the top front is about 4-inches above ground. Lay a layer of burlap, screen, or small-hole hardware cloth across the entire bottom. Then, keeping your seedlings and starts in their containers, set them on the covered gravel and gently fill in the spaces between containers with soil. This will help retain heat.
Managing the temperature in the cold frame is vital, so attach an outdoor thermometer to a shaded side wall on the inside of the frame. If you’re hardening off cold-loving plants, the temperature should remain in the fifties at night and between 55 and 65 degrees in the daytime. If you are hardening off warmth-loving plants, the temperature should be between 55 and 65 at night, 7o to 75 in the daytime. If you’re hardening off both, keep temperatures about 55 at night and 70 in the daytime.
Since the frame is usually only 4 to 8 degrees warmer than the outside air if we get a late frost, you will need to blanket the frame with linens, straw, and/or put light bulbs inside. (Make sure you use a waterproof cord and socket if you use light bulbs). If it gets too hot in the daytime, ventilate the frame by propping it open a little.
To harden off your plants, start by keeping the temperatures above 50-degrees at night and between 65 and 75-degrees in the daytime. Every few days, let the temperature range increase by no more than five degrees and increase daytime ventilation (when not too cold) until the plants are accustomed to the actual temperatures. Then plant them in the garden. But keep the blankets and straw handy. You never know when we’ll get a surprise spring frost.