There comes a point each spring when your seedling have grown large enough to move outside, but they're not strong enough to withstand the cold. That's when you harden off
young seedlings before planting them in the garden. One of the best and easiest ways to harden off plants is to use a cold frame. Cold frames are structures that insulate young plants when temperatures drop, but allow them to acclimate to the outdoor air and the strength of the sun's light slowly. You could create a cold from bales of hay and an old blanket, but these kits have brought science and convenience to the craft of cold frames.
Here’s a cold frame cover that’s designed to sit over a cold frame. It drapes over the included enameled steel frame and has zippered openings on both sides, for easier access. Made of woven polyethylene, like a heavy row cover, it gives about the same cold protection, but is easier to work under. A optional shade covering is available so you can use it during the growing season to protect plants from sun and heat. Provides about 27 sq. ft. of growing/pot area. Dimensions: 71-1/2" L x 57-1/4" W. Approx. $90.00
This snap together cold frame is great for protecting seedlings at the start of the season or for setting on top of garden plants, to extend the season in the fall. It is secured in place with spikes, so it is easily portable.
The frame is light weight aluminum. The panels are glazed polycarbonate, with some UV protection to prevent leaf burn and very good light transmission. The tops are vented and hinged for easy access. They are nice, quick ways to gain some extra growing time and to protect plants from unexpected frosts, in a pinch.
41" (L) x 22" (W) x 18" (H), priced at about $50, or the larger 41" (L) x 41" (W) x 21" (H), priced at approximately $90.
It’s called a green house, but it’s really an enclosed shelving system that serves as a cold frame, giving you several extra weeks of outdoor time in both spring and fall. It is probably more practical at the beginning of the season, for hardening off tender seedlings. This is a nice choice to place up against a house or wall for extra heat. It's a bit less practical for extending the fall season, unless you have some vegetables in pots that you want to keep going as long as possible. The steel frame has adjustable shelving, which is nice for hardening off taller plants. The vinyl cover opens with a zippered flap. The pieces push together, so it’s easy to assemble. Dimensions: 62" H x 19" D x 27" W. Approx. $35.00