Get Ready For Spring – Build a Cold Frame!
Is your mailbox starting to fill up with seed catalogs for the new year? Are you getting excited by visions of gorgeous spring flowers and delicious, fresh greens? If you don’t have your own greenhouse or just the right place indoors with enough light to get started, consider building a cold frame to get a head start on the growing season.
Cold frames are angled, bottomless box structures built to capture the heat from sunlight to give plants some protection from the cold when the temperatures dip below freezing. The hinged roof of the box is clear to allow in sunlight while the rest of the structure rests right on or buried a few inched into the soil, keeping the heat from escaping at night when the temperatures fall. The seedlings will be hardy since the cold frame is not artificially heated.
It may may only be a few degrees warmer inside the cold frame than out, but these few degrees can maintain a steady temperature to help get your seedlings started before it warms up enough to plant them outside on their own. You can start growing cold hardy seedlings for transplanting such as salad greens and root vegetables in the frame, but keep plant seedlings that really need the heat to germinate likes tomatoes, peppers, and cucumbers inside in a sunny window
You can purchase a permanent or portable cold frame or build your own. However, either way there are a few things to keep in mind about where you locate the cold frame. Ideally it should be south facing to collect the warmth of the sun. Also, look at how much wind protection that location experiences, and if water is easily available, but drains well from the site.
Cold frames can be built to any dimension, though they should be small enough to reach into easily. To a build basic cold frame by following these steps:
- 5 foot x 4 foot panel of ¾ inch or thicker plywood treated for outdoor use
- 50-x 44 inch storm window (Any size storm window will work, but adjustments to the lengths of the other pieces must be made.)
- Two (2) 20 inch and two (2) 10 inch lengths of 2×2 inch untreated lumber
- Two dozen (24) 2 ½ inch galvanized deck screws
- Two (2) 5 inch T-hinges
- 24 inch length of 2 inch board, cut with notches at 3, 6 and 12 inch intervals for a prop stick
- One quart glossy white latex paint
Cut the four sides of the box from the ¾ inch plywood. For adequate slope toward the sun the back panel is 20 inches high and the front is 10 inches. The sides are tapered to match. This sloping angle enables more sun to reach the plants, and it sheds rain and snow as well.
Lay out the four pieces of plywood and join them at the corners by short lengths of 2 x 2 untreated lumber. Use two dozen 2 ½ inch galvanized deck screws to attach the panels to the four blocks for reinforcement. This makes the cold frame sturdy enough to move to a new location in the garden later if needed.
Hinge the cover with a pair of 5 inch T-hinges, and attach a handle on the front edge of the window to easily lift the lid. The handle will fit into the notches cut into a 24 inch length of 2-inch board. The three positions (3, 6, and 12 inch notches) on the prop stick will allow for ventilation on warm days.
Finally paint the entire cold frame inside and out with white latex paint to protect the wood and reflect light inside the box. You can insulate the cold frame by adding rigid foam insulation around the insides of the cold frame and by weather stripping along the top edge.
Spread potting mix about two to three inches deep in whatever part of a frame you wish to use for seedlings. Check out those beautiful seed catalogs for: Arugula, Lettuce, Kale, Mustard Greens, Cabbage, Chard, Spinach, Bok Choy, Broccoli, Beets, Radishes, Green Onions. They will all work well in your new cold frame!
Minda Daughtry is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.