A cold frame is an old technique to start the spring cool weather vegetables — lettuce, onions, spinach and radishes — up to 45 days before the last frost. They are typically small (4′ x 4′) and can be covered with plastic sheeting, keeping plants 5 to 7 degrees warmer at night — and much warmer with sunshine.
What is a cold frame?
Finished cold frame: ready-to-plant
technique all year around to make fresh greens available, though the amount of sunlight between November 1 and March 1 will limit growth, depending on how far north you live.
The composite versions use hollow boards made of plastic. They have the advantage of being much lighter (about 1 ½ lbs.) than wood (a 3’ 2×6 of pressure-treated wood weighs about 9 lbs.) but are much more expensive.
How does it differ from a raised-bed garden?
Plastic anchor joint with stake
The brown plastic brackets fasten to each board with a pair of ½” screws and are easy to take apart. In the garden, you’ll simply drive the connecting spike into the ground and the box is formed. Note that the anchor joints are sold by the pair – and you’ll need four of them. Stackable joints are NOT necessary for a cold frame but are used with multi-level raised gardens.
If you’re using 2 mil plastic sheeting, which is commonly available as a paint drop cloth, you may want to double it – particularly if your springs are windy. A 3.5 mil plastic would be preferable but is sometimes difficult to find in small sheets.
2” x 6” cedar or pressure-treated lumber – 14’ long
2” x 2” cedar or pressure-treated lumber – 16’ long
¼” x ¾” door strip – 16’ long. Any small trim can be used here to help hold the plastic sheeting in place and protect it from wind.
2 to 3.5 mil plastic sheeting (Visqueen), 4’W x 3’D
4 L-shaped corner brackets for lid + screws
Two 2” hinges
2 hooks & eyes
Corner fasteners for base or 2 1/4” screws or Super Anchor joints)
Tools: circular saw, screw-driver, drill, stapler, hammer
- Cut your 2”x6” timber into two 4’ and two 3’ lengths. Pre-drill your fastener holes then fit the pieces together (without fastening the box together) into 4’x3’ box. This will allow you to measure for your lid – and easily transport the sections to the garden.
Frame base assembly in garden & awaiting the lid.
- Measure the base BEFORE cutting 2”x2” pieces for the lid, as the Super Anchor corners may make lid pieces 4” longer on two of the sides.
- Cut the 2”x2” wood, the assemble using the 4 L-shaped corner brackets. Screw hinges to the lid.
- Cut your door strip or other furring strip to match the lid. Tack in place with a light nail or staples that are long enough to penetrate through the stripping. This layer of stripping prevents tearing of the plastic sheeting in heavy winds. Using a thin (1/4”) door strip will enable a ½” staple to penetrate the wood and plastic, simplifying the tacking process, which is next.
Frame lid, with tacking of plastic sheet in progress.
- Cut plastic sheeting so that it is large enough to lap over the sides of the lid. This gives you extra material to hold onto while tacking it to the lid. Staples are excellent for holding the plastic or Visqueen in place. Tack one side to the lid with three staples, then draw plastic taught and tack down the opposite side. Repeat with the two remaining sides – then return and double the number of staples on each side, while drawing the plastic sheeting tight. Now, trim excess plastic from the lid.
- Mount the lid onto the base and attach the hinges. Add the hook-and-eye latch to each side of the front of the cold frame and keep the lid from lifting in strong wind.
- Save two extra wood blocks to open the lid of the cold frame slightly on hot days. You may even wish to add supports to the side of the lid to hold it open for watering or other work.