One thing about gardeners who’ve been at this for years is that we learn to never throw anything away, as someday that thing can be re-purposed. From old shoelaces to pieces of rebar, and old windows to broken tomato stakes, my garden shed at times looks like one of those roadside junk stores. But it shelters apparently useless things which one day will be put to purpose for zero investment. Much like the old window and leftover concrete blocks and bricks used to build this cold frame.

What is a cold frame?

A cold frame is an enclosure with a clear top that sits low to the ground. The purpose of a cold frame is to allow plants to capture sunlight for photosynthesis while protecting them from the unpredictable winds, frost, and damaging weather of winter and early spring. It is essentially a mini-greenhouse to extend the growing season.

The purpose of the cold frame is to create a microclimate that provides warmer air, warmer soil, and protection from wind than if the plants were set out in the garden. This allows plants to be started earlier in spring for later transplant into the garden. A cold frame can also be used to extend plant life deep into fall and to grow cold hardy plants for a winter harvest. They’re especially useful if one doesn’t have grow lights for seed starting indoors – seeds can be sown directly in starting medium in the cold frame.

Cold frames are typically used for starting seeds and hosting small seedlings for early and late season vegetables such as onions, lettuces, kale, spinach, radishes, and cabbages.

How to build an easy cold frame

A cold frame can be as elaborate or as simple as you like. You can also buy cold frames online. If you’re going to build one from found materials, the first thing you need is an old window, a large pane of glass, or rigid, clear plastic. The glass or plastic can be mounted on a wooden frame with hinges, set in place flat on top of blocks, or whatever setup makes sense for you. The design depends on where you’ll be placing the cold frame (in your yard, garden, porch or deck), how durable it needs to be (is it out in the open or under an enclosure), and the nature of the critters in your garden (how crafty are the squirrels?).

You can also create a cold frame if you have a pane of glass or an old window that fits along the edges of your raised garden bed. Lay the glass at the end of one of your garden beds and make sure it sits square on all 3 sides. Then you’ll just need bricks, blocks, lumber or a similar material to prop up and enclose the open end.

The cold frame should be designed to catch as much sunlight as possible and the ability to move the glass so it can be vented on warm days to allow heat out and release excessive moisture trapped in the cold frame.

Notes of caution when building your cold frame:

  • If you’re using wood for your cold frame, make sure it’s not treated with chemicals that may leach into your food.
  • Some websites suggest you can use a clear sheet of plastic to top your cold frame. I don’t recommend doing this, as a sheet of plastic is easy for mice and squirrels to chew through – they’ll decimate your seedlings in minutes.

What you’ll need to build this cold frame:

  • cement blocks
  • an old window
  • bricks for weights

I built my simple and easy cold frame with an old window, leftover cement blocks, and some old bricks as weights for the window. The weights are mostly to keep curious squirrels, rabbits, and mice out of it and to protect the glass from flying off with a gust of wind. Primarily I’m using it to move seedlings started on my grow rack outdoors for a few weeks before transplanting so I can make more room under the grow lights for tomatoes, peppers, basil, annual flowers, and other warm season crops.

Time needed: 20 minutes.

How to build the pictured cold frame:

  1. Size your cold frame

    Space the concrete blocks so that the window will overlap the blocks 2-3 inches on each side.

  2. Layer the blocks

    Layer your concrete blocks high enough so that the seedlings will not touch the glass – this is to avoid moisture buildup on the leaves of the plants, which could create a fungal infection.

  3. Check for gaps

    The blocks should not have significant gaps between them – any gaps on the outside of the cold frame should be smaller than 1/4″ so that mice won’t get in.

  4. Place the window on the blocks

    Lay the window on top of the blocks and weigh it down on each side with the bricks. To vent the cold frame on warm days, remove the bricks and slide the window 6-8 inches from one of the edges. Move the window back in place at night and weigh it down.