A simple cold frame is just what you need to get more out of your garden, from getting an early start coming into spring to extending your growing season well into fall and winter. Cold frames are easy to build — you can make one in an hour or two — and the materials are inexpensive. I’ll walk you through how to build a simple cold frame like the one I use in my own garden.
Slow-growing root crops like carrots and beets and greens like kale, spinach, arugula, lettuce and mustard all do well in cold frames, far beyond your local first frost date. The temperature inside the cold frame will typically stay between 5 and 10 degrees warmer than the temperature outside.
A cold frame is a box with an open bottom and a transparent lid that acts as a mini-greenhouse, collecting sunlight and trapping thermal energy. Placed over your garden bed, it creates a micro-environment that has warmer soil and is less subject to frost. Ideally, the cold frame should be positioned with the low end in a south-facing direction to maximize the sunlight and heat it collects.
For the lid, use glass (like an up-cycled used window sash), acrylic, a plastic sheet or another transparent material. Plastic sheets are convenient because they can be cut to fit any size cold frame. The same hinges that are used for a bedroom door are perfect for repurposing to attach the lid to the cold frame while still allowing easy access.
One side of the cold frame is taller than the other for two reasons. One, to provide the greatest opportunity to capture the sun’s rays, and second, to shed rain and snow, as well as leaf litter and other debris. You don’t want anything sitting atop the lid that will prevent sunlight from getting in.
Cold frames get much hotter than you’d think, especially in fall when daytime temperatures can be warm and there are still several more hours of daylight than in winter. To avoid overheating your plants, which can promote “soft growth” that is easily damaged in winter, you should vent your cold frame. All it takes is lifting the lid in the morning and propping it open a few inches with a spare brick or scrap piece of wood. (To hold the lid up while harvesting, a stick or tall stake is useful.) Return before the sun goes down to close the lid so the micro-environment will stay warmer than the overnight temperature outside.
Niki Jabbour, an author and gardening expert from Halifax, Nova Scotia, recommends venting any day the temperature outside is warmer than 40 degrees Fahrenheit. Once temperatures are consistently below 40, she keeps lids closed permanently — except for when she’s harvesting.
The first step to building a simple cold frame is to determine the size that best fits your garden. If you have a 4-foot-by-8-foot raised garden bed, for instance, a 4-foot-by-4-foot cold frame will nicely fit over half of the bed while still being small enough so that it can be easily removed and stored in spring. If you plan to use a window sash for your project, you’ll have to build to the dimensions of your lid.
No matter your lid, you will need plywood, 1”x1” or 2”x2” lumber for the four corners, hinges and screws or nails.
When cutting the plywood, start with the shortest piece first, the front. Continuing to use the dimensions of a 4-foot-by-4-foot cold frame as an example, cut a piece of plywood to be 4 feet wide and 7 inches tall. For the back piece, cut 4 feet wide and 12 inches tall. The two remaining sides will be identical: 4 feet wide, flat on the bottom, 7 inches tall on one end, and 12 inches tall on the other.
For the four pieces of lumber that will secure the corners, cut two that are 6 inches tall, for the front, and two 11 inches tall, for the back. You want these corner pieces to be just slightly shorter than the box so they don’t interfere with closing the lid completely. Screw the box together.
If you are making a lid from scratch, the plan is to secure durable, clear plastic sheeting to a lid that matches the dimensions of the top of the box. Cut the 2”x2” lumber to the desired dimensions, roll out the plastic sheet and staple it on.
Cut off the extra plastic with a utility knife. To secure the plastic sheet further, nail trim pieces on top, so the plastic is sandwiched in between.
Attach the lid to the back of the cold frame with the hinges. An extra 2″x2″ on the inside of the box will give the screws something sturdier to grip.
Your cold frame can be smaller or larger to suit your garden, and it doesn’t need to be square either. It can also be taller, depending on what you intend to grow, but understand that larger cold frames will have a harder time keeping heat in.
If you’re placing the cold frame so that it rests on top of your raised bed frame, be sure to fill the gap between the soil level and the bottom of the cold frame so that you don’t lose any heat. Extra mulch or straw tucked into place, is an easy way to fill the void.
If you haven’t watched the video on building a simple cold frame yet, scroll up to see DIY expert Todd Brock show how it’s done.
Have you built a cold frame? Let us know in the comments below.
Links & Resources
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy Essential Gardening Fundamentals: The basics on healthy soil, planting, watering techniques, composting, raised bed and other gardening methods, fertilizer, the many benefits of mulch, and more.
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