The Magic of Cold Frames: Extending Your Gardening Season with a DIY Box
A guide to growing more with DIY cold frames.
Depending on where you live in the world, frost might be in the forecast for the next couple of weeks. Or maybe you’ve already had your first frosty morning!
Fortunately, frost doesn’t have to signal the end of the growing season, and if you’ve read this article about season extension you already have a few tricks up your sleeve to prolong your growing season by several weeks, even months.
One of these techniques—cold frames—is especially effective at extending the harvest window of cold-weather crops. Plus, cold frames can double as seed-starting boxes!
In this blog post, we’ll explain what a cold frame is and the benefits of using one, as well as outline how to build your own cold frame using materials you probably already have at home.
What is a cold frame?
A cold frame is, at its simplest, a box placed over a garden bed to protect the plants underneath it from cold temperatures and frost.
Cold frames have insulating sides (typically made of wood or cinder blocks) and a transparent “lid” (usually glass or rigid plastic) that allows light to pass through but is still thick enough to retain heat.
Similarly to an unheated greenhouse, cold frames work by harnessing and retaining natural solar energy. Cold frames can warm the air underneath them by five degrees or even more, especially if the cold frame rests on top of a hotbed.
What is a hotbed?
Hotbeds are, essentially, a controlled compost pile topped with a growing medium. As the organic matter composes, it creates heat that warms the plants above it. Construct a hotbed by layering organic matter— "green" material (food scraps) and "brown" material (leaves, straw) — in a raised bed, and topping with potting soil.
Using hotbeds in combination with cold frames is an inexpensive and practical way to keep plants alive in cold weather.
Benefits of cold frames
Extending the growing season
Many gardeners use cold frames to prolong the harvest season. Most summer crops will keep producing until freezing temperatures kill them, so delaying frost allows for a few extra weeks of growing time.
Cold frames work because they create a microclimate that is more hospitable to plants. In his book, The Winter Harvest Handbook, author Eliot Coleman compares using a cold frame to gardening in a warmer hardiness zone 500 miles south!
Starting seedlings early
Not only do cold frames buy a few extra weeks of frost-free weather, but you can also use these boxes to start seeds.
Cold-weather crops like brassicas do best under cold frames, and you still want to avoid starting seeds in the dead of winter. But cold frames are the next best thing to a greenhouse and are just as good for starting seeds that don’t mind cooler temperatures.
Pest and weather protection
Other than the obvious benefit of maintaining warmer soil and air temperatures, cold frames also serve as a fairly effective barrier against some pests that are commonly spread between plants.
Some years are worse than others, but brassicas always seem to be hit the worst by flea beetles, cabbage loopers, and aphids. While it may not be the foremost purpose of a cold frame, planting brassicas in cold frames can help protect these plants against pests by creating an impenetrable barrier.
Of course, pests can get to plants when you open the cold frames, but if you check your plants each evening for pests and remove any that you find before closing the lid, the chance of a pest invasion is greatly reduced.
Cold frames can also help protect against harsh weather, including snow, hail, and heavy precipitation.
Inexpensive and easy to make
Cold frames are small enough to be portable and are relatively cheap to make. If you build one yourself, you can easily customize it to fit your garden perfectly.
Plants for cold frames
How to use a cold frame
The ideal time to use cold frames is in the cooler seasons of spring, fall, and even winter.
It’s absolutely possible to use a cold frame year-round, but cool-weather plants might get too warm during the hottest months of the year. Regardless of which plants you plant in your cold frame, be sure to vent the boxes during the day to allow humidity and heat to escape.
Selecting an ideal location for your cold frame
For cold frames to work properly, they must be situated in full sun and kept clear of leaves and other debris. If it snows on your cold frame, brush off the snow to allow light to get through to your plants. Leave snow around the sides of the boxes, as the snow will serve as extra insulation.
For an even more efficient use of naturally occurring solar energy, situate your cold frame to be south-facing and build the back higher than the front, so that the top of the box is slanted.
Build your cold frame large enough to fit mature plants, but bear in mind that the taller the box the harder it will be to hold in heat. Low wide boxes are easier to warm and hold heat longer than tall, narrow ones—but that being said, the lid needs to fit over the tops of the plants without any foliage actually touching the lid.
You can also partially or fully bury the box in the ground to add even more insulation.
Since cold frames require sunlight to keep warm, they aren’t the best season extension tool for plants that prefer partial shade.
Prepping the soil
Prepare the soil in a cold frame according to the crop’s needs. For most garden vegetables, including cold-weather crops like brassicas, you’ll want to amend the soil with organic compost and a balanced, granular fertilizer. If your soil has never been analyzed for pH or composition (or hasn’t been tested in a while) send a soil sample off to a research lab and amend it accordingly.
Alternatively, build a raised bed and fill the box with quality potting soil.
Either transplant mature plants into the cold frame as colder weather approaches, or better yet—build the cold frame around the plants so as to not disturb their roots. Just because you build the cold frame in one area doesn't mean that it has to stay there—you can always move your cold frame later on.
Sowing seeds in cold frames
You can sow seeds in a cold frame for several weeks before you can sow those same seeds outside.
You can either directly sow seeds into a cold frame or start seeds in trays or pots and place them under the lid.
Venting the cold frame
Cold frames are meant to open and close, and venting the cold frame is an essential part of keeping plants healthy. Luckily, monitoring temperature and humidity is as simple as opening and closing a box!
Either bend the lid back on itself to fully open the cold frame, or use a stick to prop the lid up several inches above the top of the box, to allow humidity and heat to escape the box while still offering some protection from the elements.
DIY cold frame ideas using recycled materials
There are two main parts of a cold frame: the box itself and the lid (and hinges, if you want the lid to swing open).
If you’ve ever built a raised bed, building a cold frame is very similar (and if you haven’t built a raised bed don’t sweat it, it’s not too hard).
The most common building material for the bottom part of a cold frame is lumber. This is a way to repurpose old pallet wood—just make it to only use untreated wood, as treated lumber can leach chemicals into your garden, which can harm you and your plants.
This basic cold frame plan from the University of Minnesota is a great place to start, and you can customize the cold frame to fit your needs.
You can build (or buy) cold frames that fit on top of raised garden beds. Since raised beds warm up earlier in the year than in-ground beds, adding a cold frame on top of a raised bed is a great way to start plants even earlier than with either tool alone.
Straw bale frame
You can make a basic cold frame in a pinch by stacking straw bales around your garden bed and placing a piece of rigid greenhouse plastic or an old window on top. As long as the “lid” spans the gap between the bales you’ll have an insulated cold frame!
Bricks or cement blocks are another inexpensive and readily available building material for the bottom part of a cold frame. Stack the cement blocks around your garden bed and place the lid on top. One added bonus of using blocks to build a cold frame is that the blocks will absorb sunlight and release heat at night, further warming the plants inside the cold frame.
Polycarbonate frame and lid
If you have the resources, you can build a cold frame complete out of rigid greenhouse plastic—and though it will heat up more quickly it won’t hold heat as well as cold frames made of more insulating materials.
Old glass doors and windows make great lids for cold frames, but you must be careful not to break them—broken glass in the garden isn’t fun. Rigid greenhouse plastic like polycarbonate makes a perfect lid that you don’t have to worry about shattering.
You can also make a more spacious lid by building a rectangular frame and attaching metal or plastic hoops. Once the hoops are secured, pull plastic film (like polyethylene) over the hoops and secure. While this isn’t the most protective of all greenhouse lids, it acts like a mini greenhouse and allows ample room for fully-grown plants to thrive underneath it.
Any of these recycled building materials are sustainable alternatives to prefabricated cold frames on the market.
In short, cold frames are small boxes placed over garden beds to protect cold-sensitive plants from frost. Although cold frames can look much different from garden to garden, they work just like small greenhouses to heat the air and soil underneath them via sunlight.
Cold frames are inexpensive and easy to build and can be used to start seeds weeks, even months, early. With a sturdy cold frame, you can even grow some plants year-round using natural solar energy.
Once you get your cold frame built, shop our collection of non-GMO cool-weather seeds and get them in the ground ASAP!