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Cold frames are a great season extension tool to help you garden in cold weather or to start warm weather seedlings long before your last frost date. Learn all about cold frames (including DIY cold frames) in this article!


Cold frames: a useful resource for cool and cold-weather gardening

Have you ever tried to start seeds indoors and ended up with anemic, gravity challenged sprigs? Bright, direct light is vital to emerging seeds and most windowsills are not sunny enough.

This leaves you with two options:

  1. indoor grow lights (like the DIY grow light system we recommend), or
  2. real sunlight with cold protection. 

Artificial lighting (grow lights) is an ideal choice if you have the indoor space. If your indoor space is at a premium but you want to start garden seedlings to get a jump start on the growing season, cold frames are your best bet.

Thankfully, cold frames are also easy and inexpensive to build! 

Not everyone has a greenhouse, cold frames are a more frugal option for both money and space.

Not everyone has a greenhouse like this. Cold frames are a more frugal option for both money and space.

What is a cold frame?

A cold frame is exactly what it sounds like: a transparent outdoor frame that protects plants from cold weather while still letting sunlight in.

This means your plants get natural sunlight and extra warmth. In most regions, a cold frame can be used effectively to grow winter seedlings and crops, though if you live in a climate zone below USDA Zone 6, you may need a more fortified version than the one I use in my garden.

A cold frame can be constructed from materials you can easily find. This one is made from lumber and some old windows.

A cold frame can be constructed from materials you can easily find. This one, photographed in Asheville NC (zone 7a/6b), is made from lumber and some old windows. Notice the rocks around the cold frame as well, which trap heat, helping to create a warmer microclimate.

Also, if you don’t feel like making your own cold frame, you can order a really good Austrian/German-designed cold frame via Amazon. It’s double sided (more space), made of durable materials, lets in light from all angles, and you probably couldn’t make it more affordably if you were to buy all the pieces/parts yourself.

How to make cold frames

Cold frames are typically unheated. All their plant-protecting power comes from solar energy stored in the structure and soil during the day.

Parts of a cold frame:

Top – A light-permeable cover such as glass, plexiglass, or greenhouse plastic is used for the top of a cold frame.

Sides – The sides are made of any material that will create a supportive structure for the cover.

Bottom – A bottom is not necessary for your cold frame. Most people just use soil. If you decide to create a base for your cold frame, make sure it allows water to drain.

Our cold frame constructed from straw bales and recycled windows.

Our cold frame constructed from straw bales and recycled windows.

We used recycled windows for our cold frame. Some of them came from a friend’s farm, some of them came from my husband’s coworker, and some of them came from our local Habitat for Humanity store.

Glass is the most transparent and permanent material you can use… unless you live next to children playing ball or position your cold frame under the Whomping Willow (or any aging tree).

Depending on how cold your weather gets you may want your straw bale joints to overlap more. In very cold areas be sure to stuff any gaps with loose straw or leaves.

Depending on how cold your weather gets you may want the straw bale joints on your cold frame to overlap more. In very cold areas be sure to stuff any gaps with loose straw or leaves.

Available materials and permanence: considerations when constructing your cold frame

Straw has been our material of choice for the sides of our cold frame. Why? Straw bales are: 

  • thick and very insulating,
  • easy to come by (garden, home improvement, and farm stores all carry them),
  • relatively inexpensive (in bad hay years the price goes up), 
  • fully biodegradable when we are finished with them,
  • extremely easy to assemble.

At this point we’ve been using straw for 3 years and have found it perfectly suited for our needs. I had given up on shelf greenhouses or ever getting around to building a wooden cold frame and was carrying my seed trays in and out of the house on warm days. Needless to say I am much happier with our cold frame!

Note that you don’t have to use straw for your cold frame! People also use lumber, bricks, and many other materials to create cold frames. Use what’s easily available and/or within your budget!

Easy to assemble, easy to disassemble

In our case we don’t have a permanent spot to keep our cold frame in, so it has moved around the past 3 years. Sometimes it is in our driveway, sometimes next to our shed, and sometimes it is out in the garden.

That means our building materials and cold frame needed to be:

  • easy to carry, and
  • easy to take apart and put back together again.

When the summer plants are dormant we could even put a temporary cold frame on top of one of our late-emerging beds.

 

Plants stay toasty warm inside, but so do the critters, watch out for slugs and other plant-munchers.

Plants stay toasty warm inside, but so do the critters! Watch out for slugs and other plant-munchers.

Recommended placement of your cold frame

You can put a straw bale (or any cold frame) just about anywhere in the yard. However, the best places get south-facing sunlight for early morning sun.

The least effective placement is on the north side of a house, building, or tall tree (not enough sun).

Seed trays and potted plants aren't the only use for a cold frame. You can put them directly over a half-hardy plant in the garden like these golden snow peas and remove the structure when the weather warms in the spring.

Seed trays and potted plants aren’t the only use for a cold frame. You can put them directly over a half-hardy plant in the garden like these golden snow peas and remove the structure when the weather warms in the spring. 

Ideal plants for cold frames

Since the plants inside will only be a little bit warmer than the outside air, cold frames are mostly used for frost-tolerant crops during the winter. Check out our article Easiest plants to grow in the fall and winter, for a complete list of frost-hardy plants. 

Spring transplants under cold frames

You can also use your cold frame to grow out your spring transplants. If you do, you’ll want to pay careful attention to the weather and bring the more tender seedlings inside on nights when it drops below freezing.

Don't forget that all sugar, snow, and shelling pea varieties have edible greens. Plant Austrian winter peas or any other type and harvest salad greens all winter long.

Don’t forget that all sugar, snow, and shelling pea varieties have edible greens. Plant Austrian winter peas or any other type and harvest salad greens all winter long.

Too cold – Although most winter crops are at least a little bit frost tolerant, they don’t grow when it is under 40 F outside. They’ll appreciate the cover of your cold frame staying on any time the weather is below 50 F.

Too hot – Likewise, cool weather crops don’t like it hot. If the thermostat says 50 F or warmer, crack the cover or take it off completely.

You’ll start to get a feel for how warm or cool your cold frame stays as time goes on! 

These tomato seedlings were grown out in our straw bale cold frame.

These tomato seedlings were grown out in our straw bale cold frame.

As the weather warms…

By spring, you’ll start leaving the cover off of your cold frame all the time. We usually leave our cold frame up until we’re well past any freak late freezes.

Then we give the straw bales a second life by spreading them under our rows of blackberry and raspberry brambles. When the berries leaf out and send out new canes they rest on top of the bales and block light from competing weeds. While the bales are under the brambles, they become hosts to mushrooms and other soil organisms.

We think the summer mushroom colonies mycoremediate any herbicides the previous farmer used. Ideally, you can find straw bales that don’t have pesticide residue so this issue doesn’t have to be a concern. 

By fall the bale weed barrier has rotted enough to lose its form. The bales get a third life in the garden when they’re dumped in the chicken run to be dismantled as a precious feast by our laying hens. At that point they become compost that we can use to fertilize our next batch of seedlings!

One of our chickens enjoying what's left of a straw bale.

One of our chickens enjoying what’s left of a straw bale.

We hope this article helps you better understand what cold frames are, how to use them, and how to build one yourself. 

Happy gardening!

-Eliza @ GrowJourney
Master Gardener, Master Naturalist, Permaculture Instructor

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