A straw bale cold frame is a temporary structure used to protect hardy vegetables in autumn and winter. They require no building skills and are quick and easy to put together. Once the bales are in place, they’re topped with a clear material like an old window or a piece of polycarbonate. With the arrival of spring, the frames are taken apart and the straw can be used for straw bale gardens, mulching, or added to the compost bin. Keep reading to learn more about straw bale cold frames.
What is a straw bale cold frame
A straw bale cold frame is a low cost temporary structure used to protect crops when the temperature drops in autumn and winter. It’s essentially a miniature greenhouse. Building cold frames is a great way to increase self-sufficiency in a home vegetable garden and extend the typical harvest season by a couple of months. The box of the frame is made from insulting straw bales and topped with a clear top to capture solar energy. It requires no carpenter skills to build and once spring arrives the straw can be used in the garden.
A straw bale cold frame is arranged in a square or rectangle shape, depending on the shape and size of the garden bed. It’s generally easier to build a straw bale cold frame over an in-ground garden bed, but I’ve also built them on top of raised beds too. I write extensively about the various types of cold frames I’ve used in my best-selling books, The Year-Round Vegetable Gardener and Growing Under Cover.
Types of straw bales
Did you know that straw and hay bales aren’t the same thing? Straw bales are comprised of the stalks of grain plants and don’t contain seed heads, while hay bales are used as animal feed and do contain seed heads. The problem with using hay bales is those seeds germinate and sprout up around your garden. When it comes to size of the bales, you’ll discover there are two main sizes available. A two string bale is 14 inches tall, 18 inches wide, and 36 inches long. A three string bale is 16 inches tall, 24 inches wide, and 48 inches long. The size of the area to be protected determines the number of bales, exact dimensions, and total window area of the frame.
I try to source my straw bales in late summer. It’s also a good idea to ask about herbicides. Herbicides may have been sprayed on the farmers field to reduce weed growth. Check with the farmer or garden centre to ensure the bales they’re selling are herbicide free.
How to use a straw bale cold frame to extend the growing season and more
I generally use my straw bale cold frames for harvesting cold hardy vegetables like kale, leeks, and salad greens. Yet there are many ways you can put this simple structure to work in your garden and here are 6 suggestions of how to use a straw bale cold frame:
- Winter harvesting – An insulating straw bale cold frame is a quick, cheap, and easy way to protect winter crops. Build it to size around a garden bed or overtop a row of vegetables to extend the harvest season by months.
- Extending the autumn harvest – A straw bale cold frame isn’t just for winter harvesting. You can also use this handy structure to protect vegetables like cabbage and broccoli from autumn frost.
- Getting a jump-start in spring – Start sowing seeds for hardy salad greens like kale, spinach, and lettuce in early spring.
- Use it to harden off seedlings – A simple straw bale cold frame is handy for hardening off indoor-grown flower, vegetable, and herb seedlings in spring.
- Overwinter half hardy plants – Depending on your region, certain vegetables and herbs may not be hardy enough to survive winter. Building a straw bale cold frame around crops like artichokes is an effective way to provide winter insulation.
- Chill flower bulbs for forcing indoors – I love forcing spring-flowering bulbs like tulips to bloom inside my home over winter. They require a chilling period of weeks to months, depending on the type of bulb. Placing the pots of bulbs in a straw bale cold frame is an easy way to do this. Learn more in this article.
Materials to use for the top of a straw bale cold frame
We know that straw bales make up the box of the frame, but you have several options for the top, or sash of the structure.
- Polyethylene – Clear plastic sheeting is widely available, but it doesn’t always make the best top for a straw bale cold frame, a lesson I learned the hard way. The first year I build a straw frame I covered it with a poly sheet and weighed down the edges. Late autumn rain and then winter snow caused the center to sag down into the frame which then froze into an iceberg. We couldn’t harvest! The next time I used clear poly I stapled sheets to the top and bottom of an empty window frame to provide strength and structure.
- Window – An old window makes an excellent cold frame sash and you can often find them for free. Larger windows are ideal, but you can use several smaller sized windows to top a straw bale cold frame too. The size of the windows often dictates the size and shape of the straw bale cold frame.
- Polycarbonate (plexiglas) – 8 mil thick polycarbonate is the material I use to top my wooden cold frames. It’s strong and durable and allows excellent light transmission. For these reasons I also like to use polycarbonate on top of my straw bale frames and unlike poly sheeting it never sags and allows easy harvesting and tending of the crops.
- Bubble wrap – Bubble wrap makes an insulating cold frame top and rolls with large or small bubbles are available. I recommend treating it like poly sheeting and stapling it to an empty window frame which prevents sagging from winter snow and rain.
How to build a straw bale cold frame
Cold frames are typically built to have a sash angle of 35 to 55 degrees. This slanted surface, which should face south, permits maximum light to enter the structure. I’ve built straw bale cold frames with angles, as well as level frames. If you’re growing crops in a straw bale frame, it is best to create an angle, but if you’re overwintering crops, achieving an angle isn’t as important and I don’t bother. Build the frame before hard frost damages your vegetables.
- Building a frame with an angle – For an angled frame, place the back (north side) and side bales on their sides and lay the bales at the front (south side) of the structure flat. This creates an angle for the top that lets in more light.
- Building a level frame – With this type of frame you can lay the bales flat or on their sides. I base this decision on what I’m growing. If I have tall crops like mature kale plants, leeks, or broccoli, I lay them on their sides so the frame is taller, but if I’m growing compact salad greens like lettuce or baby spinach I lay the bales flat.
After placing the bales, add your top and adjust as necessary to reduce gaps between the top and straw bales. You may have to shuffle the bales or move them slightly to ensure a good fit. If you’re concerned about the bales shifting over winter, you can add a wooden stake along each side to hold the frame secure. Gardeners in sites prone to high winds may also wish to strap or weigh down the tops.
Cold frame tasks
Once a straw bale cold frame is in place, there are three tasks to consider to promote healthy plant growth.
- Venting – On a sunny days, particularly in mid to late autumn, the inside temperature of a straw bale cold frame can raise very quickly. Open or remove the top to prevent overheating, replacing it by late afternoon.
- Water – I regularly water my cold frames into late autumn, or until the ground freezes. I don’t water in winter. Gardeners in milder climates will need to water from time to time in winter to maintain soil moisture. An easy way to water is to remove the top on rainy autumn days.
- Snow removal – A layer of snow on top of a cold frame can be insulating, but it also blocks light. I use a soft bristled broom to sweep away snow after a storm.
Bonus – I enjoy keeping track of the temperature inside my cold frames by adding a minimum-maximum thermometer. You don’t have to do this, but it is fun to note the temperature variations from mid autumn to early winter.
The best vegetables to grow in a straw bale cold frame
I plant my late autumn and winter frames with cool weather crops that can tolerant frost and frigid temperatures. Below are 5 of my top vegetables for a straw bale frame.
- Kale – Mature kale plants can grow tall, between 15 inches to 4 feet tall, depending on the variety. My favorite varieties to grow include Winterbor, Lacinato, and Red Russian.
- Leeks – Leeks are a long season vegetable. The seedlings are transplanted into the garden in early spring with the harvest beginning in mid to late autumn. The plants grow up to 24 to 30 inches tall making them too tall for my wooden frames. They’re ideal for straw bale cold frames.
- Spinach – Cold hardy spinach is a standout in the autumn and winter garden. I direct seed varieties like Winter Giant and Bloomsdale in early autumn and harvest until we run out in late winter.
- Carrots – Many root vegetables can be harvested throughout the cold months. My favorites include beets, parsnips, celery root, and carrots. Seed fall and winter carrots in mid-summer and harvest November through March. Top varieties include Napoli and Yaya.
- Asian greens – Asian greens like tatsoi, mizuna, mustards, Tokyo Bekana, and komatsuna are extremely hardy crops to grow in a straw bale cold frame. I direct seed in early autumn for months of vibrant greens for salads and stir-fries.
I’ve also used a straw bale cold frame to protect hardy herbs like curly and Italian parsley, cilantro, thyme, sage, and chervil.
What to do with a straw bale cold frame in spring
After a winter in the garden you’ll notice the straw bales from your frame are looking a bit worse for wear. That said, there are several ways you can use the bales or the straw in the garden. First, you can recycle the bales to make a straw bale garden, which is an easy way to grow vigorous, vining crops like pumpkins, squash, and gourds. Gardeners generally use new bales for straw bale gardens and season them for a few weeks before planting. However, the straw bales from my winter cold frames have already begun to break down. I add a bit of compost and organic vegetable fertilizer to the top and plant directly in the bale.
You can also use straw to grow potatoes. Plant seed potatoes about an inch or two deep in a garden bed and top with 5 to 6 inches of straw. As the plants grow, continue to add straw. When you harvest, you’ll find the tubers have formed in the straw making for a quick, easy, and dirt-free harvest.
I also use the straw from my cold frames for mulching crops like tomatoes, adding a 2 to 3 inch layer of straw around the plants after transplanting. Place the straw carefully, leaving a couple of inches of space between the mulch and the plant stem. If you don’t need straw in the garden, add it to the compost pile. Once it breaks down add the compost to your garden beds to enrich the soil.
For more ideas on using straw in the garden, be sure to check out these articles:
- Straw bale gardening: learn how to grow vegetables in straw bales
- Different types of mulch for the garden
- How to grow potatoes in the ground, in pots, and in straw
- How to deep mulch for winter harvesting
Are you going to build a straw bale cold frame?