How to Build a Straw Bale Cold Frame

Learn how to make a simple straw bale cold frame DIY using old windows, to either harden off your plant starts or to extend your growing season!

Today I am going to share with you how I created more room for my plant starts by simply learning how to make a cold frame. My growing rack was getting full, and I came up with this simple DIY cold frame project, making a straw bale cold frame with old windows we had lying around.

What is a Straw Bale Cold Frame?

A straw bale cold frame is a box made from straw bales and windows, and the box becomes the cold frame which protects plants from frost. The sides are made from the straw bales, which helps to insulate the plants. The window on top allows the sun in to help warm the space, keeps the heat from escaping and the sunlight is needed by the plants for photosynthesis.

Cold frames do not need to be airtight, but should also not have super large openings in it, especially at night time when temperatures are cooler.

DIY Straw Bale Cold Frame Tutorial

This straw bale cold frame DIY, is so simple, it will only take a moment to tell you how we built it. Building a cold frame like this, literally only takes a few minutes.

We bought six bales of straw. The straw will be reused later, for mulching in the garden. Don't use hay if you want to mulch with it. Hay has lots of seeds in it, straw has less.

We had 3 windows lying around, that had blown their seals. So six bales of straw was the perfect size for us. Depending on what old windows or glass doors you can get your hands on, you may need to buy more or less.

We simply placed 2 bales end to end on the long side, 1 bale on each end, and placed the windows on top. And that is how simple it is to make a cold frame with straw bales and windows!

Hardening Off Plants in a Cold Frame

Yes, cold frames are great for hardening off plants in! Especially when it is still really cold outside. Check out my article How to Harden Off Seedlings if you are not sure what this process entails. 

Before placing any plants or seedlings in the cold frame, you may want to use these natural slug and snail control methods, as those guys love to hide under the hay bales and plant trays, and they can eat all those plant starts you have worked so hard for.

If it is still cool out, this works very well. We had a heat spell this week, where it got in the high 70's and low 80's. We actually had to shade some of the starts from direct sunlight it was so hot! So be careful on warm days that your plants don't cook in there.

Also, be aware of fire hazard, if you leave the windows on and it gets very very hot. Straw is extremely combustible, so be sure that you are venting the cold frame properly in hot weather.

Venting the Cold Frame

Even on not so hot days, be sure to open the cold frame up and vent it appropriately. We only have it completely closed up at night, and on days where it is 40 degrees or less.

As it gets closer to actual planting time and continues to warm up, you should have it open more and more. Even at night as you slowly harden the plants off.

Extending the Growing Season with a Cold Frame or Greenhouse

Not only can you extend the growing season by using the cold frame in late winter/early spring for hardening off plants and housing your veggie starts, but you can actually grow in it! Especially with this straw bale design.

If you wanted to fill in the center of the cold frame with soil, you could start growing greens and other cold hardy vegetables such as brassicas or root crops in there in winter with the windows on top for a spring harvest.

You could also do the same thing in fall and early winter. Simply plant a fall crop of greens or brassicas or root crops and they should do fine temperature wise in there for quite some time. Plant growth may slow down as the daylight wanes in mid winter, but should resume as the days lengthen.

I also love my greenhouse for this reason. It is a plastic greenhouse, unheated, almost more of a high tunnel really. But it has served me well over the years. It allows me to move my tomatoes and peppers into it the first weekend in April, and they are usually still producing into October for me. Which for the PNW is pretty incredible as we have such a short growing season here.

This greenhouse I leave up all year, as the roof piece is separate from the walls, and it is too tall to easily put up and take down.

Last year, I bought a second greenhouse, that I use for selling my plants starts from, and I highly recommend that one as well. I also use it to move my plant starts into, when my seed starting rack gets too full. This one is much easier to put the plastic cover on, as it is not as tall, and it is just one piece. So we pull the cover off for winter, and store it until March when we set it up again to start moving seedlings into it.

Eliot Coleman, the Master of Winter Gardening

To learn more about 4-season gardening, extending the growing season, be sure to check out the book The Winter Harvest Handbook: Year Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses by Eliot Coleman. He lives in Maine, and is a market gardener, who grows food all year long. In Maine. Through the winter. Yes, he really knows his stuff!

Beware of Mice, Voles and other Vermin

One thing to be aware of in either a cold frame or a greenhouse, are mice, voles and other vermin eating your plants. They are known to love to nibble the tops off of your brand new veggie starts. In the dead of winter, they will find your root crops if you have them growing in there, they will eat them too.

So be sure to have some type of mouse/vole trap in place. Eliot Coleman has a great design for a DIY trap, that I found some pictures of in this excellent article: Extend the Growing Season with Coleman's Double-covered Low Tunnels. They also talk about the trap and how it works in this article. Scroll down and on the left hand side, you can see two pictures of a wood box with mouse traps inside.

Reusing Straw Bales in the Garden

The benefits of a straw bale cold frame, is that you can re-use the straw bales for other things in the garden, when you no longer need the cold frame. I prefer to use my straw for mulch in the garden. Which prevents weeds, see my article here on The Secret to Easy Gardening, No Weeding!.

It could also be used as animal bedding for goats or chickens or what have you. Or, as I mentioned above, you can fill in the cold frame with soil, and use the straw bales as your raised garden bed supports.

Another option, is to also garden right inside the straw bales themselves. Simply use a pointy trowel, dig out a hole in them, add some compost and plant your plant! This is also known as straw bale gardening.

There are lots of sources online about it and I have two personal friends who only garden in straw bales, so I know it works! See more ideas in my article Using Hay & Straw in the Garden.

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