Gardening Tips: A Cold Frame to Build

Extend the garden season with a cold frame project and other gardening ideas.

| November/December 2009

Assembled homemade cold frame

The miniature greenhouse, one cold frame design possibility, keeps the heat in at night and is sufficiently tall enough to house mature broccoli (not shown).

Paul Gardener

I love the transition seasons – autumn and spring. In early spring, I watch impatiently as the ground thaws and the dormant bulbs peek through the snow. I know that soon I’ll be able to get my hands in the ground again and begin the garden season.

Months later, watching as the summer greens fade to reds and golds, and feeling the evenings’ crisp cool makes autumn a favored time as well. I can’t help feel a bit of loss, though, that soon my garden will be sleeping, and the fresh tomatoes and garden greens will be gone until next summer. It was this combination of impatience early in the year and sense of loss at the end that finally led me to experiment with season extenders so I could get an earlier start in the spring and hold over a good crop in the fall.

Spring can be one of the most productive times in the garden if you really make an effort to capitalize on the cool weather. For several years, I tried to grow a good crop of early season vegetables like broccoli, kohlrabi, carrots and cabbages with little to spotty success at best. In northern climates like mine, we tend to have cool, wet springs that usually give way to hot and dry summers overnight. The problem always has been that by the time I get cool-season crops to sprout naturally and begin growing, the summer heat kicks in and, bam, everything bolts and turns bitter. I’ve now found that by taking some simple steps, I can get my plants started much earlier and finally have enjoyed the bounty of spring, as I knew it could be. 

As easy as you make it 

Early season hoop houses can be as simple or as elaborate as you want them to be. My first one was a couple of lengths of rebar bent into two half circles and connected at the top with a scrap piece of furring strip (1-by-2 board) with some leftover clear poly sheeting (Visqueen) thrown over it. It was largely a success.

In the hoop house, I was able to grow and enjoy a full spring harvest of broccoli and some delicious Napa cabbage. The benefits of this little setup were simple. First off, it was cheap, and it did a good job of keeping the heat in at night. It did have a few downsides, though. Because it was just a basic structure and had little in the way of support, it didn't handle our wet spring snow well. Also, by the nature of its design, it was taller in the middle and quite shallow on the edges. This wasn’t a problem at first, but after the broccoli began to get more than just a few inches tall, the shape caused the plants to bend in toward the center and, on a few of the hot days we had, the top leaves were scorched from touching the sheeting. To solve that problem in later years, I came up with a cold-frame design that would give me a good deal more room to grow and would provide significantly more structural support in case of snow. It’s like a miniature greenhouse. 

The miniature greenhouse 

When designing my new cold frame, I decided on PVC for the frame. It’s easy to find, inexpensive, and I can easily modify the structure to suit some future growing need; design flexibility is always a consideration in my garden. The PVC I chose to go with was ½-inch SCH 40 – rated at 600 PSI.

Tammie Brice
5/18/2011 3:56:06 PM

Would you be able to post some pics of your PVC frame? I would really appreciate it. Thanks.

Paul Gardener
11/10/2009 1:54:41 PM

Hi Robert, I did leave the end open for the picture of the PVC frame. The flaps of plastic that normall cover the front of the frame are folded back over the body of the frame. This is a regular morning practice right now as the weather here is still quite nice during the day. Open in the AM & close in the PM when I get home from work. If the weather were staying consistently cold I would leave the flaps closed. On the window frame cover I left the ends open because these are Cold weather tollerant plants; Kale and Kohrabi. They will do quite well with only a cover to hold the hard frost off. I some others that are not even covered and are still quite healthy. I like to leave these in over winter to feed some fresh greens to my chickens. They love the occasional treat for as long as I can give it to them. Best of luck! Paul~

Robert Blackburn Jr.
10/23/2009 10:53:42 AM

Thanks! With both the PVC frame and the window frame (pun?), it appears you leave the ends open. Is that correct or is that just for picture purposes?

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