Gardening Tips: A Cold Frame to Build

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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).
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Improvisation is just one part of gardening. A good gardener will devise a plan for extending the season, and a couple of old window frames make for an excellent insulated straw bale cold frame.
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Apply clear packing tape to the poly sheeting in the clamp locations before you add them, to avoid damaging the sheeting and prolong its usable life.
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Make sheeting clamps from short lengths of PVC pipe split off center.
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PVC 3-way connectors secure ridgepole to the rafters (rafters not installed).
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The corners and connection points can later be glued to add stability.
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Use homemade clamps to secure sheeting to the frame.

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.

If you look at the thickness of the wall of different types of PVC, you will notice that some are very thin walled (1/16 inch) and some are much thicker walled (1/8 inch). The 600 PSI PVC offers a thicker wall, much more strength and costs only nominally more. 

Supply list for a 4-by-6-foot cold frame:

  • 2 three-way ½-inch PVC connectors with 2 threaded ½-inch pipe ends
  • 4 120-degree ½-inch PVC connectors
  • 4 tee-fitting ½-inch PVC connectors
  • PVC glue (optional)
  • 10-by-14-foot piece of 6-mil poly sheeting
  • 4 10-feet-by-½-inch SCH 40, 600 PSI PVC pipe
  • 1 24-inch piece of ¾-inch PVC.

Cut the ½-inch PVC pipe to the following lengths:

  • Ridgepole: 1 68 inches long
  • Rafters: 4 26¾ inches long
  • Extensions: 4 1¾ inches long
  • Top Plates: 2 70 inches long
  • Legs: 4 24 inches long (wall height; lengthen if desired) 

Get started

Join the threaded pipe ends to the threaded ends of the three-way connectors. These pieces will help form the gable ends of the cold frame and connect the rafters to the ridgepole. Slide the ridgepole into the open end of the threaded pipes. Be sure that both gable ends are level to the ground and mark the joint with a permanent marker before gluing if you choose to glue. (I suggest assembling the entire project dry first and then gluing if you feel it is necessary.)

Next, place the center of one tee-fitting connector on each end of the two top plates. Slide an extension into one side of the tee and one end of a rafter into the other. Connect the other ends of the rafters into the open ends of the three-way connectors. That completes the roof framing.

To frame the walls, first attach the 120-degree fittings to the open ends of the extensions so they point toward the ground. Attach the legs to the open ends of the 120-degree joints and stand the whole frame up. When you first assemble the cold frame, it can be a bit awkward to keep upright because the pipe will spin in the fittings. The work is a lot easier if construction is done on either grass or dirt so the legs can be planted a bit so they don’t slide around.

After assembling the basic frame, decide and where if you want to glue it together. For my cold frame, I glued the end frames together, leaving the ridgepole and top plates loose so the cold frame could be disassembled for storage.

To attach the poly sheeting to the cold frame, I made some clamps out of the ¾-inch PVC pipe sections. Cut off a length of pipe approximately 2 inches long. Stand it upright so you’re looking down at the opening and mark a straight line that is slightly off center. Using a pipe saw (a wood saw will work too), cut the pipe at the mark. This will leave a 2-inch length of PVC that, when you look at it from the end, will look like a perfect “C.” When the open side is pressed over a ½-inch diameter piece of pipe, it will pop right over it, holding it tightly. I made seven of these clamps and use them to hold the poly sheeting to my frame.

After gluing the frame and placing it in the garden, I slid the sheeting over the top, making sure it was centered both ways. Then, after clamping the sheeting to the rafters and ridgepole – a hint: clear packing tape applied to the poly sheeting in the clamp locations will help keep the sheeting from getting holes – I folded it in at the front and back and used a few old pieces of stone around the bottom to hold the sheeting in place. You could make stakes if you like, but they leave holes in the plastic and tend to shorten its usable life. 

The straw bale cold frame 

Another option to extend the growing season can be had easily by building an insulated cold frame around a crop of cold-tolerant plants – winter greens, for instance. Many of the cold-tolerant plants, such as kale, cabbage, turnips and beets, can be grown well into the winter by using an insulated cold frame.

I made mine by encasing the outside of a garden bed with six standard straw bales, leaving a 3- to 4-foot-wide planting bed in the middle of the bales. With six bales, two for each side and one in the middle of the ends, the bed is 4 feet long. Add more bales to the sides to lengthen the bed.

To keep the heat in at night, either stretch a length of poly sheeting over the top of the bed and brace the poly in place. If you have old window frames, you can make a basic glass framed roof for the straw bale cold frame. I joined some old windows at the top with some inexpensive hinges. Placing them over the straw bale beds makes a sturdy roof and lets the heat vent out easily during the day. 

Height is nice

One of the nice things about either type of cold frame is height. Because both styles are taller, if you decide to use a cold frame to extend your growing season at the end of the year, each will fit over or around more plants than would a shorter hoop-style frame.

For instance, I’m planning to cover my okra this year since I got a late start on it., Last year, I managed to have tomatoes up until Thanksgiving simply by covering the plants at night and propping the sides open during the day so the plants didn’t overheat. Few things rival the taste of vine-ripened tomatoes in November; I couldn’t do either of those with the small hoop frames. They are too short.

Whether you decide to start small and build a simple hoop-style frame or build a more elaborate miniature greenhouse, extending your growing season helps make the most of the cool, early spring weather and holds off the cool autumn nights a little longer for those last few harvests.

Paul Gardener, a GRIT blogger, keeps his family happy with fresh vegetables during most of the year at their suburban Utah home.