Growing Vegetables in a cold frame

Cold Frames I Built To Get A Jump On This Years Garden

Cold Frames

When I was a kid, each spring we would visit my grandparents to help them plant their garden, and return during the summer to help harvest. They had a huge lot that received a lot of sun, and my grandparents took full advantage of it for gardening. They planted enough to supply each of their three kids and their families, fresh vegetables though out the summer as well as enough to can to last the winter. The time I spent with my grandfather in his garden is one of my favorite memories.

With all that gardening history it is no wonder, every year on the first sunny day we have here in Portland I start itching to get out in my vegetable garden and start planting. Then after that first sunny day, of course comes the weeks of clouds and rain to ruin my excitement. With the short growing season in Portland and my love for gardening I decided to try to extend the growing season by investing in a couple of cold frames.

After shopping around a bit I found that cold frames are expensive to buy and are limited in sizes, so I decided to build my own. I was able to build two 3’ x 10’ cold frames for under $40 and so far, I am happy with them.

I decided on a dome shape for a few reasons. The shape is easy to build, maintain, and can be easily folded and stored away during the peak of summer. The dome shape is also strong, during the winter months when it snows; the snow can easily slide off the sides. My favorite feature of this style is it is a portable cold frame. I can drive the steaks in the ground any place in the garden I want to set it up.

Another benefit to this shape is that it will conserve water by reducing the amount of times I need to water inside the cold frame. The vegetable beds are mounded kind of like raised beds without the wood sides. By not using any sides and just mounding the soil, I was able to create irrigation ditches between the beds. When the rain runs down the sides of the dome and into the irrigation ditches, the water is then absorbed into the sides of the vegetable beds. This deep watering should promote deeper root growth and a stronger plant in the end.

Uncover Cold Frame on hot daysWith the cold frame plastic attached as a curtain, I can easily open them up on hot sunny days so the plants are not burned and then close them up at night to help protect the plants from the cold night air. In the late spring and early summer when it comes time to harden off the plants and take the cold frames off. I can open them ½ way up for a few nights to help them acclimate to the cooler night’s air.

I am hoping I will not have to worry about transplant shock. There has been several times in the past when I have moved my seedlings from indoors to outside for transplant. However, during transplanting, the roots were disturbed too much, or the temperature change was to extreme and as a result, the plant took days to recover. With this setup, I can put the cold frame up, seed right into the beds eliminating transplant shock, and not lose vital days of growth in Portland’s short growing season.

How to Build These Cold Frame

The cold frame construction was pretty easy. To make the dome shape I found that ½” PVC electrical conduit was flexible enough to bend to the desired shape. To attach them to the ground and hold the dome shape, I drove ¾” PVC pipe into the ground. Then the electrical conduit slide inside for a friction fit and bent into shape by sliding the other end into a pipe on the other side of the bed. To make the PVC stakes I cut the pipe in 2 foot lengths and drove them into the ground about 18 inches deep. I have seen some people do a similar setup with rebar, but I have kids and do not want anybody to fall on the rebar steaks and be impaled. PVC is much safer. I found you have to use the 200 PSI type, if you use schedule 40 the extra wall thickness makes the inside of the pipe too small to slide the electrical conduit inside of it.

Drove PVC pipe 18” in ground200 psi PVC pipeCutting end of conduit with a pipe cutter

The electrical conduit has a flange on one end so you will need to cut it off so the conduit slides inside of the PVC pipe. A basic pipe cutter works great, but if you don’t have one a hack saw will work just as well.

4 mil Plastic sheathing for cold frame

I then used a roll of 4 mil clear plastic that was 10’x25’ and cut it in half. If you want to spend a little more money, I would suggest using a 6 mil instead of the 4 mil plastic. The thicker plastic will last longer and probably give you a bit more insulation value.

EMT Conduit straps attaching plastic to cold frame
To attach the plastic to my electrical conduits I used ¾” EMT pipe straps screwed to a small block of cedar on the other side of the plastic. I spaced them out evenly across the sheeting and used the folds in the plastic to help keep a straight line. The ¾” size pipe straps slipped perfectly over my ½” electrical conduit much like a curtain rod giving me an easy way to open and close the cold frame.

Raised vegetable beds under a cold frame
Now I just need to add the vegetables starts.
Organic gardening with steer manure
I’m a huge fan of organic gardening so before I plant anything, I’m mixing in some steer manure and will be adding in some compost mulch as the season goes on.

Growing Bell Peppers in a cold frame
I am also trying companion planting techniques this year as well, and I have seeded in some carrots around my bell peppers.
It is a good thing it is raining today to get me back into the shop and get some real work done, or I would stay out here all week as I have construction plans to build more cold frames for the rest of the rows in my garden.

Progress Update

Tomato Plants Growing in a Cold FrameWell it has been a few weeks and things are growing better than expected. Normally this is the time of year where I would just be setting out my tomato starts; I am weeks ahead at this point. My tomatoes are almost to the top of the cold frame and out of room.

I did find a flaw in my design. The EMT clamps holding the plastic to the conduit, in theory sounded good. I thought I would be able to open and close the cold frame as if it was on a curtain rod. However, that turned out to be a real pain. They just do not slid very easily. Next time I think I will just buy some spring clamps and clamp it to the conduit.

Well back to the shop to build some more furniture.

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Comments: 1

  1. David says:

    Thanks for this detailed description! I’m just starting my second year as a urban gardener, so I’m trying to learn as much as I can, and living in Chicago for sure I want to have a longer season for my vegetables.

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