This fall I built my first real cold frame. I’ve made a couple of rather rough & tumble cold frames in previous years out of salvaged materials. These were moderately useful, but this time I actually purchased the materials all new. And unlike my previous cold frames, which I used just to start plants in the spring or for hardening off, this one I wanted to use on one of my raised beds to try to get some winter harvest.
Cold Frame
Of course I may be getting a bit ahead of things. For those of you unfamiliar with cold frames, a cold frame is essentially a mini-greenhouse that you can build on the ground or on a raised bed. Cold frames can be used to extend the growing season at either end of the season, to provide winter harvests, for seed starting, and for hardening off plants you started indoors.
The Hudson Valley Seed Library has a fairly good set of directions for building a cold frame on their site (in their blog post Seed-Starting 101: Part 4 of 6, March 11, 2010). I used their directions as a starting point for building my cold frame, but discovered some issues which I’ll pass on to you.
Frame Size
The Seed Library cold frame (CF) is meant to be put on the ground and is designed to be 4’ wide x 8’ long. I make, and recommend making, raised beds designed to have a planting area that’s 4’x10’ which means the frame is 4’3” x 10’3”.
The Seed Library plans call for using 2×2 lumber for making the frame for the CF lid. This works just fine if you’re making an 8 ft long frame, but the longest 2×2 board commercially available is 8 ft long, too short for my frame.
My beds are also too wide to efficiently use the clear polycarbonate panels the Seed Library directions recommend for the CF lid. These come in 8 ft lengths (by 26” wide), so cutting one in half gives the 4 ft width of the Seed Library CF covers. I could have cut the panels to width of my beds, but that would have meant throwing away almost half of each polycarbonate panel, both wasteful and expensive.
Using a 2×4 for the back edge of my cover frame solved the problem of the bed being too wide for the polycarbonate panels. Using a 2×4 meant the distance between the front and rear of the lid frame 46 1/2” giving me room to attach the lid covers to the lid frame.
Using 2x4s could also have solved the length problem, since 2x4s come in lengths longer than 8’. To make a front 2×2 for a frame longer than 8’, I could have cut another 2×4 down the middle, lengthwise, using my table or circular saw. Because I wanted to try out two different materials to cover the lid frame, I decided to make an 8’-long frame to cover most of the planting bed and a smaller one to cover the rest.
If you want to make your life easier you could just make a special bed or beds specifically for using with a cold frame(s) and be sure the outside dimensions are 4’x8′.
Frame Cover Material
As I mentioned above, Seed Library also recommends using SUNTUF polycarbonate panels, a wavy polycarbonate plastic product. No local store carries SUNTUF products any more. Lowes, where I bought my materials, doesn’t carry that brand anymore, but they have a similar product. At Home Depot, SUNTUF products can be ordered, but they’re not carried in the store nor is any similar product. It’s also possible to get flat polycarbonate (lexan/plexiglass) sheets at Williams. I used wavy material on the main cold frame lid and the flat type on the smaller frame, so I could see which I like better.
Cold Frame Cover Materials
The advantages of the wavy material are
1) Less expensive (though not by a lot especially when the cost of the insulating strips are added).
2) When getting snow off the lid you don’t hit the screws with your shovel, since the screws are in the troughs.
3) It’s easy to extend these panels past 8’ if needed.
Disadvantages
1) I had some trouble getting the individual sections (it takes 4 sections to cover an 8’ frame) to line up perfectly with each other.
2) The spaces between the wavy cover & frame are filled using a cheap foam material that’s somewhat difficult to use.
3) If the bottom of a trough doesn’t match exactly with the lid frame edges there are gaps that need to be filled in with caulk, weather stripping or a similar product.
4) Water can only flow off the cover in the direction of the troughs, while on the flat lexan it can flow off in two directions.
I’m still deciding which I like better, though I’m leaning toward the flat sheets. I’ll give you a final analysis in the spring.
Building Cold Frame Box for a Raised Bed
So I built the CF box according to the Seed Library directions, with a 2×8 board in front and a 2×12 in back. This difference in the height between the front and back of the CF is done both to allow better sun exposure of the covered bed and to allow water to drain off the lid.
When I put the frame on my raised be frame I noticed that almost the whole bed was in shade. A first I couldn’t understand why, but I realized I hadn’t accounted for the soil being 3” below the top of the raised bed frame.
I fixed the problem by using a 2×4 in front and a 2×8 in back. This gave me the proper slope for the cover and overall height for the frame, but didn’t block the sun. So if you’re building a CF for a raised bed, make sure you take into account the soil level.
Planting Timing
A cold frame will allow plants in it to survive for the winter, but it will still be too cold for the plants in the cold frame to grow much or at all. So if you really want to get a winter harvest you need to have the plants reach harvest size before it gets too cold. From what I’ve read, and seen in my cold frame this season, that means the plants need to be harvest size by late November.
This means most of the crops need to be planted in early to mid-October. This was a problem for me as all of my beds properly positioned for a cold frame still had other plants growing in them at this time. Therefore I had to wait and my plants are still only 1-2” tall and not ready for harvest (unless I want to use them as baby greens).
Inside my Cold Frame- January 2013
So for this year the main purpose of my cold frame will probably be to give me a very early spring harvest. For next year, I’ll plan for the cold frame bed to be planted with veggies that will be done by early October.
Cold Frame Temperature Control
I know these are called Cold Frames so you probably aren’t thinking you need to worry about it getting too hot in your cold frame. However, on an sunny, but still cold day, it’s entirely possible for the temperature inside the CF to get hot enough to kill your plants.
To keep this from happening the lid needs to be opened so the hot air can come out. You can do this manually by placing wood blocks between the main frame and the lid. I’ve also seen plenty of designs for other types of manual supports. The problem with manual support is if you forget to use the supports or a cloudy day turns sunny and the lid is closed the CF temp can get to high. Also if your CF is open and you’re away until after the sun goes down it can get too cold.
To deal with this problem you can install automatic lid openers. I know people who use these and they say the work very well. You need two per CF lid to be sure they can lift the lid. I use manual supports myself, because the lid openers are fairly expensive, and so far my plants seem to be OK.
One Last Thing
The Seed Library plans tell you to mitre (cut at 45 degree angles) the 2×2 boards used for the CF lid. I didn’t do this, nor do I see any reason to do so. If your carpentry skills are limited, as mine are, make your life easier and don’t mitre the boards.
