Cold Frame Winter Vegetable Gardening

Cold frame winter vegetable gardening is a wonderful way to stay in touch with our gardens through the cold and snowy months - which otherwise leave us impassioned gardeners with little to do but impatiently drool over seed catalogs while loading them up with sticky notes. (Oh alright... we could also read good books about soil microbes or composting or regenerative agriculture, or start drawing our layout, or grow baby lettuce under lights in the basement... but where was I? Oh yeah... cold frame winter vegetable gardening).

What you can grow in your cold frame over the winter depends, of course, on how cold and long your winters are. You're not going to have as long a season or as many choices in Minnesota or Maine as you would have in Arizona or Arkansas.

It's also important to distinguish between "grow" and "survive". If you live where it freezes for long periods, once it gets well below freezing and the day length gets shorter than about 10 hours, things won't really grow in a cold frame (as in, get bigger), but they may survive (as in, won't burst cells and die if they freeze).

But even though you may not be able to grow vegetables in the dead of winter, there are several crops you can grow to harvestable size before winter sets in and then keep alive in a cold frame. In harsh climates this means that the cold frame actually becomes live-vegetable storage. This is valuable because you probably can't store a winter's worth of kale or mâche in your fridge, but you can "store" it in the ground outside under a cold frame or hoop tunnel, provided it's a very hardy crop that isn't killed by freezing.

I'm afraid my first cold frame winter vegetable gardening experience years ago was not a success, because I didn't realize I needed to start planning when it was still mid- to late summer. Once it's really cold it's too late to get seeds to germinate and grow good roots before they stop growing for the season.

Three Ways to Use Your Cold Frame

There are a few ways to use a cold frame in the winter:

  • If you have a garden bed (raised or in-ground), that is already growing cold-hardy greens (see chart), carrots, or other crops suitable for extended harvest, you can put the cold frame over it as the weather starts to cool to extend your growing season.
     
  • Starting in late summer, you can plant seeds or set out transplants of cold-hardy greens in the bed where you will place your cold frame. Depending on your climate, it may be better to actually start your cold-hardy greens indoors in seed flats under lights, because hot summer temperatures may be too high for good germination of some seeds (especially spinach). Transplant indoor starts out into the bed well before frost to give them enough grow time to get their roots well-established before daylength and temperatures get too low for more growth to occur.
  • A combination of the above: put a cold frame over a section of bed that has some sustainable harvest left in it (say, carrots), and add late-start transplants or seeds to the remaining portion of the bed.

For added frost-resistance and to slow the freeze/thaw cycles through the winter months, you can bank the sides of your cold frame with straw bales. I've even seen a cold frame itself that was made of six straw bales with a glass door on top.

The "Persephone Period"

There is a term you should know if you are planning on cold frame winter vegetable gardening (or winter gardening in a hoop tunnel), and that is the "Persephone period".

Persephone was the goddess of vegetation and harvest in Greek mythology. After the harvest Persephone would go back underground (to hang out with her partner Hades) for the winter. The "Persephone period" now refers to that period of the year when there are less than 10 hours of daylight per day, and it is important because crops stop growing without enough light. If you are going to plant transplants out or direct seed into the bed that will be covered with the cold frame, you need to know how far ahead of time to start your winter-crop vegetable seeds while there is still enough hours of daylight for them to grow and get their roots established. (See chart below.)

I live in Denver, Colorado, and our Persephone period is from November 13 to January 28. To find the dates for your location in the world, you can visit:

http://www.solartopo.com/daylength.htm

On that site, play around with the date in the upper left corner until you find the two dates (fall and spring) where it switches between more than 10 hours to less than 10 hours (and vice versa).

Plan Cold Frame Winter Vegetable Gardening On Your Calendar

I have more than one calendar in my house: one for appointments and day-to-day stuff, one for duckie stuff, and one for my garden. The garden calendar serves as both mini-journal and planner. I write down weather extremes, hail, frost and unusual temperatures, but I also write down things that should be done throughout the seasons that are easy to forget. Things like "start second crop lettuces" (May), "start fall greens indoors" (July) and "start winter cold frame seedlings" (early September). Here are some of the crops that are appropriate for cold frame winter vegetable gardening:

Some Good Choices for Cold Frame
Winter Vegetable Gardening

Crop Weeks Before Persephone
Kale 15 to 13 weeks before last 10-hour day
Carrots 13 to 12 weeks before last 10-hour day
Pac Choi 10 to 8 weeks before last 10-hour day
Cilantro 10 to 8 weeks before last 10-hour day
Mâche 9 to 8 weeks before last 10-hour day
Spinach 8 to 7 weeks before last 10-hour day
Arugula 8 to 7 weeks before last 10-hour day

If the ground in your cold frame isn't frozen, make sure to keep an eye on the moisture level in the soil. Since snow won't be melting into the ground and since the air in the frame is warmer than ambient, it may get dry in there.

So have fun! It is really cool to come in the back door with fresh greens in your hands while shaking the snow off your boots.

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