Archive for March, 2009

Winter Greens, Part II: The Cold Frame

March 22, 2009

This entry is a follow-up to my article on sprouts several weeks ago.

I planned on being able to post digital pictures before I wrote this, but that’s not happening.  In addition to sprouting various seeds, some home-grown and others from the store, I add fresh local vegetables to my winter diet through the use of a cold-frame.  I have had less than spectacular results, and a few meteorological mishaps, but the idea is worth sharing.

In my last entry, I gave a plan for an insulated straw-bale cold frame.  This year, I didn’t get enough bales before winter, so I used an alternate design–a variation on a hoop house.  I picked up about 75 ‘ of concrete reinforcing mesh–sort of an industrial-strength chicken wire with 6″ holes–to make tomato cages and a compost bin.  With the 15 or so feet I had left over, I made two sections that arced into a half-circle shape, about as wide as one of the beds in my garden.  Each section was cut to leave 6″ lengths of wire sticking out on the ends, which I stuck into the ground to secure the frame.  This left me with a wire tunnel, about 5 feet long, 4 feet wide, and 3 feet tall.  I was able to get some greenhouse-grade plastic (leftovers from a local greenhouse) to cover my frame, and weighed down the edges with some fist-sized rocks.  This left me with a miniature version of the full-sized greenhouses you’d see at a commercial-scale nursery.

The cold frame collects and retains solar energy, resulting in, well, a “greenhouse effect” that keeps the plants within at a much warmer temperature than the outside air.  Though this would only add a week or two to the life of warm weather plants, like peppers or tomatoes, it can keep cool-weather plants alive through all but the coldest Michigan winters.

Fall and Spring Planting in a Cold Frame

I stuck with cool-season greens in the fall, such as leaf lettuce, bok choy, and spinach, planted in the early fall or transplanted a little later.  These go more or less dormant when the weather gets cold, but the cold frame keeps them in a harvestable state through most of winter.  There isn’t a lot of growth, so your stock dwindles as spring approaches, but it’s a great way to keep some fresh and local food on the table all winter.  With no disrespect to the other two winter-long green vegetables, it’s way more interesting than kale, and far more versatile than brussel sprouts.

In spring, the cold frame is more of a head-start than a long-term plan.  Around the time I’m planting pea seeds, I pull out the rest of the winter greens from the cold frame (there’s usually not much left, and what’s left is starting to toughen up and flower), and plant a new crop.  This year, it was spinach, bok choy, leaf lettuce, and very early carrots.  The additional heat and humidity from the enclosed cold frame will give them several weeks of extra growing time before the open-air garden is ready to be planted.  I’ve sited the cold frame in the place I’m putting this year’s peppers, which won’t be planted until early summer, so most of these plants will have a chance to mature before the peppers go in.  I’ve considered putting the peppers in a few weeks before the last frost, hoping that the cold frame would give them the shelter and warmth they would need, but I don’t think it’s worth the risk.  I’m not sure how cold it can get in a cold frame, particularly at night.

Cold Frame Problems

Weather rarely cooperates with cold frame construction.  I’ve had my plastic sheeting blown off completely, despite weighing down the edges and clipping the plastic to the frame.  I’ve also had the entire thing collapse under the weight of snow and ice, crushing the leafy plants underneath.  Also, since the plastic sheeting is impermeable to rain, it can get desert-dry inside of a cold frame when all the ground moisture is frozen.

Access to vegetables is another problem.  With rocks weighing down the edges, water sometimes pools and freezes, making it hard to lift the plastic sheeting and reach in to harvest.  The 6″ access holes are also an obstacle.  I’ve adopted a “mass harvest” mentality, getting a week’s worth of greens in one go, just to simplify things.

Once spring starts to warm up, it can get very hot inside a greenhouse.  Think of a car parked in the sun with the windows up–opening up the cold frame on those warm, sunny days is a necessary inconvenience.

Weeds that are growing in the space where you’ve set up your cold frame are also able to take advantage of the microclimate, so they often spread throughout the winter.  Finally, keeping a bed in production all year takes a lot out of the soil, so your cold-frame bed should be refreshed with compost or fertilizer more often than others.

Possible Improvements

I’ve considered a number of options that I haven’t followed through on yet.  The first–instead of using weights to hold down the plastic, I’ve seen cold frames with trenches dug along their edges, with the edge of the plastic sheet buried under a few inches of soil.  This would hold it down a lot better, but would make it much harder to get into the cold frame.

Another modification would be to put it in so that it straddled two beds, with a path down the center.  I’d have to crawl in from the end (the taller a cold frame is, the less efficient it is, as heat rises away from the plants), but I wouldn’t have to reach, and I’d be able to bury the edges as mentioned above.

I’ve saved up a bunch of dark-colored laundry soap jugs, which I intend to fill with water and use as heat collectors.  The dark colors will absorb solar heat through the day, then release it at night.  Each jug takes up valuable square footage in the cold frame, though.

I may use a few stakes next year to reinforce the wire framework–I’ve never had a collapse before, but this year’s heavy snowfall brought the whole cold frame down.  It’s an easy enough fix, and I could have saved a few plants.

So far, using a cold frame has been more art than science, but it has kept in in the garden throughout the winter, and kept me out of the produce section of the grocery store for several weeks beyond the end of my garden’s regular season.  I’ll be building another straw-bale frame next year, provided I can round up enough free straw bales, but I’ll build another wire-frame hoop house as well, just to see if I can do any better.

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