Friday, December 30, 2011

Build a Cold Frame for Ongoing Winter Harvest





Although they have not need its protection yet this winter, my lettuces and spinach are happily growing inside the cold frame while we wait for cold weather.  I planted them inside the box in September with the lid open so the warm weather did not roast them.  Now, when freezing weather threatens, I can close the lid to the box in the late afternoon, the glass lid will hold the heat inside, and I will have fresh greens the next day that are unharmed by frost. 

My husband and I made my cold frame cover from a discarded shower door.  Any glass or Plexiglas door or window would work; the glass allows sunshine and heat to reach the plants inside the cold frame.  My shower door came with the hinge attached which we use to open and close the frame.  If yours does not have a hinge, you could slide it up and down as needed and prop it with a stick; a covering that is lightweight, like Plexiglas, would probably be the best choice.  When the lid is raised, make sure to attach it to the ground in some way so that strong gusts of wind do not suddenly close it; the glass on one of my frames shattered last winter and I will have to cover the frame with plastic this winter. 

Winter greens grow inside the cold frame


We made the sides of my cold frame out of treated lumber.  We caulked the joints and put some weather-stripping along the top of the frame to prevent drafts.  The back of the cold frame is about 18 inches high, and it slopes down to the front at about a 40-degree angle toward the southern sky; the front is about 8 inches high.  This slope is supposed to maximize the amount of captured sunlight.

Options besides boards include stacked hay bales, concrete blocks, or landscape timbers.  Plug as many cracks as you can.  Gardener’s Supply Company carries prefabricated cold frames; harvesting your own salad greens instead of buying them will offset the purchase price quickly.

 Place your cold frame directly on the ground, fill your cold frame with compost-enriched soil, and plant the seeds or transplants.  Because of the greenhouse-like moist environment, seeds sprout quickly and are the most economical choice.  Water the soil when it begins to dry out and fertilize the plants as you would in your garden. 

If the weather forecast is for temperatures in the high 40s or above with sun, prop open the lid of your cold frame to vent excessive heat that might burn the plants.  I have two sticks, one to open the lid just a crack and the other to open it wide for harvest and for very warm days.  If you will not be home, prop the lid open a crack; they have a better chance of surviving the cold weather we get than an afternoon in with temperatures in the sixties closed inside the frame. Lettuce and spinach tolerate temperatures down to the mid-twenties without harm in the open garden; I close my cold frame on nights when the temperature dips into the low twenties and teens.   

The best plants for a cold frame are lettuces, spinach, collards, and other cool-season greens.  Carrots, beets, and parsnips also like the protected environment.  Depending on your cold frame’s interior height, you might also be able to grow broccoli, cauliflower, and cabbage.  I also “harden off” transplants, or provide a protected area outside for plants to become gradually accustomed to the outdoors, before I set them out in the garden in the cold frame.
Happy salad greens

When you are home during the holidays, maybe you can find time to make yourself or the gardener on your list a cold frame as a belated Christmas gift. 

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