Keep the veggies growing with a winter cold frame

With a miniature portable greenhouse called a cold frame, winter doesn't have to put a stop to food gardening. The mild temperatures of our maritime Puget Sound-area climate provide the opportunity for year-round growing. But growing in winter is not as simple as the springtime practice of sprinkling a row of seeds into a furrow.

A cold frame is set up over a convenient section of garden soil during cooler seasons. In winter, a cold frame can help your overwintering vegetables thrive, and help you get an earlier start on planting other crops. Cold-frame basics include knowing how to site the cold frame and build it, then deciding what and when to grow.

The right site

Your first consideration is where to put it. Ideally, it should be in the warmest spot in your garden, the place where hot summer crops do best. For most light, it should get south and west exposure.

Site it away from trees or shrubs for the best access to low winter sun. A spot protected from the wind will help, too.

Finally, place it next to a stone walkway to make for relatively dry and clean access during wet weather. If you need to get muddy to work in it, you may find yourself visiting it less often, and regular monitoring of the plants is necessary.

Scrap it together

A cold frame is ideal for recycling. Various materials can be used: The top can be an old glass window in its frame, or plastic panels or sheeting. Attach it to a scrap-lumber frame. For a base, use bricks or broken-concrete pavers.

Before starting to build, visualize the finished product: An angled pane of glass or plastic will sit upon a sturdy base. The top will be removable or easy to tilt up for access from the front. You must also be able to prop it open for venting.

The easiest cold frame is a plastic-covered box that fits over a wooden raised bed. You can simply attach the wood structure to the raised bed, then wrap the frame in plastic.

However, if you want to build one from scratch, try this:

Clear a flat garden space the size of your cold frame, then lay a brick or stone base on which to set the frame.

Build the wood frame (four sides and a top, with an open bottom) using 2-by-2-inch boards, sturdy enough to support the top but light enough to move easily. The front and back will be rectangles, and the sides will have an angled top board. The top will angle down toward the front wall. The back wall should be at least six inches higher than the front wall to create at least a 10-degree angle, which will shed rain and gain more light. The angle can be up to 45 degrees.

Secure plywood or heavy, clear plastic to each side (use rigid polycarbonate or flexible polyethylene sheeting that is 6 mil or thicker). Plastic will allow in more light, but will require more attention and venting on sunny days to keep from stressing the plants.

Build each wall separately, then connect them with screws or bolts for easy dismantling and storage. Hinge the top to make it easier to open.

If using a light plastic top, fasten a sturdy cord between the top and a side wall so it won't swing open in windy conditions. Use a garden stake to prop it open when working or venting.

Further refinements include painting the wood for weather-resistance. Use a light color to reflect more light back onto the plants. If the top is made of lightweight wood and plastic, use bricks to hold it down in the wind, or add eye hooks between the top and side walls. Add a handle to the top.

Use it for seedlings

In January and February, the cold frame can be used to grow seedlings started indoors. As the soil warms and days lengthen, seeds or seedlings can be planted in the soil inside the cold frame.

If starting seeds indoors, "harden off" the seedlings by setting them out for a few days during the day before transferring them full time to the cold frame.

Seeds such as lettuce, endive, leeks and onions can be started indoors in January to transplant outdoors. By February, very hardy vegetables and herbs can be sown outdoors in a cold frame. Get an early start on peas or fava beans, radishes, arugula, beets, spinach.

On nice days, monitor conditions in the cold frame. It can get too warm for delicate seedlings, and must be vented. It can also dry out much more quickly than you'd imagine, especially if you are growing plants in small pots. Soil should remain moist but not soggy.

As seeds grow, watch for slugs and snails. They seek out tender young leaves, and love to hide under shady edges of wood or stone frames.

Check regularly for plant diseases. Seedlings should be given generous spacing to avoid cultivating the soil-borne fungi that cause damping off disease (which is the seed rotting before germinating, or the seedling stems rotting at soil level).

If plants develop long, spindly stems and/or light green leaves, they're probably not getting enough light. Maybe they were started too early for the conditions — try again!

Bill Thorness is a freelance garden writer in Seattle: bill@thorness.com.