THE GROWING season anywhere is determined not by the gardener's enthusiasm, but by the vagaries of weather. In Washington's mild but capricious climate, a coldframe can mean the difference between losing your fall garden in a freak snowstorm and growing your own food year 'round.

Basically, a coldrame is any structure that lets sunlight in while offering some protection from inclement weather. It can be a window sash set on a frame, a lean-to against a wall or a flexible plastic tunnel stretched over hoops. You can build one (most up-to-date books on vegetable gardening include detailed, illustrated examples) or purchase a self-venting kit for $35 to $60 from many garden supply outlets.

To garden successfully in a coldframe throught the fall and winter, you must consider site and soil. Face the frame south to get the most warmth from the sun, and try to provide protection to the north and west as well. A heavy mulch built up to the top of the frame on these sides will do. Before planting, dig the soil under and around the frame deeply and well. The more manure and compost added down to a dept of 18 inches, the wider range of hearty and half-hearty vegetables you can grow and the more intensively you can plant this small space. It's easy to forget to water regularly in cool weather, but fall and winter can be dry as any heat wave. A coldframe will keep much moisture in, but check the soil periodically to see if it needs watering.

Shallow root crops -- beets, radishes, baby carrots, onion sets or garlic -- do well in a coldframe. This protection will extend the growing season of cauliflower, broccoli or even cabbage. Hardy herbs (parsley, chervil, burnet, chives) will have far more flavor grown outdoors in a coldframe than in a warm house. But for a crop that can pay for the cost of your investment the first year, grow plenty of lettuce and salad makings in the frame.

Whatever you plant, plan maturities carefully. Vegetables should be somewhere between ripe adolescence and their peak by the onset of cold weather (December and January). With plenty of manure in the soil, good mulching and a little special protection (an old blanket thrown over the coldframe on especially cold or snowy nights), your vegetables will survive winter but won't grow very quickly. Hardy plants such as lettuce may appear frozen, but as long as the roots aren't damaged, they will revive with some warm sunlight.

As you harvest crops from the frame, be sure to replace them with vegetables for next year's garden. Early lettuce and brassicas can be sown in February or March for transplanting. Tomato, pepper, squash or melon plants started indoors from seed will benefit from a week or two's hardening off in a vented coldframe before setting out in May.