THE END of summer need not mean the end of a backyard vegetable patch. With Washington's mild climate, fall is more than just the time to put a garden to bed. But planning -- and planting -- must be done now.

August is the month to plant a second crop of hardy vegetables such as peas, turnips or spinach from seed. Set out Brussels sprouts and cabbage plants and build or order a coldframe to insure fresh, affordable salad greens all winter long.

Of all the traditional fall crops, only the hard-skinned squash must be picked before frost. If you had the foresight to plant some of these fall squashes with their summer cousins, you may have already discovered how productive they can be in a city garden. Hard-skinned squashes offer a much greater variety in tastes and textures. Spaghetti squash are lower in calories than pasta. Huge, delectable Tahitian melon squash or tiny acorn squash are just right for stuffing. All can be cooked and pureed for freezing, but many (particularly True Hubbard and Waltham Butternut) keep well without all that fuss. But best of all, winter squashes are appreciably higher in nutrients, offering the kitchen gardener double the vitamin A of their summer kin.

Despite their reputation for roaming, hard-skinned squashes adapt well to small gardens. Bush varieties take up little space and vining squashes or pumpkins can be trained to grow up fences, trellises or tripoded poles, though heavy fruit may need to be tied with cloth strips so as not to put too much strain on the vines. All are heavy feeders and will benefit from side-dressings of commercial fertilizer a month or more before harvesting.

Pick squash when skins are hard and the vines begin to lighten in color and wither. Clean squash, if lying on earth instead of mulch, dry and oil skins lightly with vegetable oil before storing. A cool basement or garage (45 to 50 degrees) makes a city version of the traditional root cellar, unless especially damp or humid (squash requires dry storage). Never pack them more than two deep, as ventilation is important. Pumpkins prefer slightly warmer storage (50 to 60 degrees is best). They keep well in hanging net bags.

There are as many uses for winter squashes as there are varieties from which to choose. Vegetable spaghetti squash can be boiled whole, then the "noodles" are scooped out and served with butter or a favorite pasta sauce. Acorn squash may be the most attractive for serving. When scraped of strings and seeds, their little hollows are ideal for holding sauce or stuffing.

Place skin side up in a greased baking dish and pour in enough hot water to cover the bottom. Bake at 350 degrees until meat is tender (approximately 20 to 30 minutes). This method is ideal for cooking any winter squash before pureeing and freezing (steaming or boiling cut squash or pumpkin means added work removing excess liquid before putting up). Frozen without sugar or spice, your puree will work equally well baked in pies or breads, creamed into fall soups or nutritious puddings. GINGERED ACORN SQUASH (2 servings)

Halve and cook squash as described above. For each squash you use, you will need: 3 to 4 tablespoons apricot preserve 1 teaspoon lemon juice 2 pieces candied ginger, diced fine Butter

Place ingredients in the hollow of each half and bake skin side down for another 5 to 10 minutes. Serve immediately. PUMPKIN PUDDING (6 to 8 servings) 2 cups pureed pumpkin 2 cups milk 1/2 cup brown sugar 4 eggs 1 teaspoon cinnamon 1 teaspoon ginger 1/4 teaspoon salt 1 tablespoon brandy

Preheat oven to 350 degrees. Put all ingredients in a large bowl and beat together until frothy. Pour into oiled custard cups. Place cups in pan of hot water (water should cover 1/3 of cup) and bake for 35 to 45 minutes, or until a knife inserted in custard comes out clean. Serve warm or chilled.