WASHINGTON gardeners would do well to pause briefly from collecting summer's bounty and consider the benefits of planting a fall garden. The heat waves that may wither your desire to dig once more spell drought for most of the country and another inevitable round of price hikes at the supermarket. A few hours' work in the dewy morning or cool of evening won't give you heatstroke but will guarantee fresh produce through next winter.

Of all the vegetables that can be started in August, brassicas (cabbages and turnips) offer the greatest range of flavor and the highest nutrient value. Broccoli and Brussels sprouts taste like fall itself while providing plenty of vitamins A and C and calcium. No other vegetable imparts so much body to stock as does the turnip, and peeled and sliced raw, it offers crisp, fresh flavor when most other produce lies limp and lifeless on the grocer's shelf.

Quick growing brassicas can be sown now for harvest before a killing frost. For cauliflower choose Snow King or Snow Crown hybrids and Early Super Snowball; Green Comet broccoli is excellent for freezing or cooked fresh. Both these flowering cabbages survive a light frost, but will last longer given the protection of a coldframe. For Chinese cabbage, so delicious in stir-fry cookery, plant a loose-leafed variety like Crispy Choy that's ready in 45 days. That way, you'll have time for plenty of pickings before frost kills this tender brassica.

Several rows of turnips with varying maturities will provide you with greens rich in vitamins and mnerals as well as fleshy roots for stews and soups. Tokoyo Cross hybrid is ready in 35 days. Early Purple Top Milan in 45. If you plant Just Right hybrid it will be 60 days before you dig turnips, but just one month for excellent greens and pot likker. Or try kohlrabi, the European turnip on a stem. Grand Duke hybrid (45 days) is tolerant to brassica disease and Prima Hybrid (50 days) is especially tender and fiber free.

Brussels sprouts and the non-heading cabbages are perfect late fall and winter brassicas for this area. Now is the time to sow quick-cropping mustard greens or vitamin-packed collards, which take up to 80 days from seed to table but do survive light freezes. Kale is resistant to many cabbage diseases and pests -- best of all it can grow here all winter. Its flavor and that of Brussels sprouts are actually improved by frost, so you needn't rush to plant it (most kale varieties are ready to eat within 60 days of sowing). Brussels sprouts take much longer to mature; unless you started seeds in June, you'll have to hunt for these plants in local greenhouses. Sprouts like to leave their stalks earthed up (or hilled) and staked to prevent a lot of winter damage.

All brassicas need rich, firm soil and opportunity to make quick, steady growth. Water frequently while the weather's still hot and mulch to keep weeds down (and provide winter protection later). Subject to fungus diseases, brassicas should not be planted where early cabbage or turnips grew. Fall brassicas do well intercropped between rows of onions and hot peppers (these pungent plants may keep pests away until cold weather kills all insects). Or you can dust frequently with dipel which contains baccillus thugiensis, a bacteria that kills chewing pests only.

Careful gardening is only half the secret to flavorful, nourishing fall brassicas. Don't undo all your outdoor labor by overcooking these vegetables (which protest vigorously with an overpowering odor). Whenever feasible cut up large brassicas for quick and even cooking -- cut florets off thick broccoli stems and make deep X's in bottoms of Brussels sprouts to speed cooking. Whole cauliflower can be simmered successfully and odorlessly in a mixture of milk and water for 30 minutes to an hour, depending on the size of the head. TILLIE BRADFORD'S MARINATED BRUSSELS SPROUTS (4-6 servings) 2 cups small Brussels sprouts 1 cup oil and vinegar dressing Salt and pepper to taste Lemon juice to taste

Cook sprouts in boiling water for 5 to 8 minutes, taking care not to overcook. Coat with salad dressing and lemon and refrigerate overnight. Season and serve on toothpicks as an appetizer to halve and toss in fall salads. (They will keep refrigerated for at least one week).