The biggest problem city gardeners face as planting revs up in high gear is fitting all the lovely food they want to grow into a small plot of ground. To get the best yield in the smallest space possible, nothing beats the beet.

With juicy red roots underground and tender, spinach-like leaves above, these plants double-crop themselves. Or if you're strictly a greens lover, their almost rootless cousin, Swiss chard can feed a family of four all summer from just a half dozen plants.

Both beets and chard are pratically pest- and disease-free, can be planted now through July and ask only a sweet, light soil (a pH of 6.5 is best). It is important, however, to weed and water thoroughly to get these crops established; then mulch to keep weeds down and moisture where it belongs (in your soil.)

You can speed up germination of beets and large-seeded summer greens by soaking seeds 24 hours before planting. Space seeds 2 to 3 inches apart along a row. Beet seeds actually are clusters that send up many seedlings. Be sure to thin early to give young plants room enough to grow. Try Red Ball (60 days) or "bloodless" Golden beets (55 days). Lutz Green Leaf is a popular fall beet for this area (80 days to harvest). The secret of sweet beets and tender greens is picking produce while still small. Bigger beets are not better, just woodier.

To prepare beets for the table, just pull and cut off tops, reserving leaves to cook like greens. Leave one inch of stem on the root and wash gently in cold water to remove all dirt. Beets will bleed if cut. They can then be backed in a low moderate oven or boiled for 45 minutes or until tender. Beets are ready to eat when the skin slips off easily. They are delicious served hot with butter and fresh parsley, or can be cooled to dice in salads, marinate for summer fare, or pickled for use year round.

It is the greens however that are richest in vitamins A and C and minerals. Beet greens, Swiss chard of New Zealand spinach need only to be washed well and steamed briefly in a little butter before tossing with lemon juice or vinegar and serving. Or you may steam and freeze greens for a quick vegetable year round. (Do squeeze all excess moisture out before packing.)

If you choose to grow greens without the edible root, chard and large-seeded New Zealand spinach are your best bets. Both plants are pick-and-grow-again crops -- if the gardener will cut outer leaves, the plant will produce new ones all summer long. These greens are considered tender plants, though, and are safest started after all danger of frost has passed. Chard produces frilly, nutrient-rich leaves on thick stalks, that can themselves be steamed and eaten (the flavor is a bit like asparagus). Rhubarb chard is a popular red-steamed variety; white-stalked fordhook and lucullus thrive in this area. All three varieties take 60 days from seed to table. New Zealand spinach is neither a beet nor spinach, but was discovered on Captain Cook's voyages. Of all summer greens, it is lowest in oxalic acid, making its nutrients the most easily absorbed. MRS. SUTULA'S CHEESY CHARD (4 servings) 1 Large bunch chard, or any summer green Oil 2 onions, chopped 1 clove of garlic, rushed 1 1/2 cups cooked brown rice 1/3 cup or more grated cheese Soy sauce to taste

In a wok or large frying pan, saute chooped chard stems, onions and garlic until onions are soft. Stir in cooked rice and put chopped greens on top. Cover the pan and cook over low heat for several minutes, until leaves are wilted, then stir up rice and greens. Add grated cheese and soy sauce, stirring until cheese melts and holds the mixture together somewhat. Serve immediately. BEET AND BUTTERMILK COOLER (8 servings) 2 cups cooked beets 1 quart buttermilk Fresh chives Pepper, salt to taste

Chop beets and puree in blender or food processor, adding enough buttermilk to smooth. Mix with rest of buttermild and chill for at least 2 hours. Top with chives and season, if desired.