Radishes to me are just something you plant to help keep the Mexican beetles off the string beans.

Does anybody really eat them? They turn up once in a while in a Chinese stir-fry, or as a garnish on a platter of roast beef and boiled potatoes. On occasion I have heard of their use in the making of beurre radis. But in all seriousness, I know of no vegetable that is so easy to grow and difficult to find a use for.

This, however, should not prevent you from growing at least some radishes, if only to aid in pest control. They can go in now or they can go in later; they can go in just about anytime except June and July (which will cause hot-hot radishes), and are ready for picking in 30 days or less. Sow them in a narrow row around the bean bed; this not only will help deter beetles but will ensure that the radishes get the more than six hours of sunshine they need to produce roots as well as tops, because, of course, you're not going to try to grow beans in shade.

Radish varieties that are good for this area, according to the Men's Garden Club of Montgomery County, include Cherry Belle, Champion, Comet, Sparkler and Icicle. If you're planning on putting in more than a packet's worth, it'll pay to buy them in bulk from a hardware store or nursery. TRANSPLANTING SEEDLINGS: Home-started seedlings that sprouted a month or so ago will be ready to transplant to a larger pot for added vigor before you put them directly into the garden. Styrofoam cups are excellent. Fill them with potting soil mixed with one-third vermiculite or sand. Gently take a seedling out of its current home and plant it down into the cup so that about half the stalk is below the soil surface, and the leaves are above. With cabbage-family seedlings, the bottom third of the plant can go below the soil. Don't try to put more than two seedlings in one cup or small pot. Use of larger pots, which would allow for more than two seedlings, is less functional, because the object is to be able to transplant with minimum root disturbance.

Water the transplanted seedling gently so that the soil is evenly moist. Keep them in a sunny window or, during the day, on a porch that will get some sun but little wind. Good light is important to these little fellows.

After about three to four weeks (last frost around here is around May 10), the buried stalk will send out new roots, giving the seedling strength and vigor for its transfer to the garden. CORN COUNTRY: If you've got the space for it, and I would urge you to make the space, you can put in your first corn this weekend. Plant an early-maturing variety (about 70 days) and you can eat garden- fresh corn at your July 4 picnic. There are a number of ways of planting corn for a successful harvest. My best year was when I dug shallow trenches about 18 inches apart, sowed the seeds fairly haphazardly every six inches or so, drew soil over the seeds so that the bed was fairly level and covered the whole thing with a not-very-heavy layer of straw that had quite a bit of manure in it. I put in eight 30-foot-long rows, and ended up with very high pollination and so much corn I was giving it away by the bushel.

Some people prefer doing a grid, placing the seeds a foot apart in rows a foot apart. That's fine too, I'm sure. The secret is not to crowd them, because they need sun and are voracious eaters.

If your space is limited, try putting in three 10- foot rows now. Later you can add three more rows for corn that will come in during late summer. If you have any additional space, hold off until June for the final planting. Make sure you keep the corn all together, in a clump, to ensure maximum pollination. In fact, you'll probably find some of the ears at the ends of your rows haven't been pollinated. The more centrally located the stalks are, the most likely they will bear fully filled-out ears that are delicious to eat and enough to fill the belly.

My very favorite variety of corn is the Golden Queen, but for early maturity I choose Early Sunglo, which matures in about 66 days. Other varieties recommended for this area by the Men's Garden Club are: Wonderful, Sugarloaf, Seneca Chief, Kany Korn, Mainliner E.H., Illini X-tra Sweet, Aristogold, Silver Queen, Quicksilver, Platimum Lady, White Lightning, Butter and Sugar, Sweet Sal, Bi-Queen, Honey and Cream. Buy corn by the half-pound or pound. It's not worth getting it by the package. Whatever you don't use this year will keep for next season. VIRGINIA'S HISTORIC GARDEN WEEK -- This Saturday starts the 49th annual historic garden week in Virginia, when 32 areas open for tours through May 2. Included are six homes and four historic properties in Alexandria open Saturday 10 to 4:30. Tickets, $8, are at the visitors center at 221 King Street. There's a map of all state sites open (549-0205); for details call the historic garden week headquarters in Richmond at 804/644-7776. MARYLAND HOUSE & GARDEN PILGRIMAGE -- For details on the Maryland pilgrimage, call its organizers in Baltimore (301/821-6933). The 45th annual tour also stats Saturday. AZALEAS AT FORT WARD PARK -- The sixth annual azalea festival of the arts is at Fort Ward park, where the flowers are blooming along with the arts and crafts that will be for show and sale. Music, too, Saturday 10 to 5 at the park, 4301 West Braddock Road. Free. Call 838-4343.