"Know your microclimate" should be the gardener's motto in May.

The Ides of May may be the official, Department-of-Agriculture-approved safe date for setting out your tomato and pepper plants in the District of Columbia -- and a few days later for the suburbs. But you may confidently advance the date a week or more if your yard is favored with one or more of the following advantages:

* A sunny southern exposure;

* Substantial protection from wind -- such as a house, a stockade fence or a thick row of bushes;

* Your vegetable patch runs up against a brick wall or a sidewalk or some other structure that absorbs and reflects heat. Downtown Washington has "heat islands" formed by tall buildings that radiate heat.

* Drainage of cold air is also a consideration. "The top of a slope is cool because of wind, the middle is warm, and the bottom is again cold because of the entrapment of cold air," says Pamela Marshall of the D.C. Cooperative Extension Service. "So from the point of view of spring planting, the middle of a slope is best."

* If you want to be the one with the first ripe tomato on the block, spread a black plastic cover on the ground. It will warm up the soil. Grass clippings will do the job almost as effectively.

"Microclimates are important but the biggest consideration is the weather," says Marshall. "Since the long-range weather forecasts looks good for May, you may plant your tomatoes and peppers this weekend."

The worst thing that can happen, Marshall says, is heavy, cold rains, that injure tender leaves and render them vulnerable to fungus diseases.

She believes that a hardened plant is the key to successful early setting out. She suggests that in buying tomato plants, you look for a sturdy, stocky stem, and the leaves on the stem should be closely spaced. Avoid plants with stems that are long, leggy, tender-looking and full of water -- those are signs that the plant has not been hardened off.

"Stocky is ideal," she says, "a leggy plant is worst.

"The secret is that there should be as little change in the plant's environment as possible. For instance, you should not take your plants from the comfortable, humid conditions of your kitchen windowsill to the windy, dry outside." Similarly, she argues, plants will suffer sunburns or colds when suddenly transplanted from the warm, humid greenhouse to a vegetable patch that is dry and sunny or wet and cold.

If the temperature falls below 45 degrees, she says, use hotcaps or plastic milk containers cut in half, or some other contraption that will shield individual plants.

Many gardeners believe in a kind of decompression chamber or halfway house. Keeping plants for a day or two in a shed or well-protected semi-shady spot outside is one tried-and-true method; another is to take them outdoors for the day and bring them back into the house for the night, and repeat the exercise for four or five days.

The ground in the demonstration garden at Independence Avenue and Sixth Street SW is still cold and wet, says Byron Williams, a HUD employe who is working the 50-by-60-foot lot for the third year. The garden is about 30 feet from the curb, so it does not benefit from the reflected heat from the sidewalk or the nearby Air and Space Museum.

"My split-rail fence doesn't give me enough protection from the wind," Williams says. His biggest problems, he says, are the wind and the clay soil, which is slow to warm up.

He says he will not set out his warm-weather plants until the end of May though there has been some wifely pressure on account of the crowded windowsills at home.

"It may be that I have a hangover from my West Coast days," Williams says. "I remember that in Seattle my tomatoes froze on June 5. But don't worry. My plants will catch up."

"People get anxious," says Ron Chiabotta, manager of the vegetable-and-flower section at the American Plant Food Company on River Road."Last weekend was our biggest weekend ever, selling up to 1,000 individual tomato plants Sunday. The odds of your losing tomatoes in May are pretty slim."