Watering plants, indoors or out, calls for considerable judgment and knowhow. The idea is to apply just enough at the right time.

Too much water too often can badly damage the roots of the plant: When water fills all the chinks in the soil, it keeps out air, and oxygen. Without oxygen, the roots can't function. Yellow leaves are the danger signal, and while yellow leaves can result from other causes, the first thing to check is overwatering.

Sprinkling for five to ten minutes, hose in hand, does very little good; in fact, it may actually have a harmful effect by causing roots to come to the soil surface.

But under-watering can also be a problem - it's the main cause of blossom-end rot of tomatoes, for example. The first symptom is the appearance of a water-soaked area shrinks to form a flattened concave surface, dark and leathery.

Insufficient moisture in the soil results in an inadequate uptake of calcium. The young growing tissues and fruit suffer from a calcium deficiency, even though the other tissue contains a relatively high level of calcium. Apparently calcium does not move from older tissue to younger as some other elements, such as nitrogen, do.

Blossom-end rot can also be caused by a low level of soluble calcium in the soil; some varieties of tomatoes are far more susceptible to injury than others. Usually blossom-end rot stops occurring as the weather gets warmer.

Water your tomatoes and other vegetable plants once a week during dry periods - when less than an inch of rain has fallen. Soak the soil thoroughly to a depth of at least six inches. With clay soil this may require the application of about two inches of water.

A good way to determine how long it takes to apply an inch of water is to set a row of empty coffee cans from the sprinkler to the edge of the spray. Most sprinklers apply more water near the center than on the edges. The coffee cans will show you how much to overlap when moving the sprinkler to take care of the uneven distribution.

Most sprinklers put out water too fast, resulting in runoff. Narrow areas present a problem in slow application, but in larger ones adjustable sprinklers can be set so there's no puddling.

Most lawns require an inch or two of rain every week for the grass to stay green and attractive. If there isn't enough rainfall, the deficit should be made up by watering. The grass won't die if it doesn't get the water it needs, but it will turn brown and go semi-dormant.

Don't overestimate the amount of rain that falls during a short summer shower - chances are there wasn't enough to wet the soil. THE STORE OF THE ROSS

Saturday there'll be a show of roses in Tysons Corner Center's Fashion Court, sponsored by the Arlington Rose Foundation.