Take a good look at your tomato plants this weekend to find out whether they are infested with spider mites. Use a magnifying glass and check the undersides of the leaves. Specimens of foliage received in the mail indicate mites may be particularly bad on tomatoes at this time.

They feed by sucking out plant juices. Infested foliage becomes mottled and then may turn brown and drop.

The mites construct an extensive system of webbing on the underside of the leaves, much more dense than the usual spider web, over which they travel. They multiply rapidly during hot, dry weather. A heavy infestation can greatly reduce the yield.

To control mites on food plants, specialists recommend spraying with Kelthane. It is not a good idea to spray unless mites are definitely known to be present. Directions on the label for mix and application should be followed closely.

Lilies-of-the-valley usually need to be dug, separated and replanted every three or four years. They spread rapidly when growing in good soil with light shade. When they become crowded, they don't provide many flowers. Nor do they bloom well in heavy shade.

The small bell-shaped, cream-white, deliciously fragrant flowers are the crowning glory of these perennial plants. A dozen flower stems in a glass of water will perfume a room for days. There is a pink variey but it has little fragrance. The foliage of both white and pink varieties is unattractive from midsummer on.

Early to mid-fall is the time to separate them. The pips (like daffodils) separate naturally.They can be replanted immediately. The usual practice is to plant them 1 1/2 inches deep and three or four inches apart.

Some specialists recommen discarding the old pips and buying new ones. The old ones may not be in the best of condition because of having been crowded and probably won't bloom the first year after being replanted.

They will grow and bloom in full sun but the foliage will be unattractive during the summer. In heavy shade the plants will have nice green foliage but few, if any, blooms.

Those who enjoy eating rhubarb can arrange now for a crop in late winter and early spring. Dig up two or three clumps and plant them in large baskets.

Leave them outdoors in a cold area until about the first of February and then bring them into the cellar or garage where the temperature is about 60 degrees F. Keep them in a somewhat dark place. Water them whenever the soil feels dry. The leaf stalks, blanched by lack of light, will be milder than those grown outdoors.

Research has shown that vase life of roses is not lost by letting the flowers mature longer on the plant instead of cutting them when the buds first show color, according to James C. Krone, executive vice president of Roses, Inc.

In fact, says he, there is more chance of a rose opening without a bent neck (the flower hanging down) if it is allowed to mature further on the plant.

The most important thing is to get the flowers in water that contains a preservative and to harden them immediately after cutting, he says.