Although some gardeners leave their gladiolus corms (bulbs) in the ground over winter, it's much better to dig and store them indoors.

The general rule is to lift them about six weeks after flowering, or after the first frost, or when the tops turn yellow. Actually, it's best to leave them in the ground as long as the tops stay green and capable of producing food to be stored in the corms.

Dig the corms with a spading fork rather than a spade to avoid slicing them. Loosen the soil with the fork so you can easily pull up the entire plant.

Dry the corms in bright sunlight for a day or two, clean off any remaining soil, discard all diseased plant parts, break off the tops close to the corms, and put them in a well-ventilated place about 80 degress for three weeks to cure. Then store them for the winter in a dry place where the temperature won't go below 30 or above 50 degrees.

Lilies of the valley usually need to be separated 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.

Early to mid-fall is the time to dig and divide them. The pips (bulbs) separate naturally, like daffodils. They can be replanted immediately. The usual practice is to plant them 1 1/2 inches deep and three to six inches apart. They do much better in good soil, and mixing some well-rotted manure of Michigan peat with the soil before planting should pay dividends.

Some specialists recommend discarding the old pips and buying new ones, which are usually available for purchase in the spring. The old ones may not be in the best of condition because of having been crowded, 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. GARDENERS' Q&A

Q - Last fall I potted some impatiens plants, kept them on my windowsill all winter; they were nice and green, but no flowers. What is the secret of getting them to bloom? I want to try again this winter.

A - Impatiens bloom hardly at all during the short days of late fall and winter. It isn't only that days are short, but there are so many cloudy days when there is little or no sunlight. You can get lots of flowers by growing them under flourescent lights for 12 to 14 hours a day.

- We have a large lilac bush crowded into a border with several shrubs. It needs to be moved out to an area where it will get full sunlight. When should it be done?

A - Lilac is generally planted and transplanted in early spring, although late fall after the leaves drop is almost as good. Be sure to cut the top back to compensate for the roots that inevitably will be left behind.

Q - I have pumpkins in my garden this year for the first time. When should I harvest them and how should I store them?

A - Pumpkins should be mature before harvesting. Try to puncture the rind with your thumbnail. When the rind is firm enough so it is not easily punctured, the pumpkin is close to maturity.

After harvesting, pumpkins and winter squash should be cured before storing for later use. Those sold at roadside stands may not have been cured; better check with the seller.

To cure, keep the pumpkin or winter squash at a temperature of 80 to 85 degrees for about two weeks. During early fall, these temperatures can be achieved in a garage or some other outdoor building.

After two weeks at these high temperatures, they should be stored in a dry location at between 50 and 60 degrees. A cool garage or dry basement is a good place for this. If cured and stored properly, they can be kept four to five months with little loss of quality.

If you have a question for Tom Stevenson, write to him at the Weekend section

The Washington Post, 1150 15th St. NW

Washington D.C. 20071.