MIDSUMMER IS not too late to start a vegetable garden for fall crops, says Dr. Mohammed Khan, extension agent specialist and pesticide coordinator with the D.C. Extension Service.

The way to get the most from your garden is to plan for two crops, summer and winter.

Many vegetables can be planted now and harvested in late August or early September. Some can even be started from seed, but others should be bought as seedlings in flats from a local nursery and transplanted into your garden. A number tolerate cool weather very well, especially as they mature, according to extension agent Richard Biggs of Montgomery County.

"The cole vegetable family [spinach, lettuce, cabbage and other leafy vegetables] does well in cool weather," says Bill Easley, state program leader with the D.C. Extension Service. "They'll grow through December or until the first serious frost knocks them down."

Late July is a good time to transplant tomato, cucumber and tomato seedlings from flats into the garden.

In mid-August, transplant broccoli, cauliflower and kohlrabi seedlings.

Here's a list of late vegetables you can still plant this summer and their planting times, compiled with the help of Biggs, Easley and the Alexandria extension service:

(If you live in a mountainous area, plant earlier; near the water, you can plant later.)

Beans (string and wax): between July 20 and Aug. 5

Beets: now through mid-August

Broccoli: seed in July, transplant in August

Brussels sprouts: transplant now through mid-August

Cabbage: transplant now through August

Chinese cabbage: seed now through mid-August

Carrots: seed early August

Cauliflower: seed till end of July or first week in August; transplant first two weeks of August

Collards: plant now through August

Cucumbers: plant now through mid-August

Endive: plant mid-August

Kale: plant now through August

Kohlrabi: seed before mid-August

Lettuce (bibb and leaf): plant mid-August (takes three to four weeks to come up)

Mustard: plant in late August-early September

Peas: plant in early September

Potatoes: plant late July-early August

Radishes: plant mid-August (comes out when it gets cold)

Spinach: plant early August (takes four to six weeks to come up)

Turnips: plant early August

Dr. Rick Gomez, program director os USDA's 16-City Gardening Program says summer crops can be grown in the same space as spring vegetables as long as plant debris is removed and the soil is turned over.

Bill Easley recommends buying resistant varieties of seeds from a nursery. "Grow them during the first weeks indoors, before transplanting them outside. This should cut down on bugs," says Easley.

And, warns Dr. Kahn, "Be careful of the pesticides applied to your vegetables.

"With the heavy rains that we've had, pesticides often spread to areas of your garden where they're not needed," he says. July is a heavy insect month.

"The gardener should know what they're treating and what's in the treatment -- especially with so many products on the market," he says. "If the gardener is unsure as to what's attacking a crop he should call us on the HORT-line [see below]."

Richard Biggs adds: "Using a pesticide will kill the bad insects as well as the good ones."

For answers to your vegetable gardening questions, call your local extension service: 282-7400 (District), 691-3456 (Fairfax), 838-4333 (alexandria), 558-2475 (Arlington), 952-3226 (Prince George's County), 948-6740 (Montgomery County).

If you're looking for space to garden, a limited number of plots are now available in the District, free of charge, according to Allison Brown, coordinator of the Neighborhood Gardens program. Call 576-7415.