AUGUST is usually prime time for farmers' markets in America's cities.Yet many urban residents are reaching a little less eagerly for country produce these days. The reason is that more of them are now growing their own - right within city limits. Some 190,000 residents of 23 major American cities are growing everything from spinach to corn - either in their own backyards or on city or corporate lots - under an urban gardening program sponsored by the US Department of Agriculture (USDA) and its cooperative extension program. Relying heavily on 3,000 trained volunteers and a national staff of only 120 professionals, the program is aimed particularly at low-income residents. They can get land and seedlings at token prices as well as ample free advice on everything from what to grow where and how to can or freeze it to how to treat rocky soil and get rid of garden pests. Last year the program, which began in 1977 with only six cities, produced more than $20 million worth of fruits and vegetables, says Ricardo Gomez, national program leader for horticulture at the USDA. Newark has one of the largest, oldest, and most productive urban gardening programs. It is the only one to win a White House award - from President Reagan in 1988 under his "Take Pride in America" program. Newark's gardens have also won special recognition from the America the Beautiful Fund. The more than 5,000 participating gardeners in Newark's program get not only free advice but free fertilizer: leaves collected and composted by the city. Helen Poch says the community garden of 43 plots which she shares with other residents got four truckloads of the leaves last March. "They called me to go down and keep the gate open until the truck arrived," she recalls. In April she and dozens of other local volunteers headed off as usual for the greenhouses on the New Brunswick campus of Rutgers University to plant thousands of seedlings. They are later sold to urban gardeners at a rate of six for 50 cents. "We find people take better care of the plants than if they're given free," says program associate Stanley Kamara. Rutgers also offers help on its Newark campus every spring in the form of free gardening classes where in addition to advice, success stories are shared. Newark's gardeners often rent vacant lots from the city at $1 a year and last year produced more than $700,000 worth of produce, according to Dr. I. C. Patel, county agriculture agent and program director. Some gardeners sell their produce, and last year 2,000 pounds of the city-grown fruit and vegetables were donated to senior citizens and the homeless. "We share vegetables with seniors who aren't able to get out," says Lavinia Green who lives in an apartment complex for senior citizens in the city's west ward and can look out on her garden from her window. The green beans she planted late last week are already starting to come up, she says. Last year the plot even produced one seven and a half pound tomato. Someone helps her pull weeds that's the worst part and she says she has made a lot of wonderful new friends while gardening. Highlight of the season for many Newark gardeners is the harvest dinner to be held tonight. Everyone brings an ethnic dish to pass and the best of their produce which is laid out on long tables and judged for ripeness, uniformity of size, and color. Mrs. Poch, who last year won an award for having one of the city's best gardens, will bring a California cabbage, raisin, and onion dish which she learned about while watching television recently. Poch says she has tried exotic plants like white cucumbers and white eggplants but is now back to "ordinary" crops which she says fare better. "I do a lot of canning - I keep my daughter and her five children in tomatoes and spaghetti sauce all year." She says she would much rather garden with the help of the extension service than be on her own. "You get so many benefits," she says. "If you have a lot of shade, they tell you what to grow - cabbage and lettuce, not tomatoes - and what bugs are causing what problems and what to do about them." Indeed, last year, when the Mexican bean beetle threatened to destroy some of Newark's urban crops the New Jersey Department of Agriculture released over 25,000 parasitic wasps to regain control. The problem was resolved. The only disheartening thing about gardening in the city, says Mrs. Poch, is that about this time of year a little of the best produce sometimes gets stolen. But she says she plants a lot and has "done very well."