WHEN your bumper crop of beefsteak tomatoes, zucchini, and eggplant ripen in the garden all at once, it's time to freeze or can the freshness and flavor for winter's menus."There are more ripe tomatoes and squashes than we can possibly eat right now, and they're too good to be left on the vine," says Pearl Franks, standing in her community garden in Boston's Fenway area. "I hate to see good food go to waste when there are hungry people in the city - and I just don't allow it." This is peak time for canning and freezing all kinds of produce, says Mrs. Franks, who preserves fresh foods from her own garden as well as produce that other gardeners bring to her. She does it all in a small kitchen in her apartment not far from the community garden area. The gardens, each measuring 15 by 30 feet, are part of Boston's park department and available to city residents. They are managed by the Fenway Garden Society, an organization made up of the gardeners. Constantly reminding other gardeners to think of the hungry, Franks welcomes any extra vegetables so she can deliver them to the needy or preserve them for later use. "Most people plant too much," she says. "We have more than 400 garden plots here, and almost everyone has extra beans, lettuce, tomatoes, cauliflower, broccoli - everything. So I'm in the middle of my canning season right now and my apartment looks like a demonstration kitchen for preserving food. "It's easy and you don't need a lot of space," she insists, "but you do need to concentrate and do it right." Jars of red and yellow tomatoes, orangy mustard pickles, and bread-and-butter pickles are stacked near bright red relishes and other handsome jars on the dinette table of her home. There are trays and boxes of green tomatoes for piccalilli, fresh zucchini for zucchini bread, yellow squash for freezing, and more. Huge bouquets of basil, mint, savory, and other herbs covered with plastic bags are stored neatly in the refrigerator. A large, dark-blue, speckled canning kettle sits on the stove. Franks has been cooking in restaurants and at home for many years, raising a large family in New Jersey and New Hampshire. Now that her children are grown, she lives by herself but still cooks for dozens of people - her next-door neighbors, gardener friends, church members, and the homeless - not that she's got extra money for charity work. Pearl manages on a small fixed income. When she's not baking or canning or cooking, this indomitable lady takes off in early morning for her community garden. She's there all day, weeding and working. Though she uses a wheelchair to cover longer distances, has difficulty bending over, and cannot wield heavy garden implements, she has been at work in her garden nearly every day since spring planting. This year, with help from fellow gardeners, she started another full plot just to feed the hungry in Boston - this in addition to the double plot of flowers and vegetables she tends. This summer has seen three deliveries of home-grown produce to St. Francis House in Boston, where hot meals are served for free. SALT is an essential part of canning vegetables, and Pearl says, "Be sure you do not use iodized salt. It will make the vegetables dark. ... Kosher salt and sea salt are fine if you are familiar with their use." For canning most vegetables she adds 1/2 teaspoon of salt to each pint jar. "It's the only preservative I use," says this organic gardener. Pearl likes her jars to look pretty, too. White cucumbers and red onions make an attractive jar of bread-and-butter pickles. Yellow cucumbers with red and white onions is another of her favorite combinations. For flavor, "I like to put a leaf or two of basil in almost all my tomato jars when I'm canning," she says. "Squash of any kind is better frozen than canned," says Pearl, who has already put away several kinds, including the Italian heirloom "trombone" squash called Zucchetta Rampicante. Red and green Swiss chard has been frozen and stored. There will be more to pick and freeze before fall ends. Kale will be frozen after the tomatoes are finished. Freezing and canning starts in all seriousness in August and continues through September. Then it's time to start the fruitcakes for Thanksgiving: Pearl made 24 last year for friends. "I don't eat fruitcake myself," she says, "but others simply must have it for the holidays, they tell me. I'm happy to make them - especially if they bring me some of the ingredients."