CHICAGO -- In a few days, the closing ceremonies will come to Olympic stadium in Beijing. In recent days, marathon runners, swimmers and tiny gymnasts have eked out the last ounce of energy, courage and focus. And each will know that in the end, only one may step up for the gold medal.

Meanwhile, under the sweltering summer sun in Downers Grove, Ill., Greg Sliwka will be intensely training his giant pumpkins. He hand-pollinates the flowers on his vines and covers the blooms to keep bees from casually spreading unwanted genes. He buries the vines so new roots sprout from their leaf nodes. He pinches off every superfluous stem and all but a handful of the fastest-growing pumpkins, props them up on chunks of plastic foam so they won't rot and watches for the one that might swell into a giant. "I'd be happy to break 1,000 pounds," he says.

Forty-five miles south in Crete, Joe Richards is feeding his two patches of pumpkins fish and kelp emulsion. He has reason to believe it's the breakfast of champions. After all, last year, he grew the biggest known pumpkin in Illinois, 1,159 pounds at the weigh-off at Didier Farms in Prairie View, and topped it with a 1,468-pound bruiser that traveled on a trailer to Indianapolis to beat all at the Indiana State Fair.

Among the women of the Lake Forest Garden Club, the competitive spirit may be less muscular in its expression, but it is no less intense. Earlier this year, four of them spent months nurturing a jewel-like miniature landscape of junipers and succulents inspired by an Afghan rug woven in a traditional Persian garden design. The effort won their club a blue ribbon, defeating other clubs in the Show of Summer, held at the Chicago Botanic Garden in June.

What drives these green-thumbed competitors into the arena? Why isn't it enough for them just to enjoy the flowers in the garden or bake a pumpkin pie? Why do they grind through the garden club farm system, winning novice classes for peonies and flower arranging on their way to the World Series of American horticultural competition, the yearly Philadelphia Flower Show? Why do they start, like Richards, growing a 48-pound pumpkin on a bet, and end up trying to top 1,468 pounds?

BLAME IT ON DOPAMINE

Scientists say what drives humans to compete is dopamine -- the pleasure-producing chemical our brains secrete when we win something. This reward mechanism evolved to encourage us all to fight harder for the food and sex that our ancestors' genes needed to survive, the theory goes. And now that our struggles for dinner and amour aren't all that desperate, we find other ways to win our soupcons of dopamine. Like dahlia shows.

Gastroenterologist David Sales grows 30 kinds of dahlias in his Northbrook, Ill., backyard. If he had more than 30, he would be kicked up to another competitive class at the Central States Dahlia Society's September show. As with most garden club and plant society competitions, the rules and categories can be complex. A committee of the six North Shore garden clubs that put on the Show of Summer spent months just composing the 35-page rule book that had to be followed by the 517 Garden Club of America members who entered plants, cut flowers and flower arrangements.

What do they win? Ribbons and bragging rights, mostly, though in the categories for new cultivars, some professionals are able to leverage championships for marketing purposes.

Sales doesn't go to the lengths some dahlia competitors do. He doesn't pinch off all but a single bloom on a stem to grow a larger flower.

"I'm not willing to sacrifice the beauty of the blooms all year," he says.

He doesn't splint the stalks of his plants to keep them straight, or cover them with umbrellas when it rains. But one day three years ago, he strolled into the novice class with a bloom, picked up a ribbon and got hooked. For Richards, pumpkin growing "becomes almost an obsession," he says. "I put everything I can into it." Lou Horton, president of the Midwest Regional Hosta Society, says, "Anyone who gets involved in any kind of competition has that interest in their character." Many hosta enthusiasts don't, he says; they bring plants to show off, but don't enter the contests. For them, hosta society meetings are about information, friendship and camaraderie.

Horticultural competition requires a certain amount of leisure time, and its history follows the growth of amateur gardening. The Philadelphia Horticultural Society was founded by wealthy amateur gardeners in 1827, and by 1830, it was giving out prizes for the plants they showed off, according to Janet Evans, library manager at the society. Last year, the Philadelphia show had about 5,000 entries; some gardeners bring more than 100 plants apiece.

19TH CENTURY ROOTS

In the 1840s, agricultural societies started holding fairs with competitions for plant crops as well as livestock. State and county fairs started up in the 1870s, and the competitive spirit got a boost from rural 4-H clubs that got kids growing. There still are 4-H clubs all over Illinois, including Cook County.

In the first half of the 20th century, the original Chicago Flower and Garden Show, operated by the Chicago Horticultural Society, included competitions for professionals and amateurs, according to a forthcoming history of Chicago gardening by Cathy Jean Maloney. The Chicago Tribune started holding garden contests in 1901, with categories for window boxes as well as yards.

By the 1920s, there were two national organizations of garden clubs: the Garden Clubs of America and its larger rival, the National Garden Clubs, each of which has a highly ritualized tradition of competitive flower shows that goes back to the 1920s. The Garden Clubs of Illinois, affiliated with the National Garden Clubs, will hold its Midwest show at the botanic garden Aug. 23 and 24.

Garden club flower shows are major social events, surrounded by talks and parties, and the judging circuit is a social network too. In fact, if you talk to enough people in hosta societies, ivy societies, African violet societies, lily, rose and orchid societies, the competitions start to sound like the ideal of the Olympic Village: A place for amateur enthusiasts to get to know each other and swap perspectives.

SOCIAL AFFAIRS

Garden competitions are not, it turns out, as heated as the last stretch of the triathlon. "Everybody's very friendly," Sales says. "I don't think anybody takes themselves very seriously. We don't have anybody who's going to work in a detective novel, where one murders the other over the best dahlia."

Shows are run by volunteers. Exhibitors help each other out with forgotten labels or emergency watering. In many competitions, passing judges review all entries. Their job is not to discourage entrants but to make sure they get in the right categories and help them prep their plants to show well, with no drooping petals or spotted leaves.

Once the exhibits are all set up, usually the evening before, entrants are banished from the room while teams of judges, rules pamphlets in hand, pore over the plants and argue the fine points. Then, in a mad clerical rush, points are tabulated, ribbons applied, and anxious gardeners are allowed back in to see who won what.

At tomato, pumpkin and watermelon weigh-offs, it's a little less formal. The heavyweights arrive, often by pickup truck, in the morning and, at an appointed time, are weighed with everyone looking on. For drama, canny organizers often save the heaviest-looking for last.

To get in the garden game, you don't need to dedicate your whole backyard to dahlias or nurse a tender young Japanese painted fern for 5-1/2years, as Pam Benz of Winnetka, Ill., did to win a special propagation award at the Show of Summer. Most shows have categories for a single leaf or flower. A first-timer, for example, could simply bring the best lily in his garden to the annual Wisconsin-Illinois Lily Society Show held in early summer, and passing judges would have helped identify it and set it up in a green glass wine bottle in the right spot.

Because that's the ultimate aim of all these competitions: To harness the competitive urge to spread gardening know-how and inspiration.

Maybe you'll get the urge to see how your tomato stacks up or show off a bloom of your favorite lily. Fair warning: You could end up out there hand-pollinating pumpkins, or covering perfect peony blooms with plastic bags to protect them from the rain.