Washington International World of Plants and Flowers, Capital Centre, Largo. March 9 through 12, noon to 10 p.m.: March 13, noon to 8 p.m.; Adults, $3.50, children $1.75, under 6 free. 350-3900.

Building the Capital Centre was a feat in itself, but the idea of turning it into a giant greenhouse tickles the most jaded imagination. That is what is being done for the Washington International World of Plants and Flowers, open Wednesday through Sunday. For a start, the show's sponsors are building a forty-foot volcano and fifty-foot-long lagoon, a waterfall and a jungle populated with birds and animals. A dozen cherry trees are trying valiantly to bloom. Since exhibitors are being required to build landscaped displays, the show will have golf greens and pations, thirty-foot palm trees, orchids, ferns, and the world's largest hanging baskets (as if there were many contenders for that distinction). There are hopes for a tea room and a beer garden. And the stage will have a steady stream of demonstrations on fruit tree pruning, growing bonsai trees, turf maintenance, hanging vegetable gardens and, in the evenings, a design school by florists. Yon can bring your soil for a free analysis, but whether you can run barefoot in the park, nobody is saying: