Spring has reached her peak, and the only greens that seem to make it anywhere near your apartment are in a mesclun salad. And while the idea of personally selecting those perfect petunias holds enormous charm, who's really going to take care of them? Peter Diffly and Bill Meyerson of Holly, Wood & Vine (212 Forsyth Street; 529-7365), will design, install, and maintain your city garden and give it all the love and attention it deserves. Responsible for the verdure of John Leguizamo's backyard and the florescence of Lee Bailey's terrace, Holly, Wood & Vine is large enough to be accessible but small enough to care about details. The place has that "astute sense of time and aesthetics," says landscape architect Signe Nielsen, who teaches the history of garden design at Pratt. A rooftop or yard job runs $15,000 to $20,000, though prices can be as little as $250 for a window box, and estimates are free. And when you're off in the Hamptons, Holly, Wood & Vine's maintenance ($50 per hour) will keep your blooms happy.