With its verdant meadow, gleaming palm house and saucy cherry perched on the tip of a 52-foot spoon, the Minneapolis Sculpture Garden seems forever young. And yet the garden, which turns 20 this summer, is showing youthful maturity. From the original sunny walkways lined with slender saplings, it has grown into a leafy park whose graveled allées are dappled with deep shade. Moss softens the gnarled trunks of its linden trees, dandelions puncture the grass and anthills erupt from sidewalk cracks. Nature is reclaiming its own turf, softening the pristine geometries imposed by landscape architects and museum staff. And that is all to the good. Like all edens, the garden was designed to grow and change. Sculptures have been added, moved and removed. Birds now nest in the trees where, in the early mornings, their gossip almost drowns out the whine of nearby freeway traffic. By midday, mothers push prams, teens bike through, art students sketch and preschoolers gape at a giant bunny leaping over a bell. A collaboration between the Minneapolis Park Board, which owns the land, and Walker Art Center, which owns the sculpture, the 11-acre garden is one of the Twin Cities' civic glories -- a culture magnet for kids and an urban oasis for adults. To celebrate its 20th birthday, the Walker is staging a summer's worth of activities, including an artist-designed mini-golf course.