By Mildred Alpern
With the approach of winter, visitors tend to ignore the hidden gems in the famous 91st Street Garden of Riverside Park, the community garden on the Promenade level. In spring and summer, passersby gaze in rapture at the blooms. But in December, the garden appears a jumbled mass of tangled grasses and dried stalks. Yet a pause and a close-up look reveal some surprises. Just as showy, but oftentimes minute in size, many of the plants and flowers even in their final senescence display remarkable beauty. Every season here has its marvels, thanks to the Garden People.