Mark your calendars now for the 20th annual Spring Garden Tour hosted by Soroptimist International of the Sierra Foothills, scheduled for, May 18 and 19. This event is a self-guided tour of six privately owned gardens in Grass Valley, Nevada City and Penn Valley.  Gardens are open from 10 a.m. to 4 p.m., and the cost is $20 in advance or $25 at the garden gate. Tickets go on sale in early March.  Wear comfortable shoes, bring a hat and don’t forget your camera on this four- to five-hour excursion. The Garden Tour is the primary fundraiser of Soroptimist International of …