These guidelines are all dependent on Mother Nature, of course, and your location. For the Boston area, Zone 6 hardiness puts the last frost date around May 1. For Zone 5, add a week; Zone 4, add two. But that doesn't mean cold nights are gone for good. Keep a careful eye on your local weather forecast to watch for unseasonably cold temperatures. Find your USDA Hardiness Zone at www.garden.org/zipzone.