Name: Evan Pioneer Valley south, MA, USA (Zone 6a)
Do you see the little map at the top right of the planting calendar page? I believe that shows the weather station used for "...your frost-free growing season..." It's in between Cranston and Warwick. You can always pick a nearby station (just click on it) that has a later last frost date. West Thompson Lake, CT for instance has "On average, your frost-free growing season starts May 11 and ends Oct 2. http://garden.org/apps/calenda...
