what is the difference between zone 6 and zone 6a?
Subset Zone Temperatures Each gardening zone is divided into two subsets; zone 6 subsets are 6a and 6b. ... Zone 6: This zone has a minimum average of temperatures of -10° to 0°F. Zone 6a: This subzone has a minimum average temperature of -10° to -5° F. Zone 6b: This subzone has a minimum average temperature of -5° to 0°F.