USDA Plant Hardiness Zone Map

Dave

First of all, these hardiness zones represent the average annual minimum temps at a given location. It is an average of the lowest temp recorded for each year in the database. The 1990 map used years 1974-1986 (yes, just these 13 years) and the new map uses years 1976-2005 (30 years, but 2005 is the last year). There is only ONE datapoint for each year at a location, so the new map uses an avg of 30 numbers. The 30-year avg for Zone 7b falls somewhere between 5 and 10F (hence the 5-10F range), but for several of those years, the lowest temp was likely well outside of that range. Maybe it was -8F one year or 21F another year. The AVERAGE falls between 5 and 10F.

The hardiness zones are really just meant to be a general guide to determine which perennial species are likely to survive in a given area. Severity of winter cold is one factor that determines whether or not a particular perennial will survive. The map is not intended to be useful for growing annual crops like most veggies are. It's not useful to infer other unrelated things from these hardiness maps....like earlier frost/freeze dates, global warming, etc. For ex, if in 1998 the lowest temp was -6F on Feb 5, what does that tell you about how cold that winter (Dec-Feb) was, and what the last frost date was. It says nothing about those things because there's no connection.