Growing Zones vs Climate Normals: Which Should a Gardener Use?

USDA Plant Hardiness Zones rank locations by average annual extreme minimum temperature — the coldest night you can expect in a typical year. It's a single number and it answers one question: will this perennial survive my winter?

Climate normals answer a richer set of questions. Will my tomatoes ripen? How long is my growing season? When does spring actually arrive? These require seasonal detail the zone map doesn't give.

For Boston gardeners, Zone 6b tells you that temperature drops to -5°F in a typical winter. Normals tell you that the last spring frost is around mid-April, growing degree days peak in July, and wet autumns are normal. Use both.