DibbleA hardiness zone is a shorthand for how cold your winters get. It is based on the average lowest temperature an area sees in a typical year. Colder places have lower numbers, milder places have higher ones. The system runs from roughly zone 3 in cold northern areas up through zone 11 in nearly frost-free regions.
What your zone actually tells you
Your zone answers two practical questions. First, which perennial plants, like fruit trees and shrubs, can survive your winter outdoors. Second, roughly how long and warm your growing season is. A gardener in zone 4 has a short, cool season and starts many crops indoors, while a gardener in zone 9 can grow almost year round. A single state can span several zones, which is why the Virginia frost dates page lists four, from zone 5 in the western mountains to zone 8 near the coast.
How to find yours
The easiest way is to look it up by your ZIP or postal code. In Dibble, you set your garden's place once during setup (your location, your town or a postal code) and the app shows your zone for reference, along with your typical frost dates. You can browse every zone calendar here, or look up the frost dates for your state.
From zone to planting dates
Here is the part people miss: your zone does not set your planting dates directly. It points to your typical frost dates, and those frost dates, together with the heat and rain of your climate, are what each crop is timed against. In Idaho, for example, the typical last spring frost is April 8 in zone 7 and May 18 in zone 3. Tomatoes go out a week or two after your last spring frost. Garlic goes in before your first fall frost. Dibble does this math for every crop so you do not have to.
