Hardiness Zone or Frost Dates: Which One Should Vegetable Gardeners Use?

Two panels: hardiness zones based on the coldest winter night, and the frost-free growing season between the last and first frost

 

Two numbers show up in almost every gardening guide: your USDA hardiness zone and your frost dates. They are often mixed up, but they answer different questions, and for vegetables the second one usually matters more.

What a hardiness zone tells you

A hardiness zone is based on the average coldest winter temperature in your area, grouped into ten-degree bands. It answers the question will this plant survive the winter here? That makes it very useful for perennials, fruit trees, shrubs and herbs such as rosemary, which have to live through the cold months.

What frost dates tell you

Your last spring frost and first fall frost mark the edges of the frost-free growing season. They answer a different question: when can this plant safely be outside? Most vegetables are annuals that you replant every year, so they never face the winter. What matters to them is the stretch of mild weather between the two frosts.

Putting both to work

  • Annual vegetables such as tomatoes, beans and lettuce: use frost dates for timing, and check the days to maturity on the seed packet against the length of your frost-free season.
  • Perennial food plants such as asparagus, rhubarb, berries and fruit trees: choose varieties rated for your zone or colder.
  • Borderline cases: two gardens in the same zone can have quite different frost dates, and a sheltered yard can run warmer than the zone suggests, so watch your own garden over a few seasons.

If you only remember one thing, remember this: zone for plants that stay, frost dates for plants that come and go each year.

You can look up both on the free planting calendar. For more detail, see the guides on USDA hardiness zones and frost dates.

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