Why discuss planting zones? I think we’re all coming down with “weather whiplash” as more super storms shower us with everything from hail to the kitchen sink. And it’s not your imagination.

Last year the U.S. Dept. of Agriculture updated its planting zones map. The map breaks out the country into temperature gardening zones (based on readings from 1975 to 2005).

Bottom line? We may have had a cool spring this year, but overall much of the U.S. has warmed up. Most places increased half a zone (or about 5˚ F.) from the previous map done in 1990. And some places, like parts of Nebraska and Ohio, jumped up a whole zone (10˚ F.).

While the planting zones map is informative, there are 3 things you should know about it.

1. Planting Zones Ignore Summer

Planting Zones: Shetland Islands - Same Zone as Alabama

Shetland: Same zones as Alabama?

It’s true. The USDA’s map relies on winter temps. So summer heat doesn’t determine planting zones. That means two places with very different summers can be in the same planting zone!

Take the Shetland Islands, home of the Shetland Pony. They’re north of Scotland and near Iceland. Would you believe this northern place has the same planting zone as southern Alabama?

Though far north, the Shetlands are warmed by the ocean, so, believe it or not, they have about the same winter temps as Alabama. Both average around 41˚ F.!

But that’s in winter.

Summer is, of course, wildly different. Alabama gets hot, averaging 81˚ F. The Shetlands stay cool, with average summer HIGH around 57˚ F.

BUT since USDA planting zones map ignores summer, it puts these two places in same planting zone. It’s silly, of course. There aren’t many garden plants that’ll thrive in both places.

So how do you know if a a tree or perennial will thrive in your backyard?  Check the planting zones map above, THEN check the American Horticulture Society’s heat zones. Together, they give you a good picture.

2. Planting Zones Ignore Snow

Planting Zones: Quebec City Snow - Warmer Planting Zone

Snow protects plants in Quebec City

USDA planting zones also ignore snow.

Snow insulates yards and gardens from extreme cold. It protects the roots of hibernating plants. If snow cover is reliable, the plant roots are safe and this affects how and if they thrive.

Wikipedia uses the examples of Quebec City  and Montreal. They’re both in Canada but get different amounts of snow. Quebec City is further north and in zone 4. But it gets lots of snow cover every year. That makes it possible to grow plants normally rated for zones 5 or 6. Further south and in zone 5, Montreal gets less snow. So it’s harder to grow plants there normally right for that zone.

3. Cold Snaps

Planting Zones: Avoiding Garden Frost

Avoiding Garden Frost

There are other things that affect plants NOT considered in planting zones. Soil moisture. Humidity.  The number of days of frost. And the risk of a rare catastrophic cold snap. (Remember Super Storm Sandy?)

So some risk evaluation – the probability of getting a particularly severe low temperature – often is more useful than just the average conditions.

After all this, you may be wondering what zone you’re in! There’s an easy way to find out. Check out the USDA’s new planting zones map and enter your zip code.