Changes to the USDA Plant Hardiness Zone Map

Yesterday, the U.S. Department of Agriculture released a new version of the Plant Hardiness Zone Map. This is the first revision of the map since 1990. The new version includes 13 zones, two more than before. Much of the United States is now one half-zone warmer.

The old map

The new map reflects an increase in the average temperature of the U.S. in recent decades. The 1990 map used average annual temperatures from 1974 to 1986 while the new map uses information from 1976 to 2005.

For the first time, the influence of factors like elevation, prevailing winds, large bodies of water, terrain, and city heat island effects are included in calculations. I’ve written previously about how these aspects can affect specific regions, cities, and gardens. New algorithms and more sophisticated mapping methods now account for those effects. The Associated Press points out that American cities that include St. Louis, Des Moines, Honolulu, and Fairbanks, Alaska, are now in warmer Plant Hardiness Zones.

The map changes mean that, generally, the coldest winter day isn’t as cold as in the past. Relatively mild winters of the last two decades now allow some plants to grow in areas previously considered off limits.

You can expect that seed packets and gardening advisers will soon change and use the new data for recommending what plants can grow where. Burpee will include the new map on next year’s seed packets. Gardening catalogs will identify which plants are suitable for the warmer zones.

It’s important that I point out that the Plant Hardiness Map should only be used as a guide for planting.

Climate can still vary dramatically within zones, even the new ones. While some mountain areas are now in a cooler zone, a review of the new zone map still has my neighborhood in the same zone as before, 5b.  This doesn’t adequately account for the terrain and altitude differences that my neighbors and I encounter. My garden is fully 1,000 feet higher than the city 15 miles away, also zone 5b. I’m surrounded by forest. I have inches of snow and ice still on the ground from our first major winter storm well over a month ago; the city lost its snow covering the first day after the storm.

Every gardener should learn about the individual characteristics of their region and garden. Knowing your Plant Hardiness Zone is important. It will help you choose seeds and plants, but it is only one part of gardening planning.

Get to know your garden. Just because the USDA says you’re in a new zone it doesn’t mean everything has changed. The Hardiness Zone Map is based on averages, average low temperatures. The averages may be warmer, but extreme low temperatures can still be possible. Caution should still be practiced when choosing plants that are barely suitable for your zone, old or new.

The new map is more accurate than previous versions and gardeners have a new tool as we confront climate change. Like the rest of our gardening tools it is appropriate only for its intended purpose.

If you haven’t had success with a particular plant, a new zone number probably won’t change that. If you want to seek out new plants a new zone number may give you new options, but it is only a guideline.

I accept the new map with anticipation. Many gardeners in many places will try new things and that is a great idea. Entire regions are more accurately assessed. Yet even though most of us feel like our neighborhood is warmer, experience is a harsh steward. I’ll continue to recommend that gardeners, particularly in challenging zones like mine, choose plants that are best suited for success.

It only takes one or two days of historical low temperatures to wipe out plantings that should expect to survive the new zone identifications. If you live in an area of surprise temperature extremes, it’s better to opt for plants a zone lower than yours — planting 4b plants when you live in 5b — than trying to recover from winter kill.

Of course, microclimates, cold frames, hoop houses, and other season extenders will all provide the extra protection that you may need for plants challenged by temperature. If you’re going to try new plants that may be questionable consider one of these aids. Check out some of my other articles on those subjects for more assistance.

For now enjoy better information and access. The new USDA Plant Hardiness Zone Map has an improved online interface. Easier than in the past, you only need to enter your zip code for an instant zone report. A click of your mouse will bring up your color-coded state map.

Try it at:

http://planthardiness.ars.usda.gov/PHZMWeb/

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2 responses to this post.

  1. this is definitely a hot topic today on the gardening blogs. I love the new zipcode feature on the 2012 hardiness zone map, though. It certainly eliminates struggling with the map “colors” and guesswork about where your own little city lies.

    Reply

    • I agree completely. In the past you’d have to search a number of sites before you could find an accurate number for a specific town. The new feature makes it easy.

      Reply

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