It’s time to start dreaming of a garden. You whip out your seed packets and cheerfully gaze at the little map at the back—the plant hardiness zone map.
You just want to know when it’s safe to get the seeds in the ground.
Did you know that colorful little map has been a hotbed of controversy?
It’s a new zone map, produced last year by the US Department of Agriculture.
The map is a geographic guide to plants that are suitable for your local climate based on the average cold temperatures.
USDA Plant Hardiness Zone Map
2012—The revised USDA Plant Hardiness Zone Map. Who knew that such a useful little map could be so controversial?
People have been publishing these maps since the 1920s and the USDA made its first version of a hardiness map in 1960. Unfortunately, it left out two states—Hawaii and Alaska, as well as Puerto Rico.
1960—The first USDA Plant Hardiness Zone map showed zones in Canada, but none for Alaska or Hawaii
A new map was designed in the late 1980s. The 1990 map finally included all 50 states. (Let’s face it—Alaskan gardeners need all the help they can get!) It also showed that some zones had shifted—which back then was not controversial. Most people accepted the idea that weather and climate changed over time. The 1930s had a Dust Bowl and the 1980s didn’t.
1990—The new USDA plant hardiness zone map reflected better scientific records, climate shifts and all 50 states.
As ten, then twenty years passed, most gardeners noticed that their plants were blooming earlier. There was a false start at a new map in 2002, which was dropped amid some controversy over global warming. Now, ten years later, the USDA has released new map that show that, once again, there has been some changes in climate. In general, the zones have shifted north, reflecting an average warming of 5˚F. The map reflects a 30-year average of temperatures rather than the 1990’s shorter 14-year database. It also has a finer scale that shows urban heat, two new tropical plant zones, and a cool interactive feature.
Some claim the map shows global warming while the USDA claims it merely shows more data and better information. Whatever! For now, let’s gaze at the colorful little map and dream of yummy vegetables and sweet-smelling flowers.
Did your hardiness zone shift? What are you experiencing in your garden?
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.