Let’s talk about hardiness zones for our gardens. Knowing what hardiness zone you live in is key to choosing plants that will survive our winter’s chilly temperatures. If you Google “USDA hardiness zone map,” you’ll find links to an interactive map. Plug in your Zip Code and you’ll find your zone.
A variant of the USDA map, Sunset Magazine’s “Plant Zone” map will be explained later in this article.
The first hardiness zone map for our country was published in Alfred Rehder’s Manual of Cultivated Trees and Shrubs Hardy in North America in 1927. Rehder was a noted Harvard botanist whose zone map was characterized by uniform five-degree differences in the lowest average temperature of the coldest month. It was modified and updated in the late 30s and twice again in the 1940s.
In 1960, the United States Department of Agriculture (USDA) published its own map, which showed uniform ten-degree ranges in the average annual minimum temperature across the country. This map became the standard hardiness map across the country, and was updated for 30 years, in 1990. The zones in the 1990 map resulted from using the lowest daily minimum temperature recorded for the 12 years ending in 1986.
It was not until 2012 that the USDA introduced a new map—22 years after the last update. This time, mappers examined 30 years’ data, from 1976 to 2005. Since hardiness zones are determined by a region’s extreme weather events (the lowest daily minimum temperature recorded for each year), instead of overall temperature averages, and because it only reflects the past 30 years, the USDA says it is not reliable evidence of whether there has been global warming.
Most of Aberdeen is moved from Zone 8b to in Zone 9a. Because I lived for 30 years in Fresno, California, I checked the zone for that city—it is now Zone 9b, and Montesano is now in Zone 9a.
How can a city like Fresno, with boiling-hot summers be in the same basic hardiness zone as Aberdeen, where heat units for growing tomatoes are few and far between? The answer is simple: a hardiness zone only tells us whether a plant will survive cold weather.
In Fresno, instead of using the USDA’s hardiness zones, most of us relied on Sunset Magazine’s “Plant Zones.” Plant Zones incorporate more data than just winter temperatures. They include additional variables, such as summer highs, lengths of the growing season, humidity, and rainfall patterns.
There is a real contrast between Fresno and Grays Harbor seen in the Sunset Magazine Plant Zones. The additional criteria used for Sunset’s zones results in the Grays Harbor area being placed in either Zones 4 or 5. Fresno is in Zone 9, demonstrating, among other variables, the impact of summer’s heat and meager annual rainfall in the Fresno area.
Knowing hardiness zones and Plant Zones helps us pick plants for our gardens. Even though we are in the same USDA hardiness zone (Zone 9) as Fresno, growing fuchsias in the San Joaquin Valley is virtually impossible. Why? Sunset Magazine’s Plant Zones tell the tale. Fresno’s Sunset Plant Zone 9 means higher summer temperatures start early and end late. Moreover, Fresno’s rainfall year-round averages only 10 inches, versus 84 inches in Aberdeen. No fuchsias in Fresno’s gardens, but Fresno, with all its hot, sunny days is a tomato’s happy place. I would harvest tomatoes in May in Fresno. Here? August or September, if we are lucky.
To check your USDA Hardiness Zone, google “USDA hardiness zones.”
Mary Shane, a Master Gardener since 1998, lives in the Val Vista neighborhood west of Montesano.