The latest edition of the USDA’s Plant Zone Hardiness Map, used by big growers and home gardeners alike, was released Wednesday, with zones in half the country changing.
Just not those in most of Oregon.
The map, a cooperative effort of the United States Department of Agriculture and Oregon State University’s PRISM Climate Group, is the first produced since the 2012 version.
According to the USDA’s Peter Bretting, the new iteration took the average annual extreme low temperature over 30 years (1991-2020) to assign zones to help “gardeners identify which perennials to grow.”
“It’s a premier resource,” he said. “The ability of a plant to adapt to survive mid-winter cold” is vital to someone deciding what to grow.
Most of the Portland area — and much of the Willamette Valley — remained in zone 8b, which indicates the average coldest temperature is in the 15-20-degree Fahrenheit range. The map assigns 13 zone designations (in a 10-degree range) and 26 half zones (labeled a or b, with a 5-degree range).
But, if someone were to look closely — the map is online at planthardiness.ars.usda.gov — there have been some zone changes in the state.
“There are thermal belts around Portland,” said Sean Hogan of Cistus Design Nursery on Sauvie Island, pointing out that some hills are zone 9a (20-25-degree range). “The difference between zone 8b and 9a is pretty subtle.”
He said there is even a zone 10 area — which means a person could grow avocado trees and bougainvilleas, for instance, with impunity —