Which plants will survive an Oregon winter? Map that shows gets first revision in a decade - OregonLive

Which plants will survive an Oregon winter? Map that shows gets first revision in a decade - oregonlive.com

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.
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 in the 2023 USDA Plant Hardiness Zone Map of Oregon.U.S. Department of Agriculture

“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 — off and on a 40-mile stretch on the southern Oregon coast, including Brookings.

“It’s the first time it’s been acknowledged” on the map, said Hogan, who studies the zone hardiness map closely and originally helped connect the USDA and Oregon State to launch the project.

The information in the new map, called “a very, very important agricultural tool” by Simon Liu, the administrator of the USDA’s Agricultural Research Service, was based on data from more than 13,000 weather stations (as opposed to just under 8,000 for the 2012 version).

“The addition of many new stations and more sophisticated mapping techniques led to a more accurate and detailed plant hardiness zone map,” said Oregon State’s Christopher Daly, the map’s lead author.

And even though the map focuses on low temperatures, it also found in some places, things are getting warmer.

“Overall, the 2023 map is about 2.5 degrees warmer than the 2012 map across the conterminous United States,” Daly said. “This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone. The central plains and Midwest generally warmed the most, with the southwestern U.S. warming very little.”

When asked if climate change was impacting the zones at all, Daly said it was a “definite maybe.”

“Plant hardiness is just one number and can be volatile,” he explained, but “there’s no doubt climate change will start warming the zones.”

Those going to the map online can type in their ZIP code to find what zone they live in. They also will find additional resources form the Agricultural Research Service, including a wealth of gardening tips, and how to weed and keep soil healthy.

The map is not just useful to gardeners, though.

“The USDA zones are of great importance to us, as well as to landscape architects, designers, contractors, wholesalers, independent garden centers and the national retailers,” according to Nicholas Staddon of Everde Growers, a national company with a large farm in Forest Grove. “We use the zones on a daily basis and when placing plants into production, their zone hardiness is always an important question.”

— Dennis Peck, for The Oregonian/OregonLive