New plant hardiness map shows only subtle shifts in Oregon

The first "plant hardiness" map published in over a decade — using Oregon State University climate data — shows an overall warming shift that may affect plant choices for gardeners and growers.

Why it matters: The USDA map is kind of a bible for farmers and gardeners, helping them understand which plants are most likely to survive winter cold in their region.

Catch up quick: The map was last published in 2012, based on 30 years of average annual low temperatures between 1976 and 2005.

• The new map — showing the coldest average temperatures have crept up about 2.5° F across the U.S. — taps data from 1991 to 2020 and information collected from over 13,000 weather stations.
• That's nearly double the data collection points compared to the previous map — improving the accuracy of the new one.

How it works: The plant hardiness map — first published a century ago — divides the U.S. into numbered zones representing a range of 10° F.

• Each zone is divided further into two lettered 5° F half zones.

Zoom in: The new Oregon map shows little change overall, with most of the Portland area and Willamette Valley not changing zones, Christopher Daly, director of OSU's PRISM Climate Group and co-author of the new map, tells Axios.

Yes but: Daly says some places — such as around Salem and in Eastern Oregon — have shifted a half-zone higher. A thin strip along the southern coast has also now risen from zone 9b to zone 10 for the first time, according to The Oregonian.

• It depends on exactly where you're planting — an area already close to the upper edge of a given zone would be more likely to bump into a zone with a higher average winter temperature.

The big picture: "I think over the long term, with average temperatures rising, we can expect that those plant hardiness zones will probably gradually move northward, but I think it'll take some time," Daly says.

The intrigue: Daly points out that the coldest annual temperature can be affected by one-off events, like whether an extreme cold front travels as far south as Oregon in a given year.

• The lab he leads has also gotten better at mapping cold temperatures that are affected by geography, such as valleys in Eastern Oregon where chilly air lingers in the winter.

Be smart: In addition to how cold it gets each winter — which is what the hardiness map reflects — there are many other factors that influence the best plant choices for a given region.

• Heat, light, soil moisture and humidity are some of the other factors that affect how well a plant will grow.

Zoom out: The new map shows that areas in the Midwest and Great Plains warmed the most compared to the 2012 data.

The bottom line: Here in Oregon, most gardeners don't likely need to start changing the plants they're growing — at least for now.