Be cautious in embracing new, warmer hardiness zones

Recent Changes to Hardiness Zones

In 2023, the USDA released an updated hardiness zone map, reflecting observed changes in annual temperatures. Significant portions of Oregon shifted half a zone warmer, equivalent to a 5°F increase in minimum winter temperatures, while other areas remained in the same zone.

For example: Portland shifted from 8b to 9a, Medford shifted from 8a to 8b, Hood River shifted from 7a to 7b, Eugene Roseburg and Astoria stayed at 8b, Bend and La Grande remained 6b, Klamath Falls remained at 7a, and Gold Beach remained 9b.

Temperature Extremes and Climate Change

Despite warming temperatures, plants still need to withstand cold extremes, including the below-average deep freezes which will continue to occur. Even with warming trends, climate change also makes weather anomalies more frequent, and these anomalies include extremes of cold and ice. For example, in January of 2024, a multi-day ice storm with lows around 15° F in Portland, OR was exacerbated by ice accumulation. The storm was observed to kill many zone 9 plants, while broadleaf evergreens rated to zone 8 were badly ice burned, and broadleaf evergreens rated to zone 7 were unharmed.

To build resilient landscapes, it’s wise to plan for historical low temperature extremes to continue, and high extremes to increase. This means:

• Assume periodic extreme cold: Even as average temperatures rise, cold anomalies are likely to continue.

• Select tough plants that can withstand temperature extremes: Opt for plants rated at least one or two zones lower than the USDA zone for your location to account for occasional harsh winters.

• Rising summer heat is an issue: Do select plants that can withstand heat and drought stress in summer.

Most plants native to the PNW are adequately hardy for extreme cold weather events. It is non-native ornamentals that are usually hardest hit.