Portland gardeners beware: USDA updates plant hardiness zones

PORTLAND, Ore. (KOIN) -- Plant enthusiasts and gardeners alike should be aware of the changes made to the USDA's plant hardiness zones.

The hardiness zone update by the USDA is the first adjustment in nearly 11 years. The previous data looked at the country's coldest average winter temperatures from 1976 to 2005. The new 2023 USDA Plant Hardiness Zone map looks at data from 1991-2020. It may come as no surprise that some locations, including Portland, have changed slightly as warmer winters have adjusted some zones due to climate change across the country.

The difference between Portland's zone (8b) is relatively unchanged from the 2012 to 2023 zone map. The 8b zone typically means that Portland and other areas that fall in the same zone typically see an average annual coldest temperature of 15-20 degrees. The only change between the old and new zone is that Portland is now running about a degree warmer in the winter months than when the zone was last updated in 2012. That doesn't mean that Portland can't still see a record-breaking cold snap in the winters to come, but that winter trends are running slightly warmer as a whole.

The locations with some of the biggest temperature warming trends during the winter months are Dallas, TX, New York City, Denver, CO, and Miami, FL to name a few. These locations have warmed anywhere from two to four degrees in the 2023 zone update compared to the 2012 zone update.

So, the plants that have survived year after year in your garden will continue to feel the same impacts. Watching the KOIN 6 weather team's forecast in the event of plant-threatening frost or record-breaking heat will give Oregon and Washington plants a better chance at survival.