By Brenda Francis, Colorado Master Gardener

USDA Zone Changes

The United States Department of Agriculture released an updated Plant Hardiness Zone Map (PHZM) in 2023. The new PHZM shows western Arapahoe County has warmed to zone 6a and the eastern side of the county has warmed to zone 5b. These new regional averages are five degrees warmer than ten years ago.

The PHZM reflects average temperatures. Calculations include the lowest temperatures, but do not use the coldest readings as a baseline. Average lows for zone 6a are -5°F to -10°F. Average lows for zone 5b are -10°F to -15°F.

A prolonged cold snap below the PHZM average can destroy shallow rooted perennials and damage other woody plants. Expect some trial and error when using plants rated for our new, warmer PHZM average lows.