Effect of weather changes: USDA revises planting zones for Virginia

As spring sowing gets into full swing, Virginia farmers and gardeners can utilize the U.S. Department of Agriculture’s updated Plant Hardiness Zone Map to make the most out of their plantings. 

Gardeners and growers have long used the PHZM to decide which perennial, woody and herbaceous plants will likely survive and thrive in their environment, explained Sanjun Gu, horticulture specialist. for Virginia Cooperative Extension.

“For all plant lovers, farmers and home gardeners, the release of the updated plant hardiness zone map is something they will find invaluable,” Gu said.

Released last November, the revised map is based on the average annual extreme minimum winter temperature. It is based on weather data from 1991 to 2020 and findings from 13,412 weather stations across the U.S. The map provides greater accuracy than the 2012 version, which used 1976-2005 data from 7,983 sweather tations.

The 2023 map consists of 13 areas across the U.S. displayed as 10-degree zones and 5-degree half-zones. About half the country shifted to the next warmer half zone—including a majority of Virginia.

“Simply put, the higher extreme minimum temperatures allow more plant species to survive Virginia’s winters and help adapted species thrive long term,” Gu explained.

Gardeners and growers can incorporate more flowers, shrubs, trees and other plants that would not previously have survived winter. Additionally, annual cold-hardy vegetables have a longer harvest season, some flower bulbs may not require removal before winter, and winter cover crops have more flexible sowing dates.

“Typically, when you have warmer temperatures, your last frost date will be advanced,” Gu noted. “Assume that in most regions of Virginia, you can probably plant about 7-10 days ahead of a regular planting date.”

He added that gardeners and growers should monitor regional weather forecasts about 10-15 days in advance of planting. The map is only a reference tool and can’t provide a guaranteed forecast for future variations in weather.

Additionally, climate change can result in more fluctuations in temperatures that threaten plants. Because climate changes are assessed based on data from 50 to 100 years, the PHZM does not take global warming into consideration.