Colorado gardeners eyeing the U.S. Department of Agriculture’s updated Plant Hardiness Zone Map, which moves much of the state in a warmer growing zone, should hold their horses — or trowels — before loading up on heat-loving perennials as they prepare for the upcoming growing season.
The agency’s growing map, which was updated in November for the first time since 2012, is used by gardeners and plant nurseries to tell how tolerant different plants are to cold weather, or what the USDA calls the “average annual extreme minimum temperature.”
The new map moves about half of the United States, including much of Colorado, into a warmer growing zone.
Large swaths of the Denver metro area moved from zone 5b to 6a, changing the average lowest temperature from 15 degrees below zero to 10 degrees below zero, according to the agency.
The updated map is based on data from more than 13,000 weather stations between 1991 and 2020, nearly twice the number of weather stations used in the 2012 map, the USDA said in a news release.
But gardeners who see the new guidelines as a sign to start growing plants that are less tolerant to the cold should reconsider, said Karim Gharbi, horticulture agent for Colorado State University’s Denver County Extension office.
“I would urge caution with that, because moving forward we are going to have much more unpredictable winters,” Gharbi said. “We may have winters that are warmer overall, but we are still going to have these random arctic days.”
Cold snaps like the one that plunged Colorado into a deep freeze in January will continue to threaten more delicate perennials, according to Gharbi.
“People are thinking we’re going to be a tropical paradise, but it is still very cold winters, a very arid climate and a very rough spot for a lot of plants. Don’t get your hopes up,” he said.
While most garden centers won’t start bustling with customers until April and May, several stores said they’re not yet changing their advice based on the updated growing zones.
“We’re being cautious with how crazy and unpredictable the weather has been,” said Mark Ledford, manager at O’Toole’s Garden Center in Westminster. “We’re still trying to recommend mostly zone 5 plants versus zone 6 plants.”
Most of the stock O’Toole’s carries was ordered in the late summer, Ledford said, though it could impact what the store orders for next season.
City Floral Greenhouse and Garden Center in Denver is also planning to keep recommending the same cold-hardy plants, manager Trela Phelps said.
“I think it will be the status quo, same as always,” Phelps said. “We have always erred on the side of caution and will continue to do so.”
The temperature changes “are not necessarily reflective of global climate change,” the USDA said in a news release, pointing to variations in low temperatures, data from more weather stations and better mapping methods.
Map developers involved in the project cautioned against attributing the temperature increase in some zones as an indicator of climate change, the agency said.
