Many of Maine’s plant hardiness zones have shifted to the north, reflecting an overall increase in average temperatures, according to the U.S. Department of Agriculture.
The USDA’s 2023 Plant Hardiness Zone Map, released this week, is the standard gardeners and growers use to determine which plants are most likely to thrive at a location. It’s based on the previous 30 years of temperature averages.
Most of Maine has shifted up at least one hardiness zone, according to the updated map.
For growers in Maine, the zone shifts are good news. With every increase in zones, more crop options are available for planting. Things like some varieties of winter squash, sweet potatoes, yams and other warm-weather crops can thrive in places they never could before in the state.
The higher the zone number, the warmer the average temperatures and the more options there are for planting because the growing seasons are longer. Typically, the farther north you go, the cooler the average temperatures and shorter the growing season.
The map was last updated in 2012, and the zones in Maine went from 5A in extreme southern York County to 3b in the St. John Valley. In between, much of the interior of the state was in zones 4a and 4b. Along the coast the zone was 5a.
In Aroostook County, there are now pockets of zone 4b, representing as much as a two-zone shift in some spots. The entire coast from York to Washington counties moved up from 5b to 6a. Interior Maine also is now in a warmer zone, moving from 4a and 4b to 5a and 5b.
Nationwide, the 2023 map shows half the country shifted to the next warmer zone or half zone. This means those areas warmed up to 5 degrees Fahrenheit over the last three decades.
The USDA stopped short of blaming global warming for the shift in zones, due largely to the variable temperature extremes over the past 30 years. Indicators of global warming are based on larger average temperature trends over longer periods of time.