Virtually half the country, including Maine, found itself capable of supporting crops once considered too vulnerable to cold temperatures to thrive when the United States Department of Agriculture recently updated its plant hardiness zone map.
Many of Maine’s plant hardiness zones have shifted to the north, reflecting overall increases in average and minimum winter temperatures over the last 30 years, according to the USDA.
The map was last revised in 2012.
To some extent, the new zones will change how growers in the state select and plant their crops. But Aroostook County farmers won’t be planting peanuts instead of potatoes anytime soon.
Experts warn there are too many variables in Maine for gardeners to drastically overhaul their selections in favor of traditionally warmer weather crops. They also should not make any general assumptions about the inevitability of climate change.
“The zones have continued to shift, as is expected with climate change,” said Caleb Goossen, crop and conservation specialist with the Maine Organic Farmers and Gardeners Association. “That could mean, for example, moving from 4b to 5a for some folks on the map, but that’s only a 5-degree Fahrenheit change in extreme minimum temperature. In practice it won’t change things dramatically relative to the 2012 map.”
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 data the USDA used to create the new map is based on 30 years’ worth of extreme winter minimum temperatures, said Sean Birkel, state climatologist at the University of Maine Climate Change Institute.
There are reams of data at the climate office showing Maine’s winters have been steadily warming for the last century. Birkel warns that if human behavior that pollutes the environment doesn’t change, that trend will continue and the hardiness zones will keep shifting northward.
In the short term, Goossen said it’s more important to pay attention to what’s underfoot than temperature trends when it comes to plant selections.
In Maine, the more important variable for planting seeds directly into the ground is soil temperature, Goossen said, and the new zone designations are not going to make a drastic difference there.
“A new zone designation may acknowledge that where someone lives is not getting as cold, and possibly could be warming up earlier in the spring than another area,” Goossen said. “Someone that’s been using soil temperature as their guide already won’t be doing anything differently.”
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Goossen recommends proceeding with caution with the new hardiness zones because there are so many variables in play, including extreme weather, drought and wild temperature fluctuations over short periods of time.
The idea of planting new, warmer weather crops in Maine can be exciting, said Jesse Watson, principal designer at Midcoast Permaculture. But his own excitement is tempered by what the changes represent.
“Yes, we can have new plants in this warmer climate,” Watson said. “But I fear it is at the cost of greater pest and disease pressure.”
In permaculture, Watson works with the existing natural ecosystem to build a self-sustaining garden. It’s a method that relies on nature and not external additives like chemical fertilizers or outside water sources.
In practice it’s a way of making decisions on what elements are put in landscaping, farming or forestry that creates an entire native ecosystem that is also agriculturally productive, Watson said.
Hardiness zones and weather trends are a big part of successful permaculture.
In looking at planting possibilities within new zones, Watson said commitment to native species is crucial, but the door is now open to expand on that with what he calls the native plant palette.
He suggests looking at successful native plantings in southern New England to get an idea of what can now be grown in Maine.
Several varieties of native nut trees, for example, could possibly thrive in Maine, including pecans and hickory.
At the same time, growers need to pay attention to existing climate realities in Maine, Goossen said, like the average first and last frost dates in their area. Those may not have shifted north with the new zones.
It’s not all bad news and there is some good to be found within the warming trends, Birkel said.
“Certain new crops may be more viable now — especially in Aroostook County — that some growers could use to their benefit,” Birkel said. “That is a positive in something that overall presents a major challenge and negatives.”
Like Goossen, Watson said growers should use caution moving forward.
“While we can help useful native plant species move north, new insect pests will also move,” Watson said. “I can get excited about edible plants that we can assist in migration [but] I’m mostly unnerved by the consequences of climate change and the negative effects it will bring.”
Maine has already seen new and crop-damaging invasive species spread into the state with warming temperatures, such as green crabs and non-native weeds.
It comes down to how people respond to the new zones and associated overall warming, Birkel said.
Growers will have to adapt to the changing conditions at some point, Birkel said.
But at the same time, people have the power to make choices that mitigate climate change. These include lessening their reliance on fossil fuels, doing more recycling and supporting local agriculture.
“It’s important to remain hopeful and not see just negative aspects,” he said.
