May 29—Maine farmers will most likely benefit from the warmer weather and longer growing seasons caused by climate change, but they also will face greater risks as our milder winters make conditions ripe for more harmful plant diseases and pests.

Extreme weather brought on by climate change will pose additional risks, forcing farmers to prepare for the increased likelihood of drought and heavy rains within the same growing season, according to scientists who advise the Maine Climate Council.

“In general, warming temperatures and a longer growing season should be an advantage for Maine agriculture,” said Glen Koehler, an associate scientist at the University of Maine Cooperative Extension. “That has been one of the constraints: a short, cool growing season.”

Koehler presented the latest scientific findings on how Maine’s changing climate will impact local agriculture during an online meeting of the Maine Climate Council on Wednesday. They will be included in the council’s pending update to the state climate action plan, Maine Won’t Wait.

The council got its first scientific update earlier this month on how climate change is impacting the health of Mainers. While other areas of the country will see higher temperatures, Maine is less prepared than America’s traditional hotspots for dealing with the resulting health impacts.

The plan is a four-year strategy to cut greenhouse gas emissions and protect natural resources and communities from climate change impacts. Some of its goals include adopting renewable energy sources, encouraging electric vehicle use and preparing the coast for rising sea levels.

As temperatures increase, plant hardiness zones also will shift north. The U.S. Department of Agriculture recently updated its plant hardiness zone map, which has long helped gardeners and growers determine which plants are most likely to thrive in a given location.

The USDA uses the coldest average annual temperature to define plant hardiness zones. Hardiness zones ensure that perennial plants avoid lethal winter temperatures, frost-tender annual crops are not planted too early and that cool-season crops are not harvested too late.

For example, a Monmouth farmer will see annual minimum temperatures climb from minus 15 to minus 10 degrees Fahrenheit in 2005 to minus 5 to 0 in 2050, or current Connecticut conditions. By 2085, Monmouth will feel like southeastern Pennsylvania, with an annual minimum temperature of 5 to 10 degrees.

That represents a two-zone hardiness shift, from 5b to 7b, for that Monmouth farmer.

A Presque Isle farmer will see a hardiness zone shift from 4b (minus 25 to minus 20) to Hartford-like zone 6b conditions (minus 5 to 0) in 2085, if the world continues along its “business as usual” approach to curbing the greenhouse gas emissions that are driving global temperatures up.

Climate change also is projected to extend Maine’s growing season. Central Maine is expected to see its growing season increase from 176 days in 2000 to 223 days by 2100, an increase of 47 days. northern Maine’s growing season will increase by 51 days, from 145 in 2000 to 196 in 2100.

Based on temperature alone, a turn-of-the-century Monmouth farmer might be able to grow peaches. The high-value fruit would never survive in Maine’s current conditions, Koehler said, but it might thrive when our weather feels more like Pennsylvania or New Jersey, a prime peach growing area.

Yet climate change is not all good news for Maine’s agricultural sector, Koehler warned.

FEELING PEACHY

For example, some New Jersey farmers are ripping up their once-lucrative peach orchards because of another side effect of global warming: extreme weather. High weather variability has caused spring frost damage to these fragile fruit trees and resulted in costly crop losses, Koehler said.

Future Maine peach farmers could face the same problem. He also noted that temperature is just one component of a successful farming operation — a farmer also needs the right amount of rain, good soil and enough direct sunlight.

Maine farmers tired of dealing with recent drought conditions and irrigation bills are usually happy to learn the future climate will be wetter — until they discover that won’t eliminate seasonal droughts, and that the high-precipitation storm events could wash away their seed, fertilizer and topsoil.

The same rainfall that will help young peach tree roots grow could drown mature trees and leave them unable to defend themselves against certain pathogens. Many disease-causing microbes remain dormant in frozen winter soil, but can attack dormant roots when the winter soil is merely wet.

Warm-weather pests and plant pathogens once constrained by Maine’s sub-zero winters are now expanding their ranges northward and will offset some of the agricultural sector’s net gains from climate change. Plants stressed by heat, drought or rot could prove more vulnerable to their attacks.

“It’s a good example of how tricky it can be,” Koehler said during a Maine Climate Council science panel.

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