The growing season may be getting longer in Maine, but average last frost dates in springtime haven’t changed much.
The last freeze in spring is the prime consideration when deciding whether to plant outside. An unexpected frost can lead to a lost crop, but there’s no way to say with certainty when one will arrive.
Different growers have different methods to play it safe. This year, some are waiting until after the full moon on May 23, believing it often brings the last frost. Others say not to plant until Mother’s Day has passed.
In medieval Europe, people waited for the feast days of three “chilly” saints — Saint Mamertus, Saint Pancras and Saint Servatius (also known as Gervais or Gervatius) — from May 11 to 13, according to the Lewiston-based Farmers’ Almanac.
Others use data from previous years to calculate the likelihood of a freeze occurring on certain dates. The Almanac estimates average last frost dates across the state as May 1 in Portland and Augusta, May 9 in Bangor and May 22 in Presque Isle, with a 30 percent chance a freeze will still happen after that date.
Those dates are based on the National Climatic Data Center’s records from 1991 to 2020.
Data from the same timespan were used to update the U.S. Department of Agriculture’s zone hardiness map last November. Most of Maine shifted at least one hardiness zone north in the update, meaning longer growing seasons and the possibility to plant more types of crops in Maine.
The zone maps are based on the average annual extreme minimum temperature over 30 years of data. The new hardiness zone map replaces one based on data from 1976 to 2005.
Much of that growing season extension has been seen in the fall rather than in spring frost dates, according to Sean Birkel, Maine’s state climatologist and a research assistant professor at the University of Maine’s Cooperative Extension.
Final spring frosts are on average eight days earlier in Bangor and four days in Portland, Caribou and Rangeley than they were 20 years ago, according to Birkel.
The first fall freeze is stretching further, moving into mid-October from the early September to early October dates seen across the state in the early 2000s.
“Most of the warming-driven increase in growing season length has occurred in fall,” Birkel said.
The zone shifts show overall warmer winters over that period, Birkel said. Spring freezes don’t necessarily mirror the trends in winter temperatures, though.
Late frosts or cold waves can happen late in a spring that is otherwise warm, like it did last year on May 18. Overall temperatures last spring were still warmer than the 20th century average.
If you haven’t planted yet, this is a good time for hardening off seedlings, or bringing them outside during the day to prepare them for outdoor life.
You can check forecasted temperature and other weather factors on the National Weather Service’s online forecasting graph.
