Maine’s many climate zones

By Tom Seymour | Jun 02, 2020
Photo by: Tom Seymour Flowering crab not yet blooming last pictured Memorial Day weekend.

The unseasonably cool spring has tempered plant growth, especially in coastal areas where temperatures typically run colder than even a few miles inland.

For instance, I have a flowering crab apple tree that usually attains full bloom by Memorial Day weekend. This year, though, the buds were still closed during then.

Other trees, shrubs and perennial plants also indicate nature is currently on hold.

However, conditions even 25 or 30 miles inland are entirely different. There, spring has sprung in all its glory. This demonstrates the clearly defined climate zones, or, “mini” climates, in Maine.

Here’s a for-instance: while leaves on hardwood trees in Belfast are only partially open as of May 25, the day I wrote this column, conditions in Burnham, in northern Waldo County, are entirely different, in that the season is a week or more advanced. There, people can sit under the shade of their hardwood trees, while along the coast, the best that we can find is something like filtered shade, since tree leaves are only now beginning to develop.

Hardiness Zones

Gardeners must consider these tightly compressed climate zones when choosing shrubs, perennials and ornamental trees. Most people consult the USDA plant hardiness zone maps as well as average last frost dates as shown in garden catalogs. But these are woefully inadequate in a state such as Maine, where mountains, hills, valleys, coastal plains, lakes, rivers, bays and ocean all factor in to create many dozens of different climate zones.

At my home in coastal Waldo County, the maps say the average dates of the last spring frost run between March 30 and April 30. Those figures are grossly inadequate, since we often see mid- to late-May frosts.

The same goes for first average frost dates. The map in my book indicates a first average frost date of somewhere between Sept. 30 and Oct. 30. Well, I’ve seen frosts in late August and often, we see a windshield-scraping frost by mid-September.

There is a point to be made, then, of establishing these dates on a local basis. One way to do this is to keep garden records, something I have touted for years in this column. That way, you can just tally up the totals and take an average. Interpreting the outcomes, though, is entirely another matter.

For instance, while it seldom gets more than 20 degrees below zero at my place, I once saw a low temperature of -39. It was at night, with no wind blowing. That’s about as cold as it ever gets anywhere in Maine. The question is, though, will this occur again in the near future?

It may be many decades before we see a low reading of this magnitude. So for all practical purposes, we may safely ignore that extreme low and consider it just an anomaly. This means we can go ahead and plant something with a hardiness rating of 20 below and not be too concerned about it being killed by the cold.

As per average high temperatures, the usual routine in summer is that we get no more than six or seven days, and this may be spread out over months, of 90-plus-degree days. Even heat-intolerant plants will probably not succumb to the occasional 90-degree day.

Informed Decisions

Paying attention to the local climate now helps us make informed decisions regarding buying and planting. Knowing, with a good deal of certainty, when the first and last frosts will occur and how temperatures may range between extreme highs and lows, allows us to pick and choose the perfect plants, shrubs and trees for our specific environment.

With a knowledge of our local conditions, we can then look closer at the distinctions between and among various plants. Even then, it might pay to not buy something that is only marginally hardy in your area. Better to find a plant that sits well within average high and low temperatures, since that will give a good degree of assurance that your plant will prosper and give many years of enjoyment.

By the time this column appears in print, all danger of frost in our area should be well past, so go ahead and set out even the tenderest plants.

Here’s a little something to note, something I assign great importance to. Memorial Day, formerly, “Decoration Day,” a federal holiday, was observed May 30 from 1868 to 1970. Since then, the date was moved ahead nearly one week.

An old tradition states that we should not set out tender plants until Memorial Day, but that was when Memorial Day was observed May 30. That was because frost was possible up to that date, but not on or after. So bear that in mind when planting. Remember, even if you are a week late getting out your plants, it won’t hurt a thing. They will quickly catch up. Better safe than sorry.

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