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Bulletin #2242, Plant Hardiness Zone Map of Maine

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Plant Hardiness Zone Map of Maine

Developed by Lois Berg Stack, Extension ornamental horticulture specialist

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plant hardiness zone map for Maine

Why a Plant Hardiness Zone Map?

Low temperature is one of the most critical environmental limitations for plants. Some plants, like annual flowers and vegetables, simply avoid the cold of winter by dying at the end of summer. Perennial plants (including trees, shrubs and herbaceous perennials) must be able to survive the lowest temperature each winter in order to live into the next growing season. Such plants have a threshold temperature below which they will die. These threshold temperatures determine how far north plants will survive.

The Plant Hardiness Zone Map of the United States Department of Agriculture (USDA) represents low winter temperatures in North America. It is a guide to help you to assess plant hardiness.

What the Map Shows

The map above shows the plant hardiness zones of Maine based on average yearly minimum temperatures, from data for each year from 1974 to 1986. (This time period was the most recent prior to the map’s release for which reliable data were available, with most sites reporting at least 10 years of data.) Compared to the earlier map, this map shows that many sites in Maine have become colder. This recent period of cold extremes is verified by reports of freeze damage to plants. Remember that the map is based on average annual minimum temperatures. Although there is a global warming trend, we continue to have very cold temperatures each winter in Maine.

Environmental Factors that Affect Plant Growth

Low winter temperature is just one factor that affects plant growth. A plant may be adapted to the temperature of a location, but may die because of a problem related to another environmental factor. Or a plant may be stressed due to other problems, and may then die from the extra stress caused by a very cold winter. The success of perennial plants depends on many factors, including the following:

  • Temperature. Plants survive within a more or less broad range of temperatures. They thrive within a more narrow range of temperatures. In Maine and other cold climates, minimum temperatures generally determine whether plants will survive. Plants survive very low temperatures only when they are fully hardened off. If a very low temperature occurs in late fall, before a plant has fully hardened off, it may not be able to withstand the stress. High temperatures during the summer and temperature fluctuations throughout the year also impact a plant’s success.
  • Light levels. Plants are categorized as sun, partial shade or shade plants, depending on the amount of light they thrive in. Cloudy days and shade from nearby plants and structures can greatly reduce the amount of light that reaches plants. Plants may survive in less light, but they will thrive only in locations where they get the amount of light they are best adapted to.
  • Light duration. Daylength (the number of hours of light per 24-hour daily cycle) regulates many plant functions, including vegetative growth, flower bud development and growth, and the start of dormancy in the fall. Daylengths naturally vary from one season to another. In urban areas and in many landscapes, night lighting can interfere with plants’ perception of daylength. Such artificial alteration of daylength can prevent a plant from fully hardening off. This may mean it won’t survive the winter, especially if it’s in a location where it may be marginally hardy.
  • Soil, water, oxygen and nutrients. The soil supplies the plant with water, oxygen and nutrients. The soil must be able to absorb and hold a reserve of water, from rain or irrigation. This holding ability is better if the soil has ample organic matter.

Soils must also be well aerated to supply enough oxygen to plant roots. Heavy clay and compacted soils hold too little oxygen for many plants.

Essential nutrients must be available in the soil in the right amount and balance. These nutrients are more available to plants if the soil pH (acidity or alkalinity) is at an optimum level. Each plant performs best within a range of pH and nutrient levels.

About Microclimates

The plant hardiness zone map is based on a limited amount of data, and the zones represent five degree increments of low temperatures. This averaging and smoothing of data give only a general picture of temperatures. It does not include local variations called microclimates. A low, wet location, for example, may be a lot colder than a higher, dry location. Cities tend to be warmer than nearby countryside. Large bodies of water make temperatures less extreme. High mountain areas tend to be colder than lower elevations. (The Maine data station with the highest elevation is Rangeley, 1,530 feet above mean sea level, so the mountainous areas of Maine may be colder than shown on the map.) One guideline states that a 1,000-foot rise in elevation means a 5 degree F drop in temperature.

There are two areas of Maine for which there are differences between site data and the map. The first is Houlton. Houlton data suggest that it should be in zone 4a. The map shows it in zone 3b. This rating reflects both the “smoothing” of data and the fact that data stations east of Houlton, in Canada, report much colder data than the Houlton station.

The second discrepancy is Augusta. The Augusta airport data stations suggest that Augusta be rated zone 5b, but the map shows it in zone 4b, a full zone colder. Surrounding data stations in all directions produced colder temperature readings than Augusta. It’s unclear whether the entire Augusta area should be considered a warmer microclimate, or whether the Augusta weather station, located at the airport, is artificially influenced by river and urban factors.

How to Use the Map

Maine is divided into four zones, 3 through 6. Each represents a 10 degree F difference in average annual minimum temperatures. Each zone is further subdivided into sections a and b, with 5 degree F differences between them. For example, Bangor is located in zone 5a (-15 to -20 degrees F average annual minimum temperature), and Portland is located in zone 5b (-10 to -15 degrees F average annual minimum temperature).

Many catalogs and plant references refer to the plant hardiness zone map to indicate whether a plant is likely to survive in a location. For example, eastern white pine is rated hardy to zone 3, and would be expected to do well in any part of Maine. A Japanese maple, on the other hand, is rated hardy to zone 5 or 6 (depending on the type). It would show great adaptability only in coastal areas of Maine. You can use the zone information to choose landscape plants. However, don’t forget that there is a difference between surviving and thriving. A plant exposed to the lowest temperatures it can stand may survive the winter but lose so much vigor that it does not grow well.

This map is intended as a starting point only. Experiment with a wide variety of plant materials and collect long-term weather data for your site. This will help you fine-tune your ability to select plants that will survive and thrive in your location.

The map is based primarily on average minimum winter temperatures, which means that some winters may be a lot colder than what’s on the map. Plants that are marginally hardy to a location may succumb in an extremely cold winter. Table 1 (below) shows the annual minimum temperatures recorded from 1974 to 1986 at locations in Maine on which the map is based. The average of these temperatures largely sets the zone rating. Table 2 (below) lists zones calculated from the average minimum temperatures and also from the lowest annual minimum temperature recorded from 1974 to 1986.

Of course, to be hardy long-term, a plant must be able to withstand low temperatures as they occur. A plant might do well for several mild winters, only to succumb in a very harsh winter. You might want to choose plants based on a colder zone, especially if you think your location is colder than nearby areas. Or, when selecting plants that you want to live for many years, you may want to use your site’s “Low Zone” in Table 2, rather than its “Average Zone.” That way, you’ll be selecting plants that will survive the very coldest temperatures they might encounter, even in an extreme year.

Table 1


Station Name

Lat

Long

Elev
Annual Minimum Temperatures in Degrees Fahrenheit
Extreme Minimums

Avg
74-86
Deg Min Deg Min Ft Msl ’74 ’75 ’76 ’77 ’78 ’79 ’80 ’81 ’82 ’83 ’84 ’85 ’86
Acadia National Park 44 21 068 16 470 -11 -13 -11 -13 MSG
Augusta FAA AP 44 19 069 48 350 -15 -14 -19 -8 -5 -13 -10 -12 -11 -12 -5 -7 -10 -10.8
Bangor Airport 44 48 068 49 190 -22 -21 -23 -13 -8 -11 -23 -23 -17 -8 -5 -10 -13 -15.1
Bar Harbor 3 NW 44 25 068 15 110 -12 -9 -12 -7 -6 -11 -16 -17 MSG
Belfast 44 24 069 00 20 -19 -20 -15 -23 -15 -12 -27 -27 -27 -9 -26 -12 -9 -18.5
Brassua Dam 45 40 069 49 1060 -25 -26 -33 -23 -23 -26 -24 -26 -28 -20 -32 -19 -18 -24.8
Bridgewater 46 25 067 51 420 -31 -33 -36 -36 -32 -37 -17 -35 -28 -27 -31.2
Brunswick 43 54 069 56 70 -21 -10 -10 -21 -10 -23 -9 -7 MSG
Caribou WSO AP 46 52 068 01 620 -32 -29 -32 -25 -16 -27 -23 -27 -28 -16 -27 -24 -25 -25.4
Clayton Lake 2 46 37 069 32 1000 -42 -38 -38 -30 -27 -35 -29 -31 -35 -26 -33.1
Corinna 44 55 069 16 220 -27 -25 -34 -25 -20 -19 -25 -38 -31 -17 -30 -25 -24 -26.1
Dover – Foxcroft 87083 45 11 069 15 460 -15 -12 -23 -28 -25 -29 -15 -29 -18 -18 -21.2
East Hiram 43 53 070 45 530 -26 -26 -33 -25 -25 -33 -24 -19 -28 -15 -20 -24.9
Eastport 44 55 067 00 90 -12 -10 -11 -6 -3 -7 -13 -16 -10 -9 -10 -9.7
Ellsworth 44 32 068 26 20 -12 -13 -16 -18 -10 -7 -21 -33 -18 -15 -11 -15.8
Farmington 44 41 070 09 420 -26 -26 -32 -22 -22 -23 -26 -34 -26 -15 -30 -22 -13 -24.3
Fort Kent 47 15 068 35 520 -30 -36 -37 -35 -27 -38 -33 -29 -33 -18 -38 -36 -23 -31.7
Gardiner 44 13 069 47 140 -25 -20 -28 -22 -17 -19 -24 -34 -27 -27 -18 -11 -22.6
Grand Lake Stream 45 11 067 47 290 -15 -20 -23 -15 -9 -21 -21 -25 -28 -9 -24 -15 -15 -18.4
Houlton Airport 46 07 067 47 490 -27 -32 -36 -29 -24 -29 -30 -41 -32 -16 -21 -28.8
Jackman 45 38 070 16 1180 -31 -31 -26 -21 -20 -27 -26 -30 -32 -18 -34 -20 -22 -26.0
Jonesboro 44 39 067 39 190 -15 -15 -16 -14 -9 -11 -21 -21 -22 -12 -17 -10 -13 -15.0
Lewiston 44 06 070 13 180 -15 -12 -22 -12 -9 -12 -17 -23 -18 -7 -17 -10 -8 -14.0
Long Falls Dam 45 13 070 12 1160 -26 -24 -28 -18 -19 -25 -28 -22 -16 -29 -14 -16 -22.0
Madison 44 48 069 53 260 -16 -22 -28 -20 -20 -20 -30 -30 -25 -15 -28 -22 -25 -23.1
Middle Dam 44 47 070 55 1460 -26 -34 -20 -26 -29 -28 -30 -18 -25 -22 -25.8
Millinocket 45 39 068 42 360 -22 -22 -27 -16 -11 -21 -19 -23 -23 -11 -22 -20 -15 -19.3
Newcastle 44 03 069 32 190 -13 -11 -14 -9 -7 -11 -20 -20 -15 -9 -14 -8 -8 -12.2
Orono 44 54 068 40 120 -19 -15 -20 -14 -9 -13 -14 -20 -20 -6 -19 -14 -13 -15.0
Patten 4 WSW 45 58 068 32 770 -24 -22 -19 -16 -21 MSG
Portland WSMO AP 43 39 070 19 60 -16 -20 -20 -15 -14 -13 -20 -18 -16 -9 -19 -9 -8 -15.1
Presque Isle 46 39 068 00 600 -24 -32 -35 -31 -19 -31 -22 -28 -17 -27 -21 -25 -26.0
Rangeley 44 58 070 39 1530 -34 -30 -40 -29 -32 -38 -27 -35 -36 -27 -36 -27 -23 -31.8
Ripogenus Dam 45 53 069 11 970 -26 -26 -32 -25 -24 -28 -26 -30 -28 -25 -29 -18 -25 -26.3
Rockland 44 06 069 07 40 -13 -10 MSG
Rumford 1 SSE 44 32 070 32 630 -18 -21 -22 -13 -15 -17 -17 -21 -23 -10 -28 -20 -10 -18.0
Sanford 2 NNW 43 28 070 47 280 -15 -17 -22 -12 -13 -20 -18 -18 -20 -13 -23 -13 -6 -16.1
Springfield 45 24 068 10 440 -20 -23 -27 -20 -16 -20 -25 -31 -23 -11 -24 -17 -17 -21.0
Squa Pan Dam 46 33 068 20 610 -36 -37 -38 -33 -28 -34 -31 -30 -33 -21 -35 -25 -32 -31.7
Van Buren 2 47 10 067 56 460 -36 -44 -41 -37 -27 -40 -34 -39 -34 -25 -47 -33 -26 -35.6
Vanceboro 2 45 34 067 26 390 -24 -24 -26 -14 -20 -28 -29 -25 -14 -26 -20 -21 -22.5
Waterville Pump Stn 44 33 069 39 90 -17 -18 -26 -22 -12 -15 -27 -32 -28 -13 -17 -24 -20 -20.8
West Buxton 2 NNW 43 42 070 37 150 -18 -22 -30 -27 -21 -28 -31 -33 -26 -21 -33 -20 -16 -25.0
West Rockport 1 NNW 44 12 069 09 380 -10 -15 -10 -25 -20 -18 -10 -22 -12 -12 -15.4
Woodland 45 09 067 24 140 -23 -25 -19 -30 -14 -14 -22 -12 -27 -17 -16 -19.9

Table 2: Plant Hardiness Zones Based on Average and Lowest Annual Minimum Temperatures
Station Name Lat Long Ft Msl Avg
Zone

Low
Zone

Acadia National Park 4421 06816 470 ** **
Augusta FAA AP 4419 06948 350 5b 5a
Bangor Airport 4448 06849 190 5a 4b
Bar Harbor 3 NW 4425 06815 110 ** **
Belfast 4424 06900 20 5a 4a
Brassua Dam 4540 06949 1000 4b 3b
Bridgewater 4625 06751 420 3b 3a
Bridgton 3 NW 4404 07045 520 4b 4b
Brunswick 4354 06956 70 ** **
Caribou WSO AP 4652 06801 620 4a 3b
Clayton Lake 2 4637 06932 1000 3b 2b
Corinna 4455 06916 220 4a 3a
Dover – Foxcroft 87083 4511 06915 460 4b 4a
East Hiram 4353 07045 530 4b 3b
Eastport 4455 06700 90 6a 5a
Ellsworth 4432 06826 20 5a 3b
Farmington 4441 07009 420 4b 3b
Fort Kent 4715 06835 520 3b 3a
Gardiner 4413 06947 140 4b 3b
Grand Lake Stream 4511 06747 290 5a 4a
Houlton Airport 4607 06747 490 4a 2b
Jackman 4538 07016 1180 4a 3b
Jonesboro 4439 06739 190 5b 4b
Station Name Lat Long Ft Msl Avg
Zone

Low
Zone

Lewiston 4406 07013 180 5b 4b
Long Falls Dam 4513 07012 1160 4b 4a
Madison 4448 06953 260 4b 4a
Middle Dam 4447 07055 1460 4a 3b
Millinocket 4539 06842 360 5a 4a
Newcastle 4403 06932 190 5b 5a
Orono 4454 06840 120 5b 5a
Patten 4 WSW 4558 06832 770 ** **
Portland WSMO AP 4339 07019 60 5a 5a
Presque Isle 4639 06800 600 4a 3b
Rangeley 4458 07039 1530 3b 3a
Ripogenus Dam 4553 06911 970 4a 3b
Rockland 4406 06907 40 ** **
Rumford 1 SSE 4432 07032 630 5a 4a
Sanford 2 NNW 4328 07047 280 5a 4b
Springfield 4524 06810 440 4b 3b
Squa Pan Dam 4633 06820 610 3b 3a
Van Buren 2 4710 06756 460 3a 2a
Vanceboro 2 4534 06726 390 4b 4a
Waterville Pump Stn 4433 06939 90 4b 3b
West Buxton 2 NNW 4342 07037 150 4b 3b
West Rockport 1NNW 4412 06909 380 5a 4b
Woodland 4509 06724 140 5a 4a

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 1991, 2006
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