Plant hardiness is based upon a Hardiness Zone Map created by the U.S. Department of Agriculture. Maine is rated as Zone 5 along the coast with the majority of inland Maine in Zone 4. Portions of Northern Maine are in Zone 3. In this example, the lower the zone number, the colder the region gets. These zones are separated in what is expected to be the coldest average minimum temperatures for that zone. Because Maine is at the most northern range of Zone 5, we often reach actual minimum temperatures below the average minimums. When this happens, plants that are rated as hardy in Zone 5 may die from winter cold.
Unfortunately, because Zone 5 covers such a large area of the U.S., new plant introductions often receive a hardiness rating of Zone 5 as a catch-all. When planted in Maine, these plants may not survive, especially away from the coast. In rare situations, these plants are found to be more hardy than first thought, and they are upgraded to a colder zone. Such is the case with Merrill Magnolia, a hybrid of two Asiatic species that originally was rated as hardy to Zone 5. Over the years, Merrill Magnolias have been planted ever increasingly into Zone 4, and there are presently three Merrill Magnolias growing at the Presque Isle campus of the University of Maine. These particular trees have survived temperatures approaching 45˚ below zero and flower annually. With this track record, the hardiness of Merrill Magnolia has been re-listed as Zone 4 and is occassionally listed as Zone 3 hardy.
References are full of opinions of horticultural writers, most whom neither live in nor experience a winter in Maine. Without actual experience, these writers rely on existing information to determine what they write about plants. Little of this information is reliable for Maine gardeners. Maine winters come early, stay forever and are more brutal than anyone from away wishes to believe. Interestingly, the most severe winters are not necessarily those with the lowest temperatures, but those with little snow cover and quickly lowering temperatures. Constantly fluctuating temperatures — especially temperatures that go above freezing, then rapidly back to freezing — do far more damage to plants than low temperatures alone. During winters like that, tree trunks split, roots die and flower buds freeze-dry.
There are growers in the Midwest, sitting squarely in Zone 5, who believe that any plant growing in their nursery will automatically survive in Maine’s Zone 5. Thousands of dead plants prove otherwise. Only attempting to grow a plant in Maine’s conditions will prove whether or not it is hardy.
With the ability to search for information globally on the internet, it is even more imperative that local sources of plant hardiness information exist. There are many horticulture professionals in Maine who can give you factual advice on whether or not a plant should survive in Maine. Seek out this information before finalizing your choices of plants.
Nursery suppliers routinely attach manufactured information tags to plants being shipped to garden centers. These tags have the same information regardless of where the plant is being shipped. These tags include such information as preferred soils, sun or shade and mature size. Once again, little of this information pertains to actual performance in Maine gardens. Luckily, the plants don’t read these tags, and simply grow the way nature intended them to grow under the conditions they are given.
Jeff O’Donal, a recent winner of the Al Black Commemorative Lifetime Achievement Award in Horticulture, is the owner of O’Donal’s Nursery, a full service garden center in Gorham.