University of Vermont Extension System
Department of Plant and Soil Science

Plant Hardiness                                 OH 54

Dr. Leonard P. Perry, Extension Professor

The main points to keep in mind with plant hardiness are that there are many factors that can affect it, and that the hardiness guides or zones are merely that, guides. When speaking of hardiness in Vermont and the north, we mean cold hardiness and that is the subject of this leaflet. Plants also have heat hardiness, some woody plants such as lilacs or perennials such as delphiniums not surviving well in the heat of the south. There is even a map now of heat hardiness zones, but information on heat hardiness of specific plants is not as developed as the cold hardiness ratings.

Macroclimate (Hardiness Zones) and Variables

The first and often only information one sees on plant hardiness is the cold hardiness zone rating, usually for the USDA map. These zones refer to the average annual minimum winter temperatures. Zone 4a for instance represents average minimum temperatures of -25 to -30°F. First you should make sure the zones actually refer to the USDA map, and not another such as the Arnold Arboretum map. Then you should keep in mind the USDA map has changed, and is based on temperatures over a certain period. The original map was based on several decades of temperatures prior to the mid-1970s. Temperatures during the 1970s and 1980s were colder in general, so the map was redrawn in the late 1980s to represent this change, many areas dropping to a colder zone. But data, at least in Vermont, over the last two decades suggests temperatures might be a bit warmer once again, so some suggest we should revert to the former map? Perhaps you seen now why these maps are only a guide!

As we all know, seldom are winters average. A period might stay at these temperatures for several weeks. Or it might get much colder, but generally be warmer and on average fall in this range. Or temperatures might fluctuate up and down, above and below these temperatures. All these factors--depth of cold, length of cold, and fluctuations-- can all affect whether a plant survives, even if listed for a certain zone.

Another key variable is time of the season of severe cold. A temperature in the single digits early in the season, before a plant is fully hardened, may do more damage than much lower temperatures later in winter. The main variable in the north is snow cover, snow being the best insulator against cold. More winter damage and death may occur at much warmer temperatures if no snow cover. This is especially important for tender perennials, or shrubs with sensitive flower buds.

Unlike herbaceous perennials, which die back to the ground in winter, woody plants like shrubs and trees have parts exposed above ground. Most have separate leaf and flower buds, the latter often being much more sensitive to cold and less hardy. This is one reason that in some years a plant may leaf out, but have no flowers. Or in the classic example of the forsythia, flowers bloom only a foot or so high, representing the depth of snow cover and so protection from cold.

Another variable affecting plant hardiness is the genetic makeup of a plant. This is the main reason some plants, even some cultivars of a species, are hardier than others. They may have been bred or selected as hardier clones. Or they may have adapted over time to a location. This is why some plants, particularly woody ones, propagated from a southern source may not be as cold hardy as the same plant propagated from a northern source. This is difficult to determine for plants, even for growers, since plants in the marketplace are quite mobile. Even if they are grown in an area, the original plants may have come from another area which might not even be the area of the source of seeds or cuttings!

Hardiness is a function of location in a different sense as well. A particular plant such as impatiens, torch lily, or salvia may be perennial in a sourthern or warmer climate, only to behave as an annual in a colder northern climate. Keep this in mind when books, magazines and catalogs, especially those from other countries, call a plant an annual or perennial.

Microclimate and Soil Temperatures

Hardiness zones represent large areas of states or regions, and there can be variation with any zone by one or two zones in either direction. So a property in USDA zone 4 might be able to grow plants ranging from zones 2-3 to 5-6. This is due to many variables on the property affecting the climate in small areas, or microclimate. These could be a whole acre lot, or even 100 square feet. Heat loss from a building, or sun reflected from a southern wall, will make adjacent areas warmer. Likewise, an area shaded by trees or on the north side of building may keep snow and ice much later in the spring, and so be equivalent to a colder hardiness zone. A slope is often typical of a site further in the direction which it faces. This is often seen along highways in the spring, with snow still on the south side (north facing) and green grass on the north side (south facing). A hillside may also have airflow down it, resulting in cooler evening temperatures in summer and less chance of frost in spring or fall.

For woody plants, exposure to wind and winter sun change the microclimate. Evergreens on the west or north may be exposed to winter winds, so dry out and show leaf burn later in winter. Tree trunks exposed to early morning sun may heat up too rapidly, resulting in bark cracking. For herbaceous perennials, soil temperatures are the key to hardiness and survival, and their microclimate can change quite quickly depending on snow cover and mulches. An organic mulch, such as bark, of only a couple inches can moderate soil temperatures by 5 to 10 degrees F during the spring and fall when plant roots and crowns are less hardy. So if the bare ground drops to freezing, an adjacent mulched area may still be at 40°F.

In general, tender perennials begin to have winter injury when soil temperatures drop to about 28°F, many perennials tolerate to 20°F or so, and the most hardy tolerate into the single digits. In spring and fall, soil temperatures (crown level, about 2 inches deep) vary with air temperatures and lag by a day or two, the bigger the drop in air temperature, the slower the drop in soil temperature. So if the air drops below freezing, so may the soil the following day. But if it drops by 20 or 30 degrees, it may take 2 or 3 days for the soil temperatures to drop similarly.

The deeper in the soil, the less fluctuation. Even though the ground freezes, and the surface 2 inches may drop a few degrees further, bulbs at 6 inches deep may remain at around freezing. The later in the season, or the deeper the snow, the less fluctuation. Often in USDA zone 4, soil temperatures may be 40°F around the first of November, freezing around Christmas, and 28°F around the first of the year. Due to all the latent heat reserve from the soil depths, it may not drop much further unless little or no snow cover, or extreme cold. Even then, temperatures may only drop for a few days, before rising again. Yet have an "open" December with no snow, and soil temperatures may go into the teens even if the air temperatures remain above freezing. The variables already mentioned for air-- depth of cold, length, fluctuations, time of season, and other microclimate variables-- apply to soils and will affect what happens to perennials over winter.

Realizing these facts and variables about hardiness will hopefully lead to more successful choice, and placement of plants. Always keep in mind hardiness ratings are merely initial guides to help narrow these choices.

(Also see OH53 Vermont Hardiness Zone Map, and OH3 Preparing the Garden for Winter) 

Return to Perry's Perennial Consumer Page

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture.University of Vermont Extension System and U.S. Department of Agriculture cooperating, offer education and employment to everyone, without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.

version 3.01