University of Vermont
Department of Plant and Soil Science

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PLANT HARDINESS

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont
 
Whether or not a plant is hardy in the cold north isn’t as simple as just looking at a hardiness map.  There are several factors affecting hardiness you should understand in order to know better which plants to grow, where to place them, and why some may have died.
   
Hardiness is genetic. That is why some plants are hardier than others, even why some cultivars (cultivated variety) are hardier than others of the same plant. They may have been bred or selected as being hardier clones.
   
To further confuse you, plants adapt or change genetically and slowly over time to their climate and local habitat—something called “provenance”.  That's why a species grown in southern climates may not be as hardy as the same species grown in a northern locale such as Vermont. Keep this in mind when buying plants from mail order companies or nurseries.  Your best bet is a local nursery having experience with a plant.
   
Roots, stems, and leaf and flower buds generally are hardy to different temperatures. This is why many perennials die to the ground in winter only to have their roots survive and produce new shoots the following spring.
   
Forsythias, for example, often have leaves but no flowers. The flower buds, which are less hardy than the leaf buds, are killed by the cold. If flowers appear up to a certain height, but not above this height, this represents the depth of snow cover that protected the flower buds during the killing cold.
   
So how do you determine plant hardiness?  Start by figuring out your hardiness zone.  These are geographic zones shown on maps that share the same range of average annual minimum winter temperatures. (A few references also will list hardiness zones for heat--the maximum temperature a plant can endure.)
   
These zones are averages only.  For instance, on the 2012 USDA hardiness zone map, Burlington, Vermont is in zone 5a, which indicates average minimum temperatures get -15 to -20 degrees F in any year (planthardiness.ars.usda.gov).   Over the last 10 years, average lows have fallen into that range about half the time, one year it was lower (-21 degrees), the rest were slightly warmer.
   
This 2012 map is more accurate than the previous 1990 map, being based on a broader range of data (1976-2005 vs 1974-1986 for the former), better mapping techniques, and more stations of data.  Many areas are shown as half a zone warmer on the newer map.  While much of the Champlain Valley was shown as zone 4b on the 1990 map (-20 to -25 degrees average minimum), they’re now in zone 5a on the 2012 map (-15 to -20 degrees).
   
While you mainly will find the USDA most recent hardiness zone map, especially online, in references and in catalogs, there are some other hardiness zone maps for the U.S. and other countries, often with different zone numbering (www.plantsearchonline.com/zones.htm).  One is from Sunset publications, whose zones reflect not only lowest temperatures but other climate factors such as rainfall, latitude, and elevation.
   
The 2015 hardiness zone map from the National Arbor Day foundation (www.arborday.org/media/zones.cfm) is generally similar to the USDA 2012 map, yet shows many areas in a warmer zone.  Upstate New York is mainly in zones 4 to 5 on the USDA map, but warmer zones 5 and 6 on the Arbor Day map. 
   
When selecting plants, after looking at the hardiness zones, consider more specific locales.  Warmer zones (and colder ones) can exist within a hardiness zone. Plants near a building may be in a warmer zone due to heat loss from the building or the solar heat absorbed by it. A steep southern slope may be an entire hardiness zone or two warmer than adjacent level areas.  This is called the “microclimate” effect, and may be as small as a few square feet or a whole landscape. Often I find a landscape may have microclimates representing two or three hardiness zones. 
  
In between microclimates, and the hardiness zones or “macroclimates”, is the climate of a particular terrain or area, such as near a body of water, a mountain slope, or a valley.  This can be termed “mesoclimate’.  I like to think of microclimate as a particular part of a property; the mesoclimate as the whole property, neighborhood, or town; and the macroclimate or hardiness zone as seen on maps as a whole region of a state. 
   
When selecting plants, also you need to consider culture--conditions such as soil type and fertility. If the soil is heavy, wet, and has low fertility, for instance, it may stress the plant, resulting in winter injury. On the other hand, if the soil is too fertile, the woody plants may grow late into the season and not harden off properly. Again, the result is winter injury.     Herbaceous plants such as perennials are different.  Fertilizing late in the season may actually make them more vigorous and better able to withstand winter.
           
Another important point about herbaceous plants is that being below ground, above ground temperatures as reflected in hardiness zone maps are not a very accurate indicator of plant survival.  There is much latent heat in the ground that will keep these plants warmer than the air, especially if the heat is trapped by a few inches of snow.  This is the reason some zone 6 perennials (0 to -10 degrees F) can survive fine in zone 3 (-30 to -40) if sufficient snow cover.
   
Mulching can help moderate soil temperatures, similar to snow, preventing plant injury. If you select an exposed site where protective snow cover may blow off next winter, remember to mulch your plants late in the fall. Even a couple inches of mulch usually is sufficient.
   
Sites exposed to winter winds, especially those from the north or west, can cause desiccation or drying out of evergreens, resulting in leaf burn. If possible, pick a sheltered site or plan to shield plants with a burlap screen next winter. Tree wrap or guards on trunks will help protect trees exposed to early morning winter sun from "frost cracking."
   
In summary, keep in mind these few key principles towards better survival of your landscape plants during winter.
--Hardiness zones are only a starting point in choosing hardy plants, and that the climate of smaller areas on your property can influence their survival, as can variation from year to year.
--Hardiness zones are more applicable to woody plants, with parts in winter above ground, than perennials which overwinter below ground.  These are more influenced by snow cover and mulch, both of which trap the latent ground warmth.
--Culture affects plant vigor, which affects hardiness.  Avoid plant stresses such as too wet soils, fertility late in the season for woody plants, and provide wind protection for evergreens.
 

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