University of Vermont Extension
Department of Plant and Soil Science
Anytime News
Article
PLANT HARDINESS
Dr. Leonard Perry,
Extension Professor
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. 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.)
When using a hardiness map, remember the above
definition. These zones are averages
only. In colder years, plants listed as hardy in a particular zone may not
survive. In order to be safe, you really
need to pick plants based on the coldest temperatures, not the average, in your
area.
For instance, on the 1990 USDA
hardiness zone map, Burlington, Vermont is in zone 4b which
indicates average minimum temperatures get -20 to -25 degrees F in any
year (www.usna.usda.gov/Hardzone/ushzmap.html). Yet in 1994
temperatures reached -29 degrees
F, so would have injured trees and shrubs not hardy to this
temperature.
The most recent hardiness zone map
from the National Arbor Day foundation (www.arborday.org/media/Zones.cfm) shows
many areas such as Burlington
in a warmer zone, and a new USDA map in the works also may show many warmer
areas. So with Burlington in zone 5 (-10
to -20 degrees F) on the new map, zone 5 plants may be injured in years such as
1999 or 1996 when temperatures reached -22 and -23 degrees F,
respectively.
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."
One final point to consider is that
warmer zones 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 “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.
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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