The latest (2012) USDA hardiness zone map released shows much of
the country at least half a zone warmer
(planthardiness.ars.usda.gov). A similar map from the Arbor Day
Foundation (www.arborday.org/media/map_change.cfm) shows half of
many areas a full zone warmer since 1990. A study by scientist
Loarie and colleagues in the journal Nature in 2009 shows that, on
average globally, climate zones are moving northward about 3.8
feet per day. Over the past 50 years in Vermont, the facts
(alanbetts.com) show that winters have warmed twice as fast as
summer, with winter minimum temperatures increasing even faster.
We may have more need in the north to consider the AHS (American
Horticulture Society) heat zone maps as well as cold hardiness
maps when choosing plants. These, plus a map from Sunset publishing that groups regions by climate rather
than just temperature, are online from the AHS
(ahsgardening.org/gardening-resources/gardening-maps). These maps
may become an indicator of how far the weather is from “normal”,
rather than what to count on.
Even if some winters are warmer, there may be cold spells with
little or no snow cover, causing soil temperatures to drop. This
could result in death of formerly hardy perennials. Snow is a
great insulator, even just a few inches helping to trap and
maintain residual soil heat. It’s hard to believe, but even with
sub-zero air temperatures, there is enough ground heat with snow
cover to maintain soil temperatures around freezing or not much
below. This is the reason gardeners in a cold zone 3 climate (-30
degrees or below average winter minimum) may be able to grow
perennials listed as zone 5 (-10 to -20 degrees average winter
Since we can’t rely on snow cover to protect perennials, use
mulch. Only a couple inches of mulch can trap ground heat (up to
six inches or so is even better), keeping roots warmer and
preventing rapid temperature swings in soil temperature (the key
for perennials), particularly in the more crucial fall and
spring. Make sure not to mulch too deeply around tree trunks,
which will smother them, and remove mulch covering perennials in
Tender shrubs and roses can be mounded with straw (not hay, which
often contains weed seeds), IF you don’t have rodent issues. Mice
love to live in such a habitat, and chew on stems, killing plants
as a result. If mice are a problem, or to be safe, mound with
compost, soil or even manure, to protect bases and rose graft
unions. The latter are swollen bases on plants where a desirable
variety is grafted onto a more vigorous rootstock, a plant part
While, on average, winters may show a warming trend, some may
drop lower than normal. Extremes may be the new norm. So, while
we may get a stretch of several years of milder temperatures,
during which gardeners get away with more tender shrubs and trees,
that extreme winter may kill them.
Continuing to plant the hardiest varieties will provide some
protection again such extremes, as well as from abnormal late
frosts. For instance, if you live and garden in zone 5, to be safe
choose plants hardy to at least the colder zone 4. This is
particularly important for trees and shrubs that have to survive
above ground, without the protection perennials can have from snow
and mulch. Remember that cultivars (cultivated varieties) of a
plant can vary in hardiness.
Fruit trees, in particular, may be susceptible to late frosts. So, if the climate in your area is becoming warmer, allowing you to plant cherries or peaches for instance that you weren’t able to grow in the past, an abnormal late frost may kill flower buds and so the prospect of fruit. Such frosts will affect hardier fruits such as apples and pears, too. Make sure not to plant in “frost pockets” —low areas where frosts settle. Choose fruit cultivars with later spring blooms.
Wide temperature swings, particularly in fall and spring when
plants aren’t fully hardened, may cause damage. Choosing plants
adapted over a range of wide climate zones is a starting point.
This happened to one of my forsythias—one I’d had for over 15
years—killed one year by January temperatures in late March when
spring-flowering plants were already breaking dormancy and
becoming less hardened. There are some ways you can help protect
such early bloomers, evergreens, and others.
Keep woody plants, particularly broad-leaf evergreens such as
rhododendrons, well-watered prior to winter. This will help them
survive drought and long winters, and drying winds. September is a
crucial month to make sure such evergreens (especially new
plantings) get at least an inch of water a week, preferably more,
if not from rain then from your watering.
Use protected microclimates on a property for more tender plants,
such as some magnolias, butterfly bush, less hardy rose varieties,
and some hydrangeas. These are part of your landscape that have
some protection from cold and winter winds, such as near
foundations, the south sides of slopes, or protected by a fence or
hedge where snow may accumulate.
With warmer winter temperatures, particularly up and down
roller-coaster type swings, we often now get rain during winter,
which results in ice, or heavy wet snow. If a shrub is special,
or with stems prone to breaking under ice, protect it with a
wooden structure or teepee, or even wrapped in burlap (not plastic
that gets too hot on sunny days). Don’t try to knock ice off
branches, as this may cause even more injury or break branches.
You can try and gently shake snow off branches or gently brush it
Also, you can choose plants less susceptible to ice damage. Most
susceptible to ice storms are trees with broad crowns (tops) such
as green ash and honey locust; fast growers with weak growth such
as silver maples and poplars; and others with weak branching
patterns, such as American linden and pin oak. Trees with multiple
trunks, such as clump birches, and upright evergreens such as
white cedars are more susceptible to ice damage. Even shrubs such
as lilacs, if they still have leaves on in early fall when a heavy
wet snow comes (as happened in my landscape one year), will get
Some trees more tolerant of ice and heavy snow include willows
(these tend to bend, not break), littleleaf linden, sugar maples,
spruces (and similar evergreens with a conical crown or growth
habit), and pines. Some shrubs most tolerant of ice and heavy
snow include quince, lilac, spirea, shrub dogwood, euonymus,
ninebark, potentilla, and weigela.
Keeping plants in good health, and pruned properly, will go a
long way to helping them survive such ice and heavy snows. A
great example is the ice storm in Montreal in 1998, which damaged
many stressed street trees but well-grown trees in the botanical
gardens survived with little damage.
Keep woody plants (trees and shrubs) well fertilized, but don’t
fertilize them after mid-summer. This allows them time to harden
properly before winter. On the other hand, herbaceous perennials
can be fertilized into fall. This helps them to go into winter
more vigorous (they die back to the ground in fall and harden off
in the soil), which our years of research with them in Vermont
show helps them to survive winter cold much better.
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