University of Vermont
Department of Plant and Soil Science

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CLIMATE CHANGE IN THE GARDEN: COPING WITH COLD

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

Daily the news has articles relating to a changing climate and extreme weather.  One aspect of this, important particularly to northern gardeners, relates to winter temperatures.  While cold temperatures in winter have always had effects on plants, this is even more pronounced now with extremes and fluctuating temperatures.  Choosing the right plants, the right sites, and the right care can better help landscape plants to survive extreme winter conditions.

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 minimum).

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 the spring. 

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 less hardy.

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 off.

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 damaged.

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.

Proper plant care and hardy cultivars will help your landscape survive winter’s extremes.

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