If you have looked at a rhododendron this week, you’ve seen one species’ dramatic response to severely cold weather. Few plants can look quite as pathetic as a rhodie at zero degrees. Seeing those curled, drooping, formerly fleshy leaves, it’s tempting to anthropomorphize: The plant is hugging itself, huddling in the snow to keep warm.

Your rhododendron is probably fine in spite of the drama, but many other plants may be badly damaged in winter without showing obvious symptoms. A Japanese maple has no fleshy leaves with which to “hug” itself, but its bark and roots may be incurring potentially fatal damage. A dwarf conifer is bright green now but may turn brown and drop all its needles next spring. Severe winter weather harms different plants in different ways, at different times and to a variable extent.

Because of these frigid days of early January, some of our landscape plants will be case studies for winter damage or death later in 2014.

How winter does its damage

When spring comes we will all assess how our plants came through the winter. Typically, we will blame problems on the obvious – recent weather or a new insect sighting. Or we will blame the plant or the guy who sold it to us. We tend to have short-term memories; we obliterate what happened last season, the condition of the plant going into winter and the harsh weeks of winter weather like this one. So consider now (and retain for later) how winter weather damages our plants:

1. Smashed, broken, blown-down plants: Physical damage is the most obvious. Windstorms knock down trees or take out branches, especially on multitrunk, weak-wooded, badly sited, drought stressed, poorly pruned or poorly structured trees. Ice from roofs and snow from plows and blowers crush, disfigure or uproot landscape plants. Plant selection, placement and good landscaping are the prevention and cures – all topics for lifelong learning.

2. Evergreens turned brown: In late winter many evergreens – yews or arborvitaes, for example – turn brown (especially on the windward side of the plant) or they look bleached. The primary reason is that the winter winds and sunshine promote transpiration (water loss through the leaves/needles) while the roots are frozen and can’t replace the water. Also winter sun warms up plant tissues and activates cellular activities, leading to tissue damage when the temperature drops quickly. Sometimes during sunny but cold days, leaves use up their chlorophyll and can’t replace it, so that leaves appear bleached.

3. Bark cracking and splitting: This is “sun scald.” On sunny winter days, the bark heats up on the south or southwest side of a tree causing cambial activity (movement of fluids just under the outer bark). When the temperature drops quickly the cells burst and tissues die. Thin-barked and young trees are most vulnerable. Tree wraps and plant placement (blocked from direct winter sun) are preventive options.

4. Frost heaving: The smallest or most recently installed plants are most at risk from root desiccation – often fatal – when the soil freezes and thaws, heaving the root ball out of the ground or letting air in through cracks in the soil. Mulching helps prevent this. If heaving occurs, press the plants back into the soil as soon as possible.

5. Roots’ death from severe cold: Tree roots die when the surrounding soil drops to a particular temperature – the degree depending on the species. This is the reason we rely on a hardiness zone map and refer to our region as a Zone 5 or 6 USDA Hardiness Zone. All things being equal, Zone 5a hardy plants’ roots will die at minus 20 to minus 15 degrees. (Zone 5b = minus 15 to minus 10 degrees; Zone 6a = minus 10 to minus 5 degrees.) The lowest possible temperature a plant might experience is only one factor affecting its survival, but it’s important.

Fortunately, soil temperatures drop slower than air temperatures, or we would lose lots more plants in winter cold spells. Many of our plants would die if the roots reached zero degrees, but fortunately those roots are below ground and rarely reach that temperature. The roots freeze even slower if the soil is well mulched or snow covered, or if the soil was moist when it froze.

Acclimation is key

No matter how “hardy,” no plant would survive a New York winter without going through a seasonal change called “acclimation.” It’s logical and intuitive: If you take a tree that’s blooming in a February flower show and drop it outside, it will die from the extreme change, the shock. It wasn’t ready. But how a plant gets ready is rarely mentioned: Normally in fall, the short days and cooling temperatures signal a woody plant to stop growing. Then some freezing nights stop all growth. The plant goes dormant. If the plant were not dormant (our flower show take-home), a sudden freeze would cause the plant cells – full of water – to burst; the plant dies.

An acclimated plant does not die because water had time to move from inside the cell wall to the outside of the wall, where the formation of ice crystals isn’t a problem. And this cell dehydration triggers changes in the remaining cell sap, so a kind of antifreeze forms. Voila! Your plant does not die from gradually induced, even severe, freezing. Great system!

Predicting problems

Do you see now why we will see some dead plants later in 2014, even if you chose well, planted well and cared for them reasonably well? Some plants – especially unmulched, recently planted, not watered enough in fall, not fully acclimated – were exposed to severe temperatures that were just too low, or came too soon, or lasted too long. How many die depends upon so many factors.

Next week, in Part Two of this article, we’ll look at some of the plants species – including that dear, shivering rhododendron – to see which ones are most at risk. We will also look at the container plants we have stored outside or in garages, to assess how they may fare.

In the meantime, protect your family and pets, and give the birds some seed, suet and water.

Sally Cunningham is a garden writer, lecturer and consultant.