Twenty seven years ago, when I moved from the land of long winters near the coast of Maine to the land of “wait, was that winter?” in Kentucky, everyone asked me two questions. The first was about my personal bourbon preferences — and that’s a different column. But the second was, “how long did it take for you to lose your acclimation to Maine winters?”
My stock answer was, “the flight lasted about two hours!”
Fortunately, plants are more resilient than people.
When we have a whopper of a storm like the one that hit much of the country over the last week, my phone rings off the hook with the inevitable questions. “What is this going to do to my prized sugar maple or my azalea, or my fill-in-the-blank.
And the answer is, as it usually is, that it depends.
Generally, if you grow plants that are well adapted to your local climate, you’re in pretty good shape. Plants native to your locale have evolved under the local conditions and are generally a good match for the local winter. But many plants from far-off places can also do well in our winters if they come from places with similar (or more challenging) weather patterns.
But of course we all know that those color-saturated images online or in glossy mail-order catalogs tell you exactly nothing about how a plant will fare in your particular winter. We’re lured in by the pretty pictures and the effervescent descriptions — cold hardiness zone be damned! That’s when we get ourselves in trouble.
And while growing plants with a track record of success in your or very similar climates is a great place to start, it’s no guarantee. There are plenty of other things that go into how well a particular plant will make it through any particular winter.
So, let’s look at three of the factors involved with how plants respond to winter’s challenges.
How does snow impact plants?
There’s snow. There’s wind. There’s cold. So many factors to consider.
Snow is a blessing and a curse to garden plants. The blessing is the insulating value snow brings to an event. Generally, the way our winter storm conditions tend to work is that a low-pressure system rotates counterclockwise, bringing warm moist air up from southern regions to crash into cold air masses. That’s where the big snows develop. But as the system passes our location, the atmosphere’s rotation brings frigid northern airmasses in behind the storm.
Fortunately, snow, especially the light and fluffy kind, is a great insulator. With 6 or 8 inches of snow on the ground, the air temperatures can drop to below zero but the ground below that blanket of snow can remain unfrozen. That doesn’t help the trunks, stems and buds above the snow line, but it sure is nice for tree roots, herbaceous perennials and bulbs.
Ice covers a branch as snow falls early Monday morning as Winter Storm Blair moves through the area in Louisville, Kentucky. Jan. 6, 2025.
Of course, the biggest threat to tree and shrub topsides from a winter storm is breakage caused by the weight of snow and ice. Combine a coating of winter’s best with a bit of wind and you have a real recipe for disaster.
Then there’s the cold.
How does cold impact plants?
Vehicles slowly make their way through snowfall along Goss Ave. as Winter Storm Blair impacted Louisville, Ky. on Jan. 5, 2024.
In general, plants don’t feel much in the way of windchill. A densely branched shrub or evergreen plant covered with leaves can trap a little stagnant air around the plant to help a bit if there’s no wind, but this is generally negligible. Straight air temperature is the one to follow.
When it comes to air temperature, plants fall into one of three categories based on their tolerance of low temperatures. There are those that are freeze intolerant — most tropicals and annuals. If it goes below freezing, they’re toast.
Then there are plants that supercool — a process that limits the formation of ice crystals inside their cells. Supercoolers, depending on species, can withstand temperatures as low as -40 degrees Fahrenheit. Many garden plants have a critical low temperature limit somewhere between the freezing point (32 degrees Fahrenheit) and the supercooling limit.
There is a much shorter list of plants that can go well beyond the supercooling limit — all the way to the coldest temperatures found on the planet — by exporting most of the water to the spaces outside their cells where ice can form without damaging the cells.
Irene Nieto dodged a snowball in a snowball fight at Emerson Park Monday afternoon Jan. 6, 2024
But absolute temperature is still not the whole story. In order for plants to reach their full cold tolerance, they have to have a chance to acclimate to winter. Even if a sugar maple has lost all its leaves by Thanksgiving, its stems and buds would be killed by a November night below zero because it would not have gone through its full winter acclimation process. That same tree, subjected to gradual arrival of winter’s worst could easily handle -20 degrees in the middle of January.
Also, some of our well-intentioned garden practices can reduce a plant’s cold-temperature survival. Plants heavily pruned or unnecessarily/overly fertilized late in the summer or fall can be pushed into thinking more about growing than going to sleep for the winter. In some rare cases, it has been shown that even bright landscape lighting can reduce a plant’s winter tolerance by fooling the plant into thinking it is in the middle of summer’s long daylength season.
How does winter weather impact plants?
A snowboarder glides down the floodwall in New Albany, Indiana Monday after Winter Storm Blair moved through the metro Louisville area, dropping around seven inches of snow and a covering of sleet. Jan. 6, 2025.
And then there’s just plain old weather — which we all know is somewhat variable, to say the least.
A plant’s internal system for prepping for winter is all fine and good. But that system is impacted by external forces. Gradually diminishing daylength and cooling of day and night temperatures, both impact a plant’s acclimation to winter’s lowest temperatures. But if that doesn’t go as planned, the whole system can get significantly delayed.
A few years ago when we had the now infamous Christmas polar blast, plants all across the central part of the country were massively damaged. And this happened because of the rapid temperature drop (something like 60 degrees in a few hours), because it was in December (rather than January or February) and because November and early December had been unusually warm that year. A perfect storm, if you will.
The bottom line is, this fall and winter have pretty much followed a more average pattern. The recent storm hit in January. And while it’s been cold, it hasn’t been all that cold — as far as the plants are concerned. Cold damage should be minimal.
As for me, I think I may need to find another flight a couple hours south!
Paul Cappiello is the executive director at Yew Dell Botanical Gardens, 6220 Old Lagrange Road, yewdellgardens.org.
This article originally appeared on Louisville Courier Journal: How did the recent winter storm impact the plants in your garden?