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Plants don't enjoy January thaw like we do

Was that the January thaw that we experienced last week? It had the most important feature of the folkloric thaw -- unnaturally warm temperatures -- but it certainly was a short one.

To see what else we might expect, I probed some weather statistics as well as reports from Cornell's Earth and Atmospheric Sciences Department, but basically turned up what I'd always heard. The "January thaw" is a popular reference to a period of mild weather that's supposed to occur in late January in the Northeast (the mean date Jan. 25-30).

Statistically, it happens about 80 percent of the time. The stats say it lasts for two to three days, normally, with average temperatures 10 degrees warmer than usual.

So if you were scurrying around last week, doing the chores you missed in November and early December (before the snow piled up), maybe you were indeed enjoying the "January thaw."

And maybe not. Maybe it was just the big thaw's movie-trailer, a preview of things to come.

>A benefit or a problem?

Sometimes I'm annoyed by the knee-jerk assertions of TV commentators about the "wonderful, mild weather" when it's quite the opposite for plants (not to mention skiers or people who would rather not have the kids and dogs track mud into the house).

Just because it's a little more comfortable to walk outside (in the rain) doesn't mean that 45 degrees in January is terrific. It's not necessarily so.

The ideal winter weather pattern, from the horticulturist viewpoint, is a solid snow cover, and steady, freezing temperatures -- as we enjoyed for most of December.

Snow is a wonderful mulch that ameliorates even some extreme temperature fluctuations. Under the snow, plants do not heave out of the ground, and the roots aren't at risk of drying out from exposure to subzero temperatures. Plant roots that stay solidly frozen neither rot nor desiccate.

So here's a cheer for freezing temperatures and a blanket of snow -- those are wonderful!

What happens to plants during a thaw is somewhat insidious: It may seem beneficial for tree or shrub roots, or even turfgrass, to get a "drink" in the middle of winter. And that may be fine, if the mild temperatures drop back down gradually, so the water drains out of the plant cells, and the plants harden off again.

But if those plants become hydrated, and there is a sudden freeze -- zap -- some awful things happen.

In the case of woody plants, especially thin-barked maples, the hydrated cells that warmed up in the daytime suddenly freeze and literally explode at night. Hikers report hearing loud cracks at night in the forest -- the sound of bark splitting.

What you see are vertical cracks down the tree trunks, facing the direction of the daytime sun that warmed them. Besides bark, buds and a lot of other plant tissue also are damaged.

For turfgrass, the thaw presents similar problems that are a big deal to golf course managers, sports field professionals and some homeowners. The thaws and rain leave standing water on the fields, fairways and greens, damaging the small grass plants. (Turf is, after all, a sea of small plants.)

The problem is worse when ice sheets form, which leads to plant suffocation, called "anoxia." Especially when soil isn't frozen, the microbial organisms in the soil continue to use oxygen, and the carbon dioxide level increases under the ice.

If those conditions continue for long, the grass suffers serious setbacks. Golf course managers often check under the ice for a sickly sweet smell, a clue to those anaerobic conditions. Sometimes they have to de-ice the putting greens -- not easy.

At home, if your lawn is stressed for a period of time under sheets of ice, the plants may lose their hardiness and are prone to other turfgrass diseases you won't notice until spring.

The perennials and bulbs that went to sleep in the garden are also endangered by thaws.

First, rapid temperature extremes cause the soil to heave and buckle. If that movement opens up air pockets around plant roots, or lifts them above soil level, it may kill the plant.

Newer plantings are most at risk because they don't have strong, anchoring root systems. When we lose our snow cover it's especially important to gently step or pat down the raised perennial root balls.

The second problem induced by a winter thaw is the accompanying rain and standing water in a poorly drained garden. Many plants die from poor drainage in winter. The clue in the reference book or on the plant tag was "does not tolerate wet feet" or "requires excellent drainage."

>Playing catch-up

In spite of all those concerns, it was still an irresistible pleasure to walk around the garden and catch up on some unfinished tasks. I can't be the only gardener who was out there in the light rain, in my mud boots, peeking and poking around. I love and miss my garden when it's shut down and buried.

I found a couple of recently planted perennials slightly heaved, with a slight crack evident where the potted plant met the soil around it. Oops; I should have mulched better before the snow came. (It's so inconvenient for a gardener to have a life, isn't it? Family, jobs, social events )

So this was the time to finish the job. I dumped saved-up bags of pine needles and chopped leaves over all the perennials and under the shrubs. In a couple of weedy areas, I yanked out what I could and then put thick newspaper around the plants before spreading the mulch on top.

Another discovery was that this season the deer population came closer to my house than ever before, in spite of an exuberant dog's presence. I welcome the deer and am glad to share my country acres, but still I'm determined to grow certain deer-prone plants.

Now, many shrubs tips are gone. No Witch Hazel blooms this spring. I sprinkled a liberal dose of Liquid Fence where it will still count (and it does help if you keep at it).

And -- this reads like a confessional -- I finally put a Shrub Coat teepee over a dwarf white pine that I should have protected from wind burn six weeks ago. I'm also thinking about future fences.

Gardening and landscape care is a 12-month process, and the January thaw and other winter threats are just part of the game. At least, if we get a second meltdown, I (and perhaps you) will be a little more prepared next time.


Sally Cunningham is a garden writer, lecturer and consultant.

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