Beware: Mid-Winter Thaws Can Damage Your Plants

Winters can be very variable. Mild some years, bitterly cold in others … but they often switch from cold to warm to cold again, sometimes more than once.

You’d think the “warm” part—a midwinter thaw—would be good news and I suppose it could be if you hate cold weather or feel like barbecuing on your balcony. But a thaw can often cause as much damage to your garden plants as a cold snap, sometimes even more.

The Best-Case Situation

In a climate where winters are going to be cold anyway, the ideal situation for plants is when temperatures drop gradually in the fall and then remain relatively cold throughout the winter, then warm up slowly in spring. This allows the plants to harden off (acclimatize to the cold) gradually and to remain dormant throughout the winter, remaining more or less oblivious to cold until spring comes around. And cold hardy plants—plants like trees, shrubs and perennials, the ones you probably grow in your garden if you live in zone 7 and less—do best when they have a long, fairly cold winter. That’s simply their thing!

When Things Go Wrong

But the situation is rarely ideal. After a long, mild autumn, winter can arrive suddenly, before the plants have time to harden off. Or temperatures can go up and down like a pendulum all winter: warm, cold, warm, cold, etc.

And that’s what a midwinter thaw is, the mythical “January thaw” (although it sometimes occurs in February!). An exceptionally warm period during an otherwise cold winter. By definition, temperatures have to have been below freezing for awhile, then rise above it, for it to be considered a thaw.

Seesawing temperatures are never good for plants, even in summer, but the effect is much worse in winter, especially if the thaw lasts long enough for plants to start losing the cold acclimation they so carefully acquired during the fall. The longer the thaw lasts and the warmer it becomes, the greater the potential harm to the plants.

What Happens to Gardens During a Thaw?

Here are a few of the things that can happen in your garden during a thaw.

• The snow becomes wet and heavy. When snow falls, it’s often light and fluffy or powdery, full of insulating air spaces, and it thus offers plants excellent cold protection, because the stagnant air trapped among the snow particles is a poor conductor of cold. During a thaw, however, the air spaces fill with water, either from snow melt or from the rain that often accompanies a thaw. This greatly diminishes the insulating quality of snow and its capacity to protect plants.
• The snow melts. Hopefully it won’t all melt, because snow really is beneficial for hardy plants, but it likely will melt here and there, leaving the plants in those patches even more exposed to future harsh conditions.
• The snow turns icy. When a snow charged with water (and lacking air spaces) is exposed to cold again, it turns icy, further reducing its insulating capacity. It looks like snow, but it’s essentially closer to ice: you can often walk on it without it collapsing. This causes a secondary problem. Even when plants are fully dormant, there is still a bit of respiration and ventilation taking place. If the snow turns to ice, though, any respiration and gas transfer pretty much end and this can weaken or kill the plants … something you’ll only discover come spring.
• Melting snow can lead to flooding, turning your garden into a pond. Very few land plants will tolerate spending the next few months under water and root rot can easily set in.

  

• Meltwater can cover the soil and then freeze solid with the return of cold weather. Solid ice contains even less air than icy snow and won’t let the plants breathe at all. Also, it’s a fairly poor insulator and any extreme cold to come will hit the plants full force. This is the worst possible situation and it often causes plant death through the double whammy of asphyxiation and cold damage.
• Frost cracks can appear on trees. This often occurs bark when a thaw is followed by extreme cold, especially when temperatures swing regularly between freezing and thawing. It mostly harms relatively young trees or trees with thin bark. Sometimes the split can be 3 feet (90 cm) or more in length and it never heals very readily.
• Branches caught in the snow are released. This is the only real benefit I can see of a winter thaw. If any branches were bent over or trapped in snow or ice during an earlier storm, the snow/ice may melt back enough so that they can start to straighten up. The branches too, once frozen and fragile, regain their flexibility. You can help by staking the branch so it isn’t caught a second time during an upcoming ice or snow storm.

  

• Freezing rain (which sometimes accompanies thawing) can cover branches with ice and cause them to bend or break. Don’t try to straighten ice-covered branches: you’ll only damage them. As in the previous point, if the thaw continues and the ice melts, allowing the branches to regain their flexibility, you can help straighten them out. If not, just let the branches bend all winter if necessary. Trying to break off the ice that covers branches will do much more serious damage than leaving the branch bent over.
• The plant’s cold acclimation decreases, especially if the thaw persists. Even a hardy plant, capable of surviving -40? F (-40? C) temperatures when properly hardened off, can be seriously damaged at 0? F (-17? C) if it starts to lose its cold acclimation. This will mostly be noticeable after a fairly long thaw, a week or so in duration.

  

• Plants actually wake up and start growing, even as early as January. Hey, why not? If the thaw lasts several weeks, many plants will figure it’s spring and start to sprout. Not all plants will be duped, though. Many hardy plants have a built-in calendar that keeps them dormant until they’ve undergone at least three months of cold, but others have no such restriction. Some early bloomers, like hellebores and bulbs such as snowdrops and narcissus, won’t be harmed if the cold returns even after they’ve started to grow, but other plants (most other perennials and shrubs) can be seriously damaged or even killed.

What Can You Do to Help?

There is relatively little you can do to help your plants during a thaw, but you can protect any especially fragile plants when the thaw has exposed them by covering them with snow or mulch, even a rose cone or plastic-lined geotextile. Of course, this is rather uncomfortable work to do in mushy, melting snow and nippy weather. You could have saved yourself a lot of work by mulching and protecting such plants in the fall.

If your Christmas tree is still around, chop it up and used its branches to cover fragile plants. If not, perhaps you have conifers from which you could harvest a few branches.

Try to walk as little as possible on thawing soils, because your weight will compact them terribly and can also damage plants and their roots, so stay on paths if you can. If you have to go work in the garden in the winter (ideally, you wouldn’t have to), wear snowshoes: they’ll spread out your weight considerably and help prevent damage.

Let Nature Cull the Weak Ones

If there are plants that are seriously suffering from a winter thaw, I’d personally tend to let just let them go rather than rushing out to protect them. I’m not one for preserving plants artificially. If any plants don’t appreciate my conditions, I’ll just replace them come spring with plants that do. So, I see a winter thaw as a hardiness test and highly appreciate the plants come through it in perfect shape. I guess that’s the laidback gardener in me coming out!

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However, whatever your gardening persuasion, if you’re being hit with a thaw, I wish you the best of luck!