Today is the first day of winter, and it has hardly snowed. Recent weather and the state of many plants give a feeling of October many perennial plants are still green and leafy, berries intact on the shrubs. Yet I know it's now winter because most gardens are already groomed, cut back, closed down. Even procrastinators, who avoided clean-up, bulb planting and putting away the bird baths, have finally done those things. I imagine this is what gardens look like in Philadelphia or Long Island USDA Zone 6 territory or further south.
What if winter continues this way?
Except for winter sports enthusiasts and business people depending on ski tourism, locals tend to like this weather. I'm hearing comments along these lines: "I want a white Christmas, but otherwise this is great! Everything is so much easier!"
For plants, a milder winter, temperature-wise, offers several advantages. Yet a winter without snow is not safer or better for plants. Let's consider the several ways that milder or snow-free winters can affect plant survival.
>> Temperature alone
If it were just a question of slightly milder temperatures, regional landscapes could be dotted with Kalmia, Deutzia, redbuds, Enkianthus and the most delicate southern magnolias (plants that often die when temperatures drop below minus 10 degrees). If soil pH and texture, and cultural conditions are otherwise suitable, these plants survive winters just like this December. Unfortunately, what really happens and professional landscapers will tell you the plant lover yearns for these plants, plants them and they thrive for a few years. Then zap! a winter comes with a mild spell followed by some minus-15 degree nights, and it's over for those plants.
Mild winter weather interspersed with sudden deep freezes is much more dangerous than extended, steady subzero temperatures.
For woody ornamental plants listed as Zone 6, we simply should not trust a few mild winters to remain the norm, even with a slow climate adjustment in progress. Woody plants are more expensive than herbaceous perennials, generally grow very big and can potentially live for many decades. So take your risks with perennials and tread carefully with your woody plant selection and placement. (A microclimate perhaps the east side of your house away from the wind, or a sheltered courtyard can still provide a haven for some Zone 6 or marginally hardy plants.)
>> Lack of snow
Which is more dangerous for your plants: minus 10 degrees and no snow, or minus 20 degrees and a heavy blanket of snow? Most of you know by now: The snow either prevents the soil around the plant roots from getting that cold, or it greatly slows the freezing process down so that minimal damage occurs. Plant roots are killed the quickest when they dry out, which happens when temperatures fluctuate and the soil heaves and cracks, exposing them. Gradual change is good. The snow blanket is a buffer; it makes change gradual.
Precipitation is also a factor. Snow isn't the best kind of precipitation (10 inches of snow equal to about 1 inch of rainfall), but if the air is freezing and there's no rain, snow is better than no precipitation at all. Melting snow provides water to roots that are waiting for it. Especially evergreens (that lose moisture through their needles all winter) need snow; during a slight thaw the snow provides water to the roots just in time to serve it up to the above-ground plant.
Lack of snow may also leave some plant crowns, lower trunks and branches more vulnerable than usual to animal damage. Right now the foraging deer and rabbits can nibble on all remaining above-ground plant parts; 2 feet of snow would have covered the plants. Squirrels can even dig up bulbs in the unfrozen ground this late in the year; they don't usually burrow through the snowdrifts. Most deer still have lots to eat in the fields (where habitat is still sufficient) so they may not be as dependent on your landscape shrubs yet, but they will eventually move in closer to yards with great foraging material. It's a good year for animals so far, and a smart year for us to figure out our management and repellent strategy.
While a snow-free winter has all the drawbacks horticulturally speaking that I have mentioned, here is a reason to rejoice: We will not see broken branches and cracked crowns caused by heavy snow landing on our shrubs. No snow-laden ice dams crashing on the foundation plants; no snow-blower tonnage on the holly; no snowplows ramming through the hedge at least if this weather continues.
>> What to do now
As I finish writing this piece a heavy rain is falling. The plants are soaking it up. If winter weather ever comes they will enter dormancy thoroughly hydrated. Groundcovers (Epimedium, lamium), mosses and leathery-leaved perennials (hellebores, euphorbias, Brunnera and Bergenia) have spread and are showing off their shiny, deep green leaves. The ground is still soft. Gardeners can still take steps to protect gardens and landscape trees or shrubs.
* Mulch: The most-touted advice is to mulch the soil over plants roots when the ground has frozen, but many gardeners do this sooner just to be finished for the season. If you have piles or bags of wood chips, pine bark, straw, pine needles, compost or leaves standing by, go ahead and spread it now, 3 inches thick around your plants. We can still bet that the soil will freeze soon. In two weeks you can pile on the Christmas tree branches and boughs from garlands or indoor decorations.
* Repel or block wildlife from dining and digging: Keep sprinkling products such as Deer-Away, Liquid Fence, coyote urine, garlic spray or whatever you use to repel animals. Lay chicken wire over bulb patches. Tie on the soap bars and scented dryer strips. Studies show that deer eventually get used to strange scents (or sounds or sights) so the best strategy is to keep changing the products.
* Use wraps, cages or Shrub Coats/Covers on vulnerable plants.
Thanks to a gentle start this winter, most gardens and landscapes are in great shape so far. We can improve their odds with a few simple, protective tactics. We are just (barely) in time.