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Great Gardening by Sally Cunningham: Green Christmas may be problematic for some plants

Bing Crosby dreamed of a white one and Elvis had a blue one, but this Christmas is extraordinarily green. After the White Christmas nostalgia passes, and the skaters and skiers leave the room, most folks comment that they have been loving these weeks – no shoveling, no snow days, no snow piles crushing our shrubs, no plows in the night. A mild winter may also seem easier on our landscapes – but experienced gardeners and horticulture professionals know it is not necessarily so.

Ideal winter template

Considering the plants we use, both native and imported, in USDA Zone 5 (in which winter temperatures sometimes reach minus 20 degrees), the preferred winter weather pattern would go like this: First would come frosty nights in autumn. Then temperatures would lower gradually. The soil would freeze solid, and sometime in December snow would arrive – steadily and gradually. Preferably a few feet of snow would cover the ground for several months. Snow acts as a mulch that buffers extreme temperature changes. Finally the snow would melt steadily and gradually, the soil would thaw, and – voila! — Spring! No January thaw. No ice events followed by windstorms. No sudden April freezes.

Weather doesn’t usually cooperate to suit our wishes or our plants’ needs. Accordingly, right now December 2015 appears to be setting us up for some landscape and garden problems.

Here is how mild winter affects plants:

• Bulbs pop up prematurely: Most fall-planted bulbs need at least three months of dormancy. It’s the rest period. As daylight and temperatures both increase in spring, the bulbs start to grow. When weather stays warm too long in fall or warms up early in spring, many bulbs start pushing upward – yikes!, you say.

Among many out-if-sync plant behaviors this is the one most people ask about, but actually it’s the least of our worries. New bulb shoots are quite tough, the cell walls containing a kind of antifreeze. When the temperature drops again, growth ceases. Sometimes the tips of leaves or shoots look blackened, but we rarely lose the plants or future flowers.

• Perennials don’t quit or regrow: My perennial gardens have many plants that have green leaves much of the winter, and they look especially great now – hellebores, bergenias, epimediums, lamb’s ears, heucheras, gingers and more. Other perennials have new growth sprouting at the crowns. These will all survive, but I wish for the ground to freeze so I could finish mulching.

You may have spread mulch already, and that’s fine too. The reason to wait is that fungus diseases and rodents can thrive in warm, damp, well-mulched places (around plant crowns). So I’d rather wait. But one time or another, do mulch.

• Fungi thrive in moisture: Most plant diseases are fungus diseases, by far. (Bacterial diseases and viruses are much less common.) Most fungi thrive in damp conditions. For that reason we clean up debris around the roses and rake leaves off the lawn to discourage black spot, powdery mildew, and snow mold, for instance. When the soil is frozen, fungi stop reproducing but in a moist garden they sporulate like crazy. It’s too soon to worry about it, but a mild winter could mean more apple scab or other diseases of that ilk.

• Woody plants can de-acclimate: Plants are not usually killed by extremely cold temperatures unless they are planted way out of their hardiness ranges. Even a Japanese maple labeled for USDA hardiness Zone 6 usually does well even when soil temps dip far below zero.

Woody plants need to “acclimatize” and go dormant in fall, and it takes time. What kills plant roots is not the temperature but the temperature dropping suddenly. This can happen now or in January when the soil warms up and there is no snow cover. Then the plant can “de-acclimate,” get ready to grow, and a sudden temperature drop does massive damage to the roots. A solid mulch of snow or any kind of organic mulch (3 inches of bark mulch, for instance) makes all the difference.

A similar thing happens to tree bark in many species (especially thin-barked trees like some maples): The sun warms the bark one day, the cell walls change, the sap begins to run, and then – zap! – the temperature drops sharply and the cell walls burst making the bark split.

• Soil heaves and roots dry out: Desiccation is a major cause of plant death in all seasons. In a winter without snow, if the soil freezes and cracks, plant roots are exposed to the air. They dry out and die. Mulch usually keeps soil from cracking and heaving. If you find perennial or woody plant root systems pushing upward or tipping, you must press them earthward, to reconnect the roots to the soil. Consider keeping an unfrozen bag of compost in the garage to fill in any cracks.

• Shrubs or trees bud too soon: Recent shrub cultivars such as the lilac ‘Bloomerang’ were hybridized relying on the fact that spring flowering shrubs can bloom again if they have a long enough season. This fall many people reported forsythias reblooming and roses continuing to flower all through November. I’ll bet those ‘Bloomerangs’ did well too.

Will you lose the spring flowers? Maybe a few. Generally that acclimatizing, dormancy pattern will kick in and most buds will go to sleep until a more propitious moment sometime in late spring. Similarly, sometimes in spring the weather warms up too soon, tree buds (especially fruit trees) flesh out and then a freeze damages them. Budding too early followed by a typical May freeze may mean no or little fruit.

More worries?

If a person is inclined to worry there are even more downsides to a mild winter. Without a harsh, long freeze, we’ll probably have more fleas, ticks and emerald ash borers next year. Ice storms followed by wind are much more destructive than soft, lovely snow. The warm lake will produce massive lake-effect snowfall eventually. And worst of all: no frozen grapes = no ice wine.

So stop celebrating and worry about this terribly green Christmas. Or trust that most of our plants will be fine – and take a walk without your hat. Your choice.

Sally Cunningham is a garden writer, lecturer and consultant.