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December presented us and our plants with less severe weather than usual. Snow was a Christmas no-show, and temperatures were mostly moderate, so the soil has not yet frozen solid. Such a mild winter could increase the odds of survival for many garden perennials, trees and shrubs.

If this keeps up (which is unlikely), even plants left in containers will get through winter with a lower than usual mortality rate. Still, snowlessness isn't a guarantee of plant survival. We still have to help some plants survive -- just differently this winter.

>How winter works

The best of all possible winters would go like this: Ample rainfall in autumn would be followed by a steady pattern of nighttime temperatures below 32 degrees. The temperature would then drop gradually until the ground is frozen solid. The chill would continue, soon to include a long-lasting blanket of snow.

Snow acts as an insulation or mulch for landscape plants, so extreme temperatures don't damage their roots. Each plant species has its own level of tolerance for cold, but most of our landscape plants (Zone 5 hardiness) experience some root damage at minus 10 degrees. At minus 20 degrees -- rarely reached here, almost never when the plants are mulched -- many plants die.

Snow also protects dormant plants from the warmth of the sun. First, the snow keeps the plants from breaking dormancy: new growth during winter, as in a January thaw, wastes plants' energy and leads to extra dieback. Second, snow prevents the destructive pattern of freezing and thawing, expanding and contracting, that happens during a pattern of sunny days and freezing nights. As soil thaws and refreezes, plants become dislodged, and their crowns or roots are heaved upward, exposed to drying air. Death usually follows. A mulch of snow -- or any mulch -- can prevent that.

Snow also protects some plants from animal browsing, although this can work both ways. In a snowy winter, many short plants survive because their crowns and buds are hidden from nibbling bunnies and deer. We've all seen short hydrangeas or azaleas with their tops chewed off above the snow line, the only surviving flowers around their bottoms. On the other hand, a severely snowy winter may bury so many of the natural habitat plants that animals may be forced closer to homes, in search of exposed landscape shrubs. Right now, most animals still have accessible browse material to eat in more open areas.

>When to mulch

If there were ever a winter to have gathered and spread mulch, this is it. Whether you mulched already, or just stashed the leaves, straw bales, compost or wood chips for after the hard freeze, you're ahead of the pack. There is no hard and fast rule about the timing.

You may have heard it is best to mulch after the ground is frozen, because more water gets to the plant roots or because fungus diseases and nesting rodents might thrive under damp mulch. But if you took one of those last bright, glorious days to work in the yard and spread your mulch, you did well.

For those who haven't mulched, Christmas trees and boughs now offer an ideal answer. Even if you haven't used them for mulch before, the possibility of a snowless January would be reason enough to take some of these steps:

Cut off the Christmas tree boughs and lay them over the perennials, low shrubs and on the root areas of taller shrubs and trees. Ask for your neighbor's tree, too. Don't forget the greens on the mantle and in the planters, after the holiday arrangements become irrelevant.

Rake leaves you have missed. Leaves shouldn't remain packed on the lawn, anyway, as they foster some turfgrass fungus diseases. Spread leaves 2 to 3 inches thick on the garden.

On open landscape or flower beds, just lay used Christmas trees right on the beds to provide a weather buffer.

To protect rhododendrons, azaleas, Japanese pieris and vulnerable evergreens, create a winter windbreak with discarded Christmas trees. Even in a mild winter, windstorms damage evergreens, because they lose moisture through their leaves (needles) and their roots can't compensate by taking moisture in. Shrub coats or burlap barriers are alternatives, but Christmas trees are now free.

>The unplanted ones

You probably wouldn't believe how many people confess their plant sins to me, starting with, "I have these (plant names, or bulbs) still sitting in my driveway, I know I should have planted them. Now what should I do?"

Frankly, I have some leftovers, too, and it doesn't make us bad people. (We just couldn't resist that treasure at the fall fair or garden center or plant exchange!)

For best odds of survival, put the unplanted ones in one of these protected spots: against the house out of the wind (with some temperature moderation from the foundation), behind a Bilco door, in a trench stuffed with leaves, or in an unheated garage. Cluster the plants together. If they're outside, after they have frozen solid, put mulch on and around them. Check them at least once in January and February to see if they've dried out entirely and add a little water -- especially if there is a thaw and no snow. Most perennials will not make it as houseplants, as their life cycles require a cold, dormant period.

As for those unplanted bulbs, you still can stick them in the unfrozen soil and get them growing -- even if they won't produce the same huge flower as an October-planted bulb. Eventually they'll catch up, and it's better than storing them in the house. Or, for now, put them in the garage or refrigerator, labeled "Not for eating."

Merry, merry mulching for a happy new year.

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Sally Cunningham is a garden writer, lecturer and consultant.