Protect tender plants with water, mulch, and shelter
Everyone knows that plants die if winter temperatures are too frigid for them to endure. But severe weather can pose a threat even to hardy plants. An early-season burst of bitter cold can shatter the cells of woody plants that haven't yet hardened off. Later in the season, those same plants could march through a similar cold snap in stride. Deeper into winter, cold, dry winds can draw the life from conifers or broad-leaved evergreens. Even warm spells can be perilous. High temperatures can evaporate the last reserves of moisture from the transpiring leaves of evergreens whose roots, locked in frozen ground, are unable to draw replenishing moisture from the soil.
Most hardy perennials could sleep through winter peacefully if tucked under a thick blanket of snow. But where snowfall is iffy, exposure to Jack Frost's full force may kill marginally hardy plants. In poorly draining soils, winter wet can rot the crown of hardy perennials. And the churning freeze-thaw cycles of early spring can easily heave plants—roots and all—from the ground. To complicate matters further, the tissues of some plants, particularly trees and shrubs, are more susceptible to cold temperatures in their youth or their first year or two after transplanting. Only when they've reached a certain level of maturity are they fully hardy.
My garden is subject to just about every one of those threats. So, to prepare marginally hardy or recently planted perennials, trees, and shrubs for winter, I make sure at-risk plants are deeply watered before the ground freezes. In addition, any recently transplanted or marginally hardy evergreens get a spray of an anti-transpirant, like Wilt-Pruf, to seal the microscopic openings in their leaves. When the ground has frozen, I give new plants—even those rated bone-hardy for my garden—a 2- to 4-inch blanket of mulch, either ground bark or, preferably, shredded leaves. I also use pine boughs or branches cut from the Christmas tree. These make an excellent, airy mulch for young hellebores or any fledgling evergreen perennial because they help moderate temperature changes and offer protection from the winter wind and sun.
Plants in need of special coddling—anything unlikely to survive winter's cold and wet—should be tucked into a custom, seasonal shelter before cold weather settles in, usually about late November in my garden. It doesn't have to be elaborate. I've used overturned plastic pots, lengths of burlap, shredded leaves, even a heavy-duty paper bag. Unless you make the effort to build an artistic shelter, chances are that an array of protected plants is going to look like a hastily abandoned campground. But I can live with the less-than-good looks for a year or two until a newly planted tree or shrub is well-established. Even so, any plantings that will need long-term coddling shouldn't be positioned prominently in the stark winter landscape. To avoid aesthetic crises, I tuck my tender treasures at the bottom of a gentle slope in the backyard, where they can't be seen from the house.