In November 2006, I wrote about the coming winter, wondering if there would be sufficient snow cover to protect the fringe tree’s roots from freeze damage. Now I look out the window at that tree, a bit taller than last winter, its main trunks and branches trimmed with small white lights, and ask the same question, a foot of snow lying over the garden and more to come.

I wish for a winter-long blanket of snow over the garden, footprints and paw prints filled over and over again until I know the location of the garden’s shrubs only from memory of their proximity to the taller trees. When I shovel out the narrow path from porch steps to car, the white walls will rise until I cannot see over them.

Back in university days, I spent a winter comparing soil temperatures at a depth of 6 inches with and without snow cover.

I learned that temperatures in the root zone never dropped more than a degree or 2 below the freezing point of water if the soil was covered with a few inches of snow. On the other hand, roots were experiencing lethal temperatures in adjacent plots where snow was excluded.

Gardeners need to know that the fine nonwoody roots of many plant species, the roots most active in water and nutrient uptake during the growing season, are not as freezing-tolerant as woodier roots and stems. Winter survival of these fine roots in many species depends on insulation of the soil by snow.

Recent winters have taught us that there is more to plant winter survival than the average annual minimum temperature. We seem to be entering a period of more erratic winter weather, including more frequent thaw-freeze cycles. The insulating cover of snow that we once depended on to protect plant roots from freeze damage is lost or severely reduced during a midwinter thaw. If the thaw is quickly followed by extreme cold, plant roots can be killed by freezing temperatures that penetrate the bare ground.

Thus temperatures well within the range for a given hardiness zone may be lethal to the roots of plants recommended for that zone. When roots are lost to winter freezes, water uptake by the plant the next growing season may be severely limited, leading to branch die-back or death of the plant. Plant losses that we might blame on summer drought may, in fact, be related more to root loss in winter.

It is also important to realize that plant winter survival is not dependent solely on the average annual winter temperature; plants can be severely damaged or killed by a single freeze. More erratic winter weather patterns can produce a single Zone 4 night in an otherwise Zone 5 garden. The successful gardener knows the garden’s microclimates, sites such as garden beds located near warm walls where temperatures are moderated and marginally hardy plants could thrive.

As weather patterns change, current hardiness zone boundaries can serve only as a starting point in selecting plants with adequate winter hardiness. If you garden near the border of two hardiness zones, it would be wise to build your garden around plants recommended for the colder zone.

Let us all hope that this winter is more like those remembered by the oldest of gardeners, a winter when there is snow on the ground until warm April winds begin a slow melt. Meanwhile, I send to you, faithful reader, this image of the fringe tree in Marjorie’s garden, a constellation come to Earth at the edge of the garden. May your roots stay warm through the coming months.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.