Which roots will survive winter?

It is November. Last week’s winds blew the last water-soaked leaves from all deciduous trees except oaks that still hang on tenaciously to their russet foliage and beeches with papery copper-colored leaves that give voice to winter winds.

It is November, and my thoughts about the garden turn to survival of its inhabitants, the winter hardiness of each different plant. Will the flower buds of the redbud survive the coldest night to come? Will there be sufficient snow cover to protect the fringe tree’s roots from freeze damage? There are endless uncertainties.

Many of us put a lot of stock in the hardiness zone ratings of our garden plants; we know in which hardiness zone we garden. In Maine there are seven official hardiness zones, ranging from a small wedge of Zone 3a in the northeastern corner to a balmy Zone 6a at Lubec and Eastport. Borders between zones imply a change in the average annual minimum temperature. And based on the performance of garden plants around the country, recommendations are made about which plants should survive winter conditions in our gardens. If you live in Zone 3a, for example, it would be wise to avoid plants that cannot take -35 degrees F. In Zone 6a, however, -10 degrees is the lower limit. Or, it is supposed to be.

Recent winters have taught us that there is more to plants’ 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, much less tolerant of freezing than stems or leaves, 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 more related 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 freezing event. More erratic winter weather patterns can produce a single Zone 4 night in an otherwise Zone 5 garden. The serious 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, developed under old weather patterns, can only serve as a starting point in selecting plants with adequate winter hardiness. Certainly, 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.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.