February 21, 2018
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Hardiness zones key to winter survival

By Reeser Manley

We sit indoors and talk of the cold outside.
And every gust that gathers strength and heaves
Is a threat to the house. But the house has long been tried.
We think of the tree. If it never again has leaves,
We’ll know, we say, that this was the night it died.
It is very far north, we admit, to have brought the peach.
What comes over a man, is it soul or mind —
That to no limits and bounds he can stay confined?
You would say his ambition was to extend the reach
Clear to the Arctic of every living kind.
Why is his nature forever so hard to teach
That though there is no fixed line between wrong and right,
There are roughly zones whose laws must be obeyed?
There is nothing much we can do for the tree tonight,
But we can’t help feeling more than a little betrayed
That the northwest wind should rise to such a height
Just when the cold went down so many below.
The tree has no leaves and may never have them again.
We must wait till some months hence in the spring to know.
But if it is destined never again to grow,
It can blame this limitless trait in the hearts of men.

                                      — Robert Frost, “There Are Roughly Zones”

It is October. The garden has a month or so to go before winds blow the last water-soaked leaves from deciduous trees, except the oaks that hang on tenaciously to their russet foliage and the beeches with their papery copper-colored leaves that give voice to winter winds.

It is October and my thoughts turn to survival of the garden’s inhabitants, the winter hardiness of each plant. Will the flower buds of the redbud (Cercis canadensis) survive the coldest night to come and grace our early spring with its small purplish-pink, pealike flowers? It flowered this past spring, a rare joy in a Maine garden, but there have been years when it did not.

Our redbud is a selection from the northern limit of the species’ native range, central Illinois. A tree propagated from Georgia stock would have died the first winter, but this tree has grown into a large spreading specimen in the nine years since we planted it in Marjorie’s garden. I know why, having spent a winter in central Iowa; none of my 10 winters in Maine has been as cold.

But for a redbud, flower buds are not as cold hardy as stem tissue and vegetative buds. The plant may survive, but not flower. We can only hope.
There are other uncertainties. Will there be sufficient snow cover this winter to protect our fringetree’s roots from freeze damage?

Gardeners put a lot of stock in the hardiness zone ratings of garden plants; we know in which hardiness zone we garden. But recent winters have taught us that there is more to plant hardiness than just surviving 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 freezing tolerant than stems or leaves, can be killed by freezing tempera-tures 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 gar-den’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.

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