The eucalyptus thrives in Florida and southern or central California, but winter kills in Atlanta, Ga. The strawberry tree, Arbutus unedo, can be found in Atlanta but not in Richmond. The evergreen magnolia is grown as far north as Cairo, Ill., Washington, D. C., or Cape May, N. J.
The sequoia can be planted in Louisville, Philadelphia, and New York. In the prairie provinces of Canada, where winter temperatures drop to 30° and 40° degrees Fahrenheit below zero, the choice of deciduous trees is sharply limited. WHY?
In the late fall in northern Virginia, I have bedded out young plants of Ligustrum coriaceum, the evergreen round-leaf Japanese privet, and lost almost everyone.
But mature plants at Westminster, Md., near the Pennsylvania line, came through the winter with no injury. WHY?
In November 1955, Pacific Northwest thermometers plunged from 70° to 20° degrees Fahrenheit within 24 hours.
Night running temperatures dipped to 10° and 20° degrees Fahrenheit for four days. Thousands of azaleas and rhododendrons, which had survived far lower temperatures in other years, were killed outright. WHY?
Two gardenias were set out, one at the bottom of a slope in full sun, the other in a sheltered nook on the north side of the house. The first died when winter set in, and the second lived. Once again, WHY?
The answer to all four “why’s” is contained in the meaning of winter hardiness.
Winter injury to plants is essentially the destruction of cell tissue, but it is not always due to sap freezing and the ice crystals breaking the cell walls.
More often, the cell dies from lack of water, desiccation resulting from the dehydrating effects of wind and sun, and water shifting within the cell.
Process Of Plant Hardiness
Jacob Levitt of the University of Missouri, a leading authority on plant hardiness, explains the process this way: in the fall, growth slows down, as does food manufacturing through photosynthesis, but some starch accumulates.
Activated by the progressive shortening of the day, enzymes within the cell begin to convert the insoluble starches to soluble sugars, which collect in the cell vacuole.
The freezing point of sugar in solution is below that of water, so the higher the sugar content, the lower the chance that ice will form within cell walls.
As the sugar concentration rises, water is drawn from the protoplasm into the vacuole, resulting in changes in the protein-water relationships in the protoplasm.
This increases the plant’s elasticity and becomes more resistant to the strains of expansion and contraction caused by temperature changes.
While this happens, water continues to move out of the cell and into the intercellular spaces. Here it can freeze solidly, which often happens, without damaging the cells.
For these transformations to occur, sufficient water in the tissues must remain more or less constant throughout the winter.
Water intake does not cease in cold weather, but frozen ground, intense sunshine, high winds, and evaporation through the bark, and in the case of evergreens, leaves can drop the water level too low.
Answers To The Four “Whys”
Applying these principles to our four “why’s,” we get the following answers:
Plants differ in their genetic (built-in) capacity to convert starch into sugar and render protoplasm elastic.
Whatever their winter hardiness for a given area, plants of a particular variety become more tolerant of low temperatures as they mature, partly because older plants stop growing earlier in the season and partly due to the law of survival of the fittest. The best-adapted plants survive the longest.
Winter Kill
Winter kill following sudden temperature drops is caused by insufficient sugar in the sap, which leaves the unprepared protoplasm brittle. Even in a normal winter, late summer or fall growth is likely to winterkill for the same reason.
With winter almost over, mild weather followed by a cold snap will injure and perhaps kill usually hardy plants.
The conversion of sugar back to starch and the increase in the water content of the protoplasm reduces a plant’s frost hardiness.
Plants of borderline hardiness will survive if protected from the dehydrating effects of the sun and wind.
How To Minimize Winter Damage To Plants
You can’t control the weather, but you can minimize the chances of winter damage to your plants. Here’s how:
1) Do not encourage late growth by summer or early fall fertilization, pruning, or watering in early fall. There is some experimental evidence that late growth caused by end-of-summer and early-fall rains may be hardened off and saved by applying potash and superphosphate. Nitrogen in the summer and early fall fertilization does the damage in most cases.
2) Broad-leaved evergreens like camellias, particularly susceptible to winter burn or flower bud damage, should have a northern or eastern exposure or be planted where there is high shade.
3) The trunks of newly set out trees should be spiral wrapped to prevent sunscald and splitting of the bark – which results from desiccation.
4) Plants of questionable hardiness should be planted on slopes where colder air will flow past them, never in low spots where frost pockets may develop on a cold night.
5) Mulch the ground under trees and shrubs, not to keep the soil warm, as some belief, but to stabilize its temperature and water supply. When exposed alternately to the sun’s heat by day and frost at night, soil undergoes rapid fluctuations which have unsettling effects on plants and make unavailable, through freezing, the moisture in the upper layer of soil.
6) If the season has been dry, water well, especially evergreens, just before the ground freezes.
7) Protect newly transplanted broad-leaved evergreens with burlap screens. Their ability to take winter temperatures may have been affected adversely by root injury when dug up. It is wise to spray them with WiltPruf, a plastic base liquid that leaves a thin, transparent film on the foliage, reducing transpiration.
Common sense will tell you that a healthy plant, adequately watered, fertilized and pruned, and growing in well-drained soil, is in better shape to come through the winter unharmed than a neglected plant in poor health.
44659 by Jacob Fisher