When winter comes, we who live in the temperate zone button up our overcoats, don stadium boots and perhaps even wear earmuffs to keep out the cold. That’s our way of coping with the climate in which we live. But how do plants living in the temperate zone cope with the cold season?
Plant species have evolved four distinct ways of meeting frosty winters: annuals die, leaving survival to their seeds; herbaceous perennials withdraw their sap into food-stored roots, which are insulated by the surrounding earth, and grow new stems the next spring; deciduous trees and shrubs withdraw into their woody stems, which are filled with anti-freeze sap, and abandon their leaves; evergreens create anti-freeze conditions even in their leaves, some of which are so well protected that they survive even in the Arctic Circle.
For each group of plants there is a limit beyond which its methods fail. The willow and the mimosa are both deciduous trees able to survive frost by the same methods, yet the first grows within the Arctic Circle, the second not much north of Virginia. This varying ability is the measure of the natural hardiness of the particular species, beyond which it is not reasonable to expect it to thrive.
Plants at the extreme limit of their hardiness, although they stay alive, may be valueless for their particular purpose.
There may not be a sufficient number of growing days when the temperature is above 40 degrees F. during the summer. In England, for instance, grapes seldom fruit, even though the winters are mild and the vines persist year after year. In northern New England, chestnuts and hickories grow but seldom ripen their nuts.
Trees and shrubs make a limp and sap-filled growth in spring, but by autumn this has become stiff and woody. It is then said to have “hardened off” and can be expected to go through the winter safely. Much less actual moisture is left in the stems and fewer ice crystals can form within them to cut and tear the tissues. The liquid which “remains contains more sugar in the fall than it did in the spring. Like the winter mixture in your automobile radiator – it requires a temperature fur lower than 32 degrees F. to freeze.
Cold weather is harmless only when it comes at the accustomed time, when plants have prepared for it. As spring advances. however, plants can support less and less cold without damage. Apple trees, for instance, can live through zero winters, but their flower buds just opening will blight at 25 degrees, open flowers at 27 degrees and immature green fruit at 29 degrees. If by some unlikely freak a ten-day blast of February cold descended on a mid-June countryside, it would be as great a cataclysm as the world has ever known. It might take a million years to evolve from the few plant survivors a new flora as rich as the present one.
Your particular garden climate is not permanent. The general average may be unchanged, but an average year may have a particularly cold winter, with an unusually warm summer to offset it. Even a difference of 5 degrees in February temperatures may kill a prized garden favorite. The shade of a tree. the shelter of a rock, even the frost-free elevation of a foot-high terrace can make all the difference between life and death. A spring frost frequently kills only low-lying plants; it has been known to kill only the lower buds of a plant. leaving the top buds unharmed.
The draw of wind up a valley may throw much more rain on one hillside than on the opposite one, only a mile or two distant. The banks of small ponds are often shrouded with morning mists. which can delay spring flowers, avert fall frosts and mitigate summer droughts in nearby gardens. Whole city and suburban areas may be affected in much the same way by smoky fog. If you live on a hill. you should allow for winter tempera tures about 1 degree colder for every 250 feet your garden may lie above the surrounding countryside. By knowing your climate, with its local factors, you can keep damage to a minimum. but of course you cannot provide for every possible extreme of weather.
How can you protect your tender plants from frost? You have six simple, inexpensive weapons: shade, drainage, windbreaks, mulching, pruning and watering.
First of all, choose a site a little higher than ordinary garden level to avoid frost pockets. – Cold air is heavy, runs off like water from even a slight slope or elevation and collects in low places. Shade or a northern slope will delay spring growth and thus avoid late frosts. Drainage will encourage autumn ripening. Windbreaks will frustrate drying winds. Avoid pruning and watering late in the summer. Avoid having the soil too fertile around a doubtful plant. The last three factors induce a soft growth dangerously late in the season.
Warm, sunny corners are only for extra-hardy plants which can stand rigorous conditions. Wrapping gives both shade and windbreak?very useful tools indeed. In the north boxwood is commonly covered with a little house of burlap. This does not increase the warmth but tends to keep the plant cool despite occasional winter thaws. Thus its dormancy, which protects it from winter’s cold, is not disturbed.
Too much shade has no easy remedy unless. plants can be transplanted to sunnier sites. or intervening branches pruned away. Some benefit results from pruning out branches to open bays which will let light into the interior of a plant; this does not increase the amount of sunlight which falls on the plant but can increase the number of square inches of leaf surface which receive sunlight.
Too much sunlight is not only useless, but positively harmful. It destroys the food-making substance (chlorophyll), and causes excessive evaporation. The more light. the greater the loss. In the case of sun-loving plants, this is more than made up in increased growth. Tolerant plants such as the spiderwort can easily take care of it. Real shade lovers. however. may die if set in full sun. There is a group of plants, which includes the ladyslipper, for which at least part shade is utterly essential, a quality quite different from mere tolerance of shade. Light requirements for the same plant vary with the location. In the weaker sunlight of the north some shade-loving plants may demand more light; in the hot South, sun-lovers may do better with a little shade.
Because lawns fail under old elms and maples it does not follow that grass will not survive shade. Starvation and thirst. not shade, usually cause the lawn to fail there; the trees simply take everything from the soil. Because the leaves of some broadleaf evergreens. particularly Rhododendron maximum, may become badly burned and discolored in full sun it does not follow that the plant is a real shade-lover. The burns are caused by hot March sunshine on frozen leaves and while unsightly are not seriously harmful to the plant. Rhododendrons have poor foliage in sun, poor flowers in shade. A compromise – light shade, airy and open – solves this particular problem. Even plants which seem fully tolerant of shade may not really be so. Although tomatoes may flower and fruit in half-shade apparently as well as in full sun, their vitamin content drops.
Not only how much light but how long – how many hours a day – is important to some perennials. On the Equator one can depend upon 12 hours of daylight all year round; summer days grow longer as one goes north. When southern plants such as cosmos, chrysanthemums and dahlias are brought north into 15 or 16 hours of daylight, they are unable to produce their normal midsummer blooms. Not until autumn, when the days shorten to their familiar 12-hour length, do they flower, although they can be tricked into it earlier by artificial shading. Similarly, northern plants such as bearded iris and evening primroses. which need long days to produce blossoms, never bloom at all in the Deep South.
Some flowers open and close with startling regularity at certain times of the day, like garden clocks. Careful timing in a garden will give a whole series of overlapping intervals. but the timing may he quite different next year. It will certainly be different for another gardener 100 miles away or at a few hundred feet greater elevation. Portulaca, for example, clocked by the botanist Loudon as opening at 9:10 A.M. and closing at 11:12 P.M., may actually open anytime around ten o’clock in the morning and close anytime after five o’clock.
Perhaps one might better attempt a garden thermometer than a clock, for heat has just as much effect as light on plant movements. In winter, rhododendrons of different species might give the temperature within a few degrees. Rhododendron carolinianum curls its leaves long before R. maximum. Curling depends also, however; on how dry the air is; drought makes leaves curl, too.
During a drought a plant adapts itself to scarcity of water; the leaves stop growing, the entire chemistry of the cells is profoundly altered, even the roots become preternaturally sensitive to moisture. Now, suppose you give the plant a good hosing, in casual pity. Its whole chemistry slowly changes back to an economy of plenty. But no local saturation lasts long and when it is gone the roots are instantly parched; there are no remaining outlying sources to be reached. It takes days for the former drought preparations to be restored, and the plant cannot do.it in time. Leaves curl, stems droop. Too late now for anything but the epitaph: killed by kindness. Mulching and shade are far safer than watering.
On the other hand, very light spraying – not enough to wet the soil – is very effective against heat. A fountain or a lawn sprinkler will cool a summer evening by 10 degrees. It is still more effective on frosty spring nights. for it stirs the air and prevents frost from forming. By taking a variety of precautions a foresighted gardener can thus guard his tender early vegetables against freak weather.
