Many gardeners, even experienced ones, think of winter as a down time for their gardens. The flowers are gone, the leaves have fallen, evergreens huddle against the cold. Nothing is happening. In fact, quite the opposite is true. While blizzards rage and freezing winds lash the landscape, many other things are going on. Below the ground, in the roots and soil, the garden is stirring with activity. Early bulbs are swelling and putting out their first shoots, preparing to burst into the open air. Perennial plants, aided by the cold, are undergoing vital chemical changes that will equip them for a new cycle of growth. Seeds are similarly reacting to the cold and getting ready to sprout. Nutrients from fallen leaves and dead stems are working their way down into the soil with the water of melting snow and winter rain, enriching the underground environment.
Above-ground, winter is a time of special beauty. Berries sparkle on some shrubs, while the leaves of others take on the appearance of burnished bronze. The leafless branches of trees etch filigree patterns against the leaden sky or cast hard-edged shadows across freshly fallen snow in the slanting rays of a late afternoon sun. Their bark, hidden in summer by leaves, comes into its own in winter: silvery gray, white, green; deeply fissured, sleek as a seal or curiously pocked by a peeling surface.
Color is everywhere, and not always subdued. The winter stems of some young dogwood shrubs are bright red, others are yellow, purple or green. The wheat gold of dried grasses stands out in brilliant contrast against a backdrop of dark evergreens. As an unexpected bonus, there is even the surprising yellow of ribbon-like witch-hazel flowers, which bloom in midwinter, or the delicate lavenders and blues of tiny species crocuses crowding up through the January snow. “Lots of people don’t believe me,” one winter gardener reports, “but I have plants blooming outdoors every month of the year.” He lives in the cold Northeast.
In the forecourt of a Baltimore home, winter emphasizes the graceful form of a yellowwood tree and the interplay of textures and greens in clipped mugo pine, creeping juniper and dwarf Norway spruce.
Nature’s winter palette can be as handsome as the brighter colors of summer, as in the warm tobacco brown of the leaves of a European beech hedge in Buffalo, New York. The leaves will remain until new growth in spring forces them off.
Whether brightened by flowers or colored with a more subdued palette of greens, grays and browns, the sights of a garden in winter are so rewarding that one couple in a New York suburb set aside a corner of their garden simply for winter viewing. It’s framed by the window of an upstairs sitting room where they often talk and read, and its scale is small. At its back is a clifflike ledge into which they have tucked a few winter-hardy rock-garden plants, evergreen candytuft and thyme. At the base of the ledge they have constructed a small pool and surrounded it with creeping lily-turf, a grasslike plant that stays green all winter. Christmas ferns, a few Johnny- jump-ups and some winter aconites complete the garden’s horticultural elements, but there are still other visual attractions.
A JANUARY WATERFALL
There is in this garden a small stone sculpture that collects a picturesque mantle of snow, and a redwood garden chair — a reassuring sight in the dead of winter and an inviting one on a sunny day during a January thaw. Feeding the pool is a waterfall that can be turned on at any time during the winter, its water warmed by a heating cable wrapped around a pipe where it passes through the garage. The trickling water adds movement and sound to the garden, and also attracts birds. At night the garden can be gently floodlit, but this is a feature its owners seldom use. “There is a family of raccoons living up on the ledge,” one explained. “At night they like to come down to the pool to drink — quite a sight when the moon is out and there is snow on the ground.”
Creating a. garden to be enjoyed in winter, as this couple has learned, means coming to terms with nature. Where winters are mild this accommodation is relatively easy. Southern gardeners some times contend with sudden changes in the weather at least as treacherous as those of the North, but their lawns remain green and so do many of their plants. Leaves linger longer on shrubs and trees and something is always in bloom — indeed, there are some plants that bloom at no other season. In the North, however, gardeners must provide landscape interest with far fewer plants and rely far more on the garden’s “bare bones” — its basic plantings, such as hedges and tall trees, and constructed elements such as walls and paths — to satisfy the eye.
GARDEN GEOMETRY
A winter garden in the North is seen largely as a composition of rectangles and circles, horizontals and verticals. There are the stark outlines of plant beds, the sinuous curves of terraces and hedges, the bold uprights of fences, trees and walls. For its decorative effects it may draw upon the fleeting patterns of melting snow on an undulating lawn or the long shadows cast by the slanting winter sun. A well- conceived plan allows enjoyment of the garden from a cozy vantage point in the house but also lures the viewer out for a wintry stroll. The 19th Century English poet John Clare evoked the quiet pleasures of a snowy garden,…
….where gravel pathways creep between
…Arches of evergreen that scarce let through
A single feather of the driving storm;
And in the bitterest day that ever blew
The walk will find some places still and warm
Where dead leaves rustle sweet and give alarm
To little birds that flirt and start away.
A leisurely walk also brings into view subtler, finer details — the angular twigs of the winged euonymus or the swelling catkins of the pussy willow — that elude the arm-chair gardener.
With a rhododendron next to the door, this home has no need of a thermometer: the plant’s leaves curl up when the temperature dips below freezing. Swelling buds of two other plants, a red-twigged dogwood and a pussy willow, will signal winter’s end.
The plants themselves, with their varied colors and textures, give life and harmony to the geometrics of the basic design. Ever greens are appreciated for the differences in their colors — green, purple, silvery gray — or for the way their branches catch and hold the snow. The bare silhouettes of deciduous trees are admired for their distinctive characteristics, whether staunchly upright, gracefully pendulous or strangely contorted. And when frost strikes, beauty is to be found in what is left behind — in the reddish-bronze foliage of the drooping leucothoë, the angular lantern-like seed pods of the golden rain tree, the cinnamon-brown patches exposed by the peeling bark of the paper-bark maple. A broad expanse of shiny-leaved bearberry dusted with snow is a study in bronze, green and white.
WEATHERING THE COLD
But this winter beauty is accompanied by responsibilities. For a garden to get through the cold months in good health, an extra measure of care must be taken to give the plants a fighting chance against the elements. In the North, plants need to be protected from the dangers of sudden thaws, winterkill and sunscald; in the South they need protection from sudden frosts. Indeed, a sudden winter chill can be more devastating to gardens in South Carolina and California than to those in Minnesota and Maine. In the North, cold is relatively uniform; in the South it fluctuates so much that plants have a hard time adjusting to it. Gardeners in western Georgia still remember the winter of 1950-1951, when the hardest freeze in 52 years sent the temperature plummeting 72 degrees in one 24-hour period. “There we were, enjoying 68° weather one day,” recalls one Georgian, “and by the following morning the thermometer had gone all the way down to four below zero.” The sudden cold snap blew in on harsh north winds, and many broad-leaved evergreens main stays of Southern gardens were so badly injured that they lost all of their leaves. Some plants recovered, but many others did not survive the glacial temperatures.
All these winter-related concerns — as well as the measures for dealing with them — touch upon the question of hardiness, the ability of a plant to survive the lowest temperatures that occur in the region where it grows. What makes a plant hardy? How does it adjust to cold? What actually happens when it’s hit by frost? And how do plants “know” when it’s safe to emerge from dormancy?
For most hardy plants, the ability to survive cold is closely linked to dormancy, a condition roughly comparable to human sleep. During dormancy plants are alive; indeed, they are somewhat active. Evergreens, for example, continue to transpire moisture through their leaves or needles. A dormant plant’s metabolic processes continue but at a very slow rate. A dormant plant is also far less responsive to the external stimuli that promote new growth. What blocks this response continues to baffle scientists, but they do know that this period of reduced activity occurs in all plants; even those in the tropics have a period of dormancy during the dry sea son. Wherever it occurs, dormancy enables plants to survive a pro longed period of stress.
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Winter’s heaths and heathers:
The bonny heather that covers the moors of England and Scotland has long charmed American tourists. Now these hardy evergreens are gaining popularity in this country. They grow best in light, acid soil, and because they need a lot of moisture, they do well in areas that are snow-covered all winter but not bitterly cold.
In these climates, similar to those just south of the Great Lakes, their wide range of foliage colors makes heathers ideal for winter gardens. Some keep the same color all year, but in late October the green foliage of others turns purple, and their gold becomes deep orange, red or rust. These hues stand out in spectacular contrast against the snow.
If you prefer flowers in your winter garden, try heaths. These low-growing evergreens are so closely related to heathers that many gardeners call them by that name (page 80). But heaths include many winter blooming varieties, like those shown opposite. Hardy heaths bear white, lavender, pink and purple blooms throughout winter and early spring.
Pink clusters of King George heath and the pale flowers of Springwood White heath perk up a snowy landscape.
Bright hues for gray days:
Heathers and heaths (or Calluna and Erica, to give their generic names) are strikingly effective when interspersed in mass plantings of irregular shapes, so that their different foliage, flowers and heights create billowing mounds of varied colors and ragged outlines. The plants will do best in partial shade, sheltered from icy winds.
They should be deeply planted in the early spring, in moist but not wet soil, and watered several times weekly until their roots have taken firm hold. Early in spring, all winter-damaged wood should be trimmed away. Heathers and heaths lend them selves well to casual gardens that have winding paths along which winter wanderers can stroll to enjoy these bright shrubs on a leaden gray day.
Clusters of bell-shaped blooms spring from the foliage of Erica tetralix, a heath so hardy that if grows in Iceland.
The cheerful flowers of King George heath bloom from December to April.
(l) Unlike heathers that change color, Calluna vulgaris alba remains bright green throughout the winter. (r) In a late-arriving winter, the normally dark foliage of Bronze Beauty heather retains its green coloration.
(l) Feathery sprays of red foliage rise from a bed of Blaze Away heather to form a pattern in the snow. ( r ) Double flowers of a soft rose color cover H. E. Beale, a heather that grows up to 18 inches high.
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THE SPAN OF DORMANCY
Dormancy lasts in plants for varying lengths of time and begins at varying seasons of the year. Spring-flowering bulbs, for example, start to become dormant when the foliage dies back in midsummer and don’t emerge from their period of rest until midwinter, when the chilling of the soil coupled with the increasing amount of sunlight stimulates them to resume their annual cycle of growth. But however long it lasts, dormancy always occurs in several stages, the first few of which are more or less invisible.
To prepare for their rest period, plants enter a period of intense activity. First they manufacture and store extra proteins, sugars and fats in their cells to tide them over the lean months and give them the energy they need to burst forth in the spring. Then if they are to bloom early the following year, they will set flower buds so they will be ready to move quickly as soon as their dormant period is over. Leaf buds too are formed, many of which contain in miniature all the preformed leaves and stems for the next year’s growth. Only after these preliminaries do they begin to show signs of slowing down: stems stop growing, and deciduous plants shed their leaves. As winter sets in, some perennial plants vanish from sight, leaving only the crown to indicate their continued presence in the garden.
WINTRY ADAPTATIONS
Underlying these various physical maneuvers is a series of chemical changes in the plant. Some of these changes are central to hardiness and help to explain why certain plants can survive sub freezing temperatures while others, the tender plants, cannot. Like much else in nature this ability, or lack of it, is a product of evolution. Scientists think that the ancestors of all of today’s plants originated when the earth’s climate was warm and humid and temperatures varied little in the course of a year. In the eons that followed, the climate cooled, and plants that could adapt to cyclical changes in temperature were the ones that persisted and reproduced. Some plants, the annuals, survived the winter in the form of seeds, which the parent plants produced and distributed in a single season before they died. Others persisted in the form of bulbs into which were packed in embryonic form all the flowers, stems, leaves and other plant parts. Still others kept their woody structure intact but went through a hardening process that protected them from being injured by cold.
In exuberant full bloom, Vivellii and Springwood White heat/is cover the terraced slope of a Seattle home. The bare trees above are Italian plume.
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Turning plants off and on:
Evolution has programmed cold-climate plants to survive winter by going into a period of dormancy. Tissues gradually harden as days grow short and temperatures decline in autumn. Chemical activity continues within the plant at a slow pace until the growth-inhibiting hormones of fall and winter give way to the growth-promoting hormones of spring. This prevents the catastrophe of a seed or bud sprouting too early, only to greet a killing frost. Evolution has also ordained that the winter-to-spring chemical changeover be completed only after a specific period of chilling. Without enough hours of the right degree of cold, a plant cannot fully break its dormancy.
DRESSED FOR THE COLD
As winter approaches, plants protect embryonic tissues with tight coverings over compact shapes: buds with scales (near right), bulbs with tunics (center right) or seeds with coats (far right). These coverings contain hormones that inhibit germination of embryonic parts until the danger of freezing has passed. They also maintain a precise water balance that reduces susceptibility to frost and keeps the plant from drying out. [BUD BULB SEED]
A CHILLING REVIVAL
The chemical changes that enable plants to break dormancy occur at temperatures ranging from 32° to 49° (near right); each plant needs a measurable period of chilling in this critical range to produce full growth in spring (far right). The hours of chilling needed depends on the climate of the plant’s native region. Insufficiently chilled plants produce weak growth, bloom sporadically, and die after a few seasons.
NIPPED IN THE BUD
When temperatures suddenly drop before a plant is fully dormant, the heart of a bud (near right), the cambium tissue or even the pith of a branch (far right) dies and turns brown. These tissues won’t produce new growth in spring. Plants need a gradual drop in temperatures in order to harden properly, a process involving reduction of water content and storage of enough nutrients to carry them through dormancy. HEART; DEAD CAMBIUM TISSUE/PITH; LIVING CAMBIUM TISSUE
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Not all plants of a particular kind are equally hardy every- VARIATIONS IN HARDINESS where. To be more precise, they are hardy in their natural environment. If this happens to be in an area where winters are extremely cold, they may also be hardy in areas less cold. But there is a risk in trying to grow many northern plants too far south. A crab apple tree in Florida, for instance, will fail to flower and fruit because it actually requires a certain period of cold weather to enable its normal growth cycle to continue. It may live for a few years, but will eventually succumb to the balmy climate (page 18). And a plant grown in an area where winter temperature readings go below those to which it’s accustomed will also have difficulty surviving. Some tropical plants, for example, cannot tolerate temperatures in what is called the chilling range, between 32° and 49°. In the subtropics, roughly equivalent to Zones 9 and 10 on the hardiness map, plants suffer if temperatures fall below 20°. On the other hand, there are plants such as species of primrose, lupine and cinquefoil that live in the frigid cold of Antarctica and survive temperatures as low as -30°, bursting into bloom in the relative 34° warmth of the brief Antarctic summer.
Certain favorable conditions — a moist, well-drained soil, for instance, and shelter from the wind — will occasionally combine to let a plant grow in temperatures that drop below its usual hardiness limit. Gardens frequently contain a plant or two like this, being pushed to the limit of its endurance. It may live through several winters, albeit with protection, but then succumb during an especially severe one. The term half-hardy is sometimes used to describe such plants, but strictly speaking it’s erroneous — a plant is either hardy in a particular climate zone or it’s not. (If a plant is not hardy, the correct term for it’s tender.)
BRANCHES BARED BY COLD
The terms semi-deciduous or semi-evergreen are often used to describe plants that normally lose their leaves, but will hold them if grown in climates warmer than those to which they are accustomed. An evergreen plant such as abelia, hardy to Zone 6, may lose its leaves in winter at the northern limit of its hardiness and may even be killed to the ground, then send up new stems the following season. The critical temperature in determining hardiness is slightly below the freezing point of water. Water is the main component of most plant cells, which also contain chemicals that act rather like antifreeze, lowering the freezing point a bit. Most gardeners are aware of the swift damage a sudden frost can bring: overnight, plants go limp and may even turn brown or black. It was once thought the plants were being killed by ice forming in their cells and expanding, rupturing the cell walls. But botanists now think that the damage is actually caused by dehydration. Ice forms between the cells, rather than inside them, and the water inside the cells passes through the cell walls and into the spaces between cells. Thus plants are injured or killed not simply by cold but by a lack of water caused by cold.
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MAPPING WINTER’S CHILL
A temperate-zone plant requires a certain number of hours at temperatures from 32 to 49 degrees to fully break its period of dormancy. This map, developed by Harry Swartz, a professor of pomology at the University of Maryland, charts the average hours of effective winter chilling in various parts of the United States. A plant grown too far south of its native area will grow poorly or die because it does not get sufficient chilling; a plant grown too far north of its range will suffer winter injury.
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Plants that are hardy routinely resist this water loss — and sometimes supposedly tender plants do too, surviving temperatures that dip as low as 20 degrees. What saves them in many cases is the larder of proteins, sugars and fats that the plant stores for winter in its cells. These chemicals make the cell sap more concentrated than it’s in other seasons of the year, and water is less likely to be drawn out of the cells. In addition, tender plants sometimes survive freezing temperatures if the weather has been dry. Annuals, for example, often live through the dry cold nights of Indian summer, even though temperatures drop to freezing.
DWINDLING DAYLIGHT
A plant’s ability to endure cold also varies with its condition at the time cold strikes. Indeed, if you subjected a supposedly hardy plant to freezing temperatures in midsummer, it would not be hardy at all but would die. Plants become hardy only when they have had a chance to acclimate themselves gradually to cold, a process that takes a long time. For most woody plants the prime trigger that sets the hardening process in motion is not the falling temperature, as most people suppose, but the shortening of days and lengthening of nights. Even if an autumn is unusually warm, these plants will prepare themselves for winter. This is clearly a better arrangement for plants, since the seasonal change in the length of days is constant, year in and year out, while temperatures are not. If plants took their hardening cues from the warm days of Indian summer they would be in deep trouble. On the other hand, the number of hours of daylight on September 1 in Des Moines, Iowa, is exactly the same every year. On September 2 that daylight will always diminish by exactly the same amount, and by, say, October 18, the day’s length will be identical to that of every other October 18.
The light-activated changes in a plant’s growth process affect many functions besides hardening. So hardening can scarcely be separated from the processes that cause plants to set leaf and flower buds for the next year and to ripen seeds. In the particular changes that affect hardiness, the plant shifts from the production of flowers and vegetative growth to the development of protective mechanisms. This is why fertilizing and pruning in the late summer are ill-advised — both encourage plants to keep on producing vulnerable new growth that won’t have sufficient time to harden before the arrival of winter.
A PROCESS COMPLETED
In the final stages of the hardening process, leaves of deciduous woody plants turn color and then drop; the plant is dormant and its hardening is complete. The danger now is not from winter cold but from thaws. As the days lengthen after the winter solstice and the plant begins the long process of preparing for spring, it will be most vulnerable during periods of midwinter warmth. Alternate freezing and thawing cause the soil to heave, thus breaking or exposing the roots, which are more sensitive to cold than the parts aboveground. For plants, as well as for virtually all other forms of life on earth, the daily weather report is of profound interest — and sometimes a matter of life and death.
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