Winter Gardening

Winter Gardening—Design Tips that Work
by Patricia Jonas*

Rosa sericea pteracantha Like most New Yorkers, I have a lot of practice utilizing every available inch of living space, and for me that includes a garden—a 560-square-foot roof terrace that is more than a third of my total space, and the principal reason I live where I do. I don't put away the garden for winter: given the price of Manhattan real estate, I can't simply forget about that much space for three or four months. During the cold season, I do try to catch up on gardening books, study catalogs, and plan for spring, of course. But with over 200 different perennials, trees, and shrubs in containers, I always find reasons to be outdoors.

Through a decade of trial and error, I've learned how to make my rooftop a four-season garden, a place of beauty that entices me to abandon the warmth of a fire and climb up two flights of stairs on even the shortest, bleakest days. Unless you hibernate or live in the very coldest and snowiest of climates, your garden, too—whether rooftop, container, or more conventional—can be designed to be inviting in winter.

Without Bounds

Every garden is particular to its site, so a good place to begin is with a description of mine. Enclosed on three sides, it is protected on the east and west by fences, and on the north by the white stucco wall of a penthouse. The view south is unobstructed, but undistinguished—not a view one rushes to the parapet to admire. Nevertheless, there is a natural forward movement to the edge that I have slowed by using groupings of containers to break up the space and create diagonal lines and paths through the garden. The deck is on two levels with a one-foot change in elevation, and raised beds built into three corners (a door opens into the fourth).

I might have stopped there if I had heeded typical plant palette recommendations for a rooftop. I stubbornly believed that although my garden might be contained, it would not be limited; and I would make a garden as rich and botanically interesting as a garden in the ground. So, I added more containers: four large oblong cedar planters (two hold a rose arch); three half-barrels; and about 65 additional planters of all sizes and shapes in wood, terra cotta, ceramic, stone, fiberglass, aluminum, plastic, and wire.

Container Considerations

Like most gardeners, I love terra cotta. It is reckless, maybe profligate, but I gamble and leave the sturdier pieces planted and outside in winter. Manufacturers advise protecting terra cotta from frost, but in my Zone 7 container garden, only a few pots have disintegrated or cracked in ten years, and only one was so expensive that I had any regrets—luck, maybe. I place the more fragile pots in the shadow of the parapet, protected from direct sun and the precipitous freezes and thaws that take a toll on both plants and pots.

The largest containers (for trees, shrubs, and roses) are long-lasting oak half-barrels, and cedar and redwood planters. It's a quick slide from beautifully weathered to falling apart, so I line wood planters with plastic landscape fabric to extend their life. An added benefit is that the plants dry out more slowly.

I also favor fiberglass planters that are reproduced in many pleasing classic shapes and finishes, and are a fraction of the weight of terra cotta and stone. Metal-impregnated fiberglass planters (available by mail-order from Claycraft Planters, 212-242-2903) start with patinas of exceptional character and actually age—a cast iron finish blends particularly well with terra cotta.

Containers lifted an inch or two above the deck permit more air circulation and prevent the bottom of the pots from freezing to the surface below. This is what pot feet are made for, but I improvise by evenly wedging pieces of scrap wood, branches, and pot shards under the pots so they don't wobble. One of the great advantages of container gardening is the ability to move plants into the limelight when they are looking their most glorious, and backstage when they are fading. I rotate my pots for best display during spring and summer, but in late fall I pull them all together in protective circles for winter, and use bubble wrap to fill the gaps and insulate between them. In December, I mulch with compost and lay evergreen branches over the pots.

While hardy plants remain in their containers throughout the year—as they would in the open garden—I over-winter tender perennials indoors in the pots too fragile to be left outside. I love the red blush the cold puts on the leaves of scented geraniums, so I usually procrastinate before bringing them in—sometimes until it is too late and they can't be separated from a frozen lollipop of earth.

Nigella damascena Conspicuous Pods, Distinctive Vines

Annuals are yanked up and composted during fall clean-up—no frost-blackened stems, withered leaves, and flowers turned to mush to churlishly advertise the end of the growing season. The only annuals that earn a reprieve are those with conspicuous seed pods to decorate the garden throughout the fall and winter. The clear sky-blue flowers of love-in-a-mist (Nigella damascena) give way to fragile seed capsules surrounded by whiskery bracteoles. Well after the reddish purple leaves of Perilla frutescens 'Atropurpurea' have fallen, the aromatic maroon seed spikes of the square-stemmed, three-foot-tall plants remain erect, and rustle in the wind through winter. Perilla self-seeds so abundantly that it can be a nuisance, but this is another great advantage of container gardening: most of the seed will not fall where it can germinate.

I apply the same rule to herbaceous perennials that die back to the ground that I do to spent annuals. During fall clean-up, I rid the garden of debris that might harbor overwintering insects and diseases. But I spare lots of seed heads and stems to remind me this is a quiet, not a dead, season, with plants living in seeds and under their mulch of boughs. I allow hips to ripen brick-red on the fall-blooming roses; the tiny, fluffy seed heads of Aster lateriflorus to linger on mounds of wiry stems; the polished mahogany rattles of Iris sibirica to tower on their sturdy stalks; and the garnet-brown, seedhead-topped spears of Penstemon digitalis 'Husker Red' to stand above semi-evergreen purple foliage.

Annual vines also do double duty as winter showpieces. Love-in-a-puff (Cardiospermum halicacabum) will cover a trellis in summer with bright green ferny foliage, but the real display begins in autumn when inconspicuous white flowers are followed by inflated seed capsules -- little balloons that cling to their vines all winter. The large pure white night-blooming flowers of the moonflower (Ipomoea alba) are not its only striking feature: left to ripen and harden, the fig-shaped seed capsules and twining vine decorate a fence and hang over the bare branches of a small peach tree.

Good Bones

A great deal has been written about how winter reveals the "bones" of the garden. Every garden needs a framework, of course, but the container garden can look especially desolate in winter with no trees or shrubs to anchor it. Noble shapes of conifers are particularly important in winter, and can add color as well as mass. The blue-gray foliage of Juniperus scopulorum 'Wichita Blue' and yellow-gold foliage of Chaemacyparis obtusa 'Crippsii' are an electrifying combination in full sun, and there are many other widely available, slow-growing conifers quite adaptable to container culture. If you are a container gardener, stay away from conifers not hardy to at least one zone colder than yours, and pay special attention to the species' sensitivity to wind.

A protected site for evergreens is extremely important. Variegated Daphne burkwoodii 'Carol Mackie' is beautiful throughout the year, but glows in winter light in a partially shady spot it shares with a diminutive Daphne cneorum 'Pygmaea Alba', whose fragrance scents the air in early spring. All evergreens continue to transpire through winter, and broad-leaf evergreens, especially, will be scorched if they lose excessive moisture to cold, drying winds. If a particularly harsh season is predicted, erect windbreaks. Or wrap the most vulnerable specimens entirely with burlap or a new polypropylene fabric called Bush Jacket (manufactured by Circlemoon Products and available through nurseries and garden centers), which is lighter, easier to handle, and more durable than burlap. Applications of anti-desiccant spray will also help. Above all, give plants a chance by keeping them well watered as they go into the cold season, and water on warm days during dry winters.

Bark and Branches

Choose deciduous trees and shrubs for interest in every season: not just for flowers and foliage, but for fragrance, interesting bark, fruit that persists through winter, and stems and branches that gracefully trace the sky. Complex branching patterns of an Acer palmatum hold the eye when the trees are bare, and the texture of papery exfoliating bark (the orange-brown bark of Acer griseum or the light tan and darker cinnamon-brown bark of Heptacodium miconioides, the seven-son flower) suits the season. Seven-son flower also has white flowers in autumn followed by remarkable small purple fruit held on its graceful peeling branches. After its leaves and purple-pink flowers fall, the arching, five-foot silvery stems of Lespedeza thunbergii 'Gibraltar' cascade like a frozen fountain. In very early spring, spring snowflake (Leucojum vernum) emerges with the tips of its white bells adorned with tiny drops of spring green.

Helleborus niger Bewitching Witchhazel

There are plants at other times of the year that equal their perfume, but nothing commands the stage like witchhazels when they debut in winter. If I had space I would grow three species: fall-blooming Hamamelis virginiana with bright yellow flowers, followed in January through March by the coppery orange-flowered Hamamelis x intermedia 'Jelena' and deep yellow- and bronze-flowered Hamamelis mollis. An underplanting of evergreen or semi-evergreen plants like barrenwort(Epimedium species), bigroot geranium (Geranium macrorrhizum), or Bergenia—all of which color bronze to red in winter—complement the scale and shades of the witchhazels. Patience is required to get them established, but hellebores are also good companions for witchhazels. They are among the very first herbaceous plants to flower; Christmas rose (Helleborus niger) begins blooming as the witchhazels fade, and the more uncommon Helleborus argutifolius is a standout all winter. Its green apple-colored flowers look especially fine with faintly scented white snowdrops (Galanthus nivalis).

There are very few plants I won't try in a container, and I often put their adaptability to the test. In fact, the smaller size of some trees and shrubs that will grow larger and more vigorously in the ground is perfectly suited to the scale of my garden. The most important thing I look for is interest in four seasons, and I haven't even begun to scratch the surface of plants that can pay the rent. What I've learned over the years is that there is no reason to put your garden away for the winter. With a little imagination and design savvy, a garden, no matter what its size or style, can offer beauty and pleasure all year round.

*Patricia Jonas, as Director of BBG's Gardener's Resource Center, Pat, who has a certificate in horticulture, enjoys the challenge of sharing her expertise with fellow plant fans.