The art and science of growing wildflowers beneath the trees
A spectacular mid-spring display of wood phlox (P. divaricata) and the rare double form of showy trillium (T. grandiflorum ‘Flore Pleno’). This kind of lavishness is possible only when the plants receive sufficient light, moisture, and nutrients.
Over half of North America would be covered by some kind of forest if left to its own devices, but not all forests offer the same growing conditions for gardening. Deciduous trees like maples, ashes, tulip trees, and many oaks allow much of the sun’s energy to pass through to the ground during winter and early spring, and the wildflowers in these forests respond with a frenetic burst of activity before the trees leaf out. Evergreens—mostly conifers like spruce, pine, and fir, which dominate forests in much of Canada, the mountainous and maritime West, and some of the South—are another matter. Since these trees do not lose their leaves in winter, the pace of life in the herbaceous layer is more subdued in spring. Many of the wildflowers, such as bunchberry, are evergreen as well, and most are adapted to the cool, acid soils that form under needle-bearing trees. It is important to take into account the type of trees you have while planning your garden. If maples, ashes, basswoods, tulip trees, and beeches predominate, incorporate spring ephemerals that do much of their growth and bloom in early spring. If your trees are mainly evergreen, choose wildflowers that are adapted to cool, acid soils and a slower pace of life. If oaks and hickories form your canopy, a mix of the two should work well.
Even so, remember that even a single tree takes up an enormous amount of space, light, and water, and that its roots can spread out 50 feet or more in all directions. Gardening in these conditions can only be done on the tree’s terms. A successful woodland garden must be responsive to the needs and rhythms of the trees, but provide just enough light, good soil, and water for the wildflowers to thrive.
LETTING IN LIGHT
Although shade is a given in woodland gardening, it is a difficult thing to quantify. While sun is sun, shade can be anything from the light shadow of a veil to the darkness of a closet. When planning a woodland garden, I like to aim for dappled shade, where the canopy is thin enough to allow sparkles of sunlight to dance across the forest floor. Limb your trees up as high as you can with a pole pruner, or if an arborist is available, remove the lower half of the branches that line the trunk. If your woods are dense with young trees or thick conifers, thin out a quarter of the weakest trees to leave gaps in the canopy. (You should have to squint when looking up on a sunny day.) Understory trees and shrubs add depth to the woodland garden, but use them sparingly—not much can grow underneath the double-layered shade of the woody understory. These larger plants are best grouped or scattered lightly through the woods with gaps for herbaceous plants.
BY WILLIAM CULLINA
BUILDING GOOD SOIL
Woodland soils grow up, not down. The slow deposition of leaves and wood adds organic matter that fuels decomposition, recycles nutrients, and builds up topsoil. The topsoil in many forests (with the exception of those where water has deposited sediments in the past) is fairly shallow, and tree roots do their best to occupy every inch of it. In order to provide space for wildflowers, you will likely have to build the soil up. Quite simply, the deeper your topsoil, the better will be both the quality and quantity of wildflowers. The easiest way to do this is to apply a yearly layer of organic mulch such as shredded leaves, bark, or aged wood chips—either in the fall or early spring. Adding a few inches of this material (beyond what the trees would supply naturally) will tip the balance, building up in a few years what would otherwise take decades. Layers of decomposing material have the further benefit of feeding beneficial fungi and providing perfect seedbeds for slow-to-germinate plants like miliums and orchids. In general, woodland plants are not heavy feeders, and once the soil is healthy enough, no additional fertilizer is necessary. Until that point is reached, however, a light dressing of a balanced organic fertilizer will help immensely. Apply the fertilizer in spring just as the understory begins to emerge—that way, smaller plants can take up what they need before the trees become active.
Fifty years ago, water restrictions and concerns about pollution were pretty much nonexistent. Today, however, we’re much more conscious of what spills out of the hose. When we design new gardens, it makes sense to match the plants to the site, selecting plants that can handle summer drought when necessary. Fortunately, many woodland wildflowers are active mainly in the spring, so summer dry spells and even an occasional drought will not kill them. (See “Woodland Wildflowers That Can Tolerate Moderately Dry Soil,” page 92.)
THE DIFFERENT KINDS OF WOODLAND PLANTS
Woodland gardens put forth a feverish burst of energy in spring. By the summer solstice, though, the garden has transformed into a thousand shades of green, where texture replaces color as the dominant element of design. When planting a woodland garden, therefore, try to follow this rhythm. Plan for and celebrate the early burst of color that evolves gradually into a predominately textural garden in summer—a place of calm, cool shelter from the heat, punctuated by occasional bloom.
Many of the most colorful woodland wildflowers are spring ephemerals, plants that make their aboveground growth early and then retreat underground as the tree canopy fills out and the temperature climbs. Ephemerals provide a quick burst of color when we need it most and require no upkeep in the summer, but they leave gaping holes if used to excess. The most successful woodland gardens use these early players as the first act, relying on longer-lasting wild-flowers,
Left: When planting a woodland garden, the most satisfying results come from using a combination of early-blooming ephemerals; mid-season “specimen” plants such as iridiums and orchids; and ferns, groundcover plants, and shrubs for long-term interest. Below (clockwise from upper left): Red trillium (T. erectum); bunchberry (Cornus canadensis); double bloodroot (Sanguinaria canadensis f. multiplex); celandine poppy (Stylophorum diphyllum) and creeping phlox (P. stolonifera); Hepatica americana; Dutchman’s breeches (Dicentra cucullaria).
Woodland Wildflowers That Can Tolerate Moderately Dry Soil
Anemonella thalictroides (rue anemone) Aquilegia canadensis (Canada columbine) Asarum virginicum (Virginia wild ginger) Chrysogonum virginianum (golden star) Gaultheria procumbens (wintergreen) Geranium maculatum (wild geranium) Hepatica americana (round-lobed hepatica) Iris cristata (crested iris), pictured above Mertensia virginica (Virginia bluebells) Podophyllum peltatum (mayapple) Sanguinaria canadensis (bloodroot) Smilacina racemosa (false Solomon’s seal)
ferns, and woody plants to fill in after they have left the stage. I call this cohabitational planting, and it is a technique that mirrors closely what happens naturally on the forest floor. Aim for a mix of ephemerals for early-season drama, some specimens like trilliums and orchids for a mid-season epiphany, and enduring structural plants such as ferns that will provide interest into the fall.
All the plants mentioned below have been reliably hardy for us in USDA Zone 5, and most are hardy well into Zones 3 and 4.1 have tried to pick plants that will perform well over most of the forested parts of the United States and southern Canada.
The yellow trout lily (Erythronium americanum) was a familiar sight in forests near my home growing up, with its ground-hugging leaves mottled in gray and brown like the side of a brook trout. This species can be frustrating in the garden, producing mats of leaves but very few flowers. A bit of fertilizer in the spring (try one formulated for bulbs) and a bright spot will encourage heavier bloom. The western mountains are blessed with a number of larger species that have freely hybridized in cultivation, producing some outstanding garden plants. Erythronium ‘Pagoda’ is a readily available and very satisfying plant that, under good conditions, will form temporary clumps of 8- to 12-inch glossy leaves and flaring, two-inch flowers spaced elegantly on tall stems. Another hybrid, ‘White Beauty’, is smaller, with creamy white trumpets that flair and curl back at the tips. All the trout lilies are excellent ephemeral companions, quickly retiring underground as the trees leaf out.
As the trout lilies are coming into bloom, Dutchman’s breeches (Dicentra cucullaria) unfurl filigreed foliage that carpets the ground temporarily in soft blue green, punctuated by arching, six-inch stems of white flowers that look like upside-down pantaloons. The flowers are effective for about two weeks if the weather remains cool, by which time its close cousin, squirrel corn (D. canadensis), takes the stage. Both species spread quickly by seed and cormlets, which can be dug and scattered about after the plants go dormant (just be sure not to disturb them after Labor Day, since they begin to form new shoots for next season that can be easily damaged).
The carpeting spring ephemerals pave the way for the stars of the spring woodland. Many of these, including trilliums, bloodroot, lady’s slippers, and phlox, are not truly ephemeral because their foliage persists well into summer and fall, but spring is certainly their finest hour. By the time the dog days of summer arrive, they are looking a bit tattered and sleepy, and so I like to think of them as vernal exclamations, accents that don’t need to hold their place all season. Two of the first to bloom, often when frosts still trace your windshield, are bloodroot (Sanguinaria canadensis) and hepatica (both H. acutiloba and H. americana). Bloodroot lofts blink-and-you’ll-miss-them flowers of radiant white that spring from the unrolling leaves. Its bold foliage is quite attractive for most of die summer, especially if moisture falls regularly. Hepaticas are one of my favorite spring wildflowers—little mounds of many-petaled violet, blue, or white flowers. However, after a wet summer, the usually evergreen foliage becomes tattered and disfigured by the black spot fungi that plague many in the buttercup family. Place hepaticas in a spot with good air movement and drainage and they really shine.
Virginia bluebells (Mertensia virginica) emerge from the ground as leafy rosettes stained a deep purple that unfold rapidly and flower for about four weeks with ethereal bells of sky blue (or rarely white or rose). They mix beautifully with the buttercup-yellow, four-petalled celandine poppy (Stylophorum diphyllum)—an equal match in size and vigor. Both species will liberally self-seed if not deadheaded and may eventually crowd out smaller companions.
As these flowers fade from view, the first of the trilliums begin to bloom. Every spring I am bewitched once again by these elegant wildflowers. I have grown many of the species, but here I will focus on the most satisfactory and easy. Showy trillium (T. grandiflorum) is the queen of the genus, with full, white flowers up to three inches across that truly light up the woodland. It is a vigorous species that, once settled in, can form sizable clumps, and if you can think in terms of decades, will seed itself around as well. Its counterpart in the Pacific Northwest is T. ovatum, with somewhat smaller white flowers. Red trillium (T. erectum), with its faintly fetid, burgundy flowers, is equally easy. Catesby’s trillium (T. catesbyi) is a good choice for heavy clays in the South. There is a large group of trilliums whose sessile (stemless) flowers nestle in a whorl of typically mottled leaves. The foliar patterns of gray, silver, green, and burgundy are at least as interesting as the long-lasting flowers, and I wonder it this camouflage evolved to help hide the plants from colorblind but ravenous deer. Three of the easiest and most readily available are whippoorwill (T. cuneatum), with burgundy flowers, yellow trillium (T. luteum), with lemon-scented blooms, and T. chloropetalum and its varieties, with flowers in a range of reds and white.
If you choose carefully, it’s possible to have a succession of colorful woodland phloxes in bloom for almost two months. Wood phlox (P. divaricata) seeds its way agreeably around the miliums, providing drifts of blue for several weeks as the trilliums reach their peak. Creeping phlox (P. stolonifera) follows on the heels of wood phlox, and its evergreen leaves make a passable ground-hugging carpet for the rest of the year. Ozark phlox (P. pilosa var. ozarkiana) is next to bloom, sporting lavender pinwheels on taller stems. Finally, Alabama phlox (P. pulchra) rounds out the display, flowering for us well into June. This species prefers a brighter spot on the edge of the woods.
Lady’s slipper orchids remain rarities in gardens, mainly because of the high costs involved in their protracted propagation, but advances in laboratory seedling production should make them cheaper and more readily available over the next tew years. Nevertheless, they will continue to have an otherworldly aura about them—they seem almost too intricate and complex to be of this time and place. I have tried most of the native species with degrees of success, but the least demanding are the large and small yellow lady’s slippers (Cypripedium pubescent and C. parviflorum) and the spectacular Kentucky lady’s slipper (C. kentuckiense). They are closely related plants, sending up stems of alternating, pleated leaves, each tipped with a flower or two in spring. These three species bloom sequentially, so you can have orchids in bloom for a solid month in spring.
Large yellow lady’s slipper is the first to open, gracing our gardens for Mother’s Day every year. By the end of May, its blooms are fading and the smaller but more intensely colored small yellow lady’s slipper is reaching its peak. It has a chrome-yellow pouch as big as a quail’s egg and chocolate petals, and blooms appear in spikes ot two or three, unlike the typically solitary flowers of its larger relative. Around the second week in June, the magnificent Kentucky lady’s slipper begins to bloom. Its flowers are the largest in the genus, with a moonlight-yellow pouch as big as a hen’s egg and chocolate petals suffused with raspberry. When people spot it in the garden there is a kind of whispered awe, as though a movie star had just entered the room. As long as you grow the yellow-flowered species in dappled shade and a moist but gritty soil, they should thrive and double or even triple in size within a reasonably short time,
Groundcovers excel in the shade, filling in gaps around larger plants and often remaining evergreen throughout the winter. Two of the aristocrats among native groundcovers are wandflower (Galax urceolata) and its cousin, the rare and lovely Oconee bells (Shortia galacifolia). Both are slow to establish but well worth the wait, clothing themselves in lustrous, deep green leaves.
If you want elegance but don’t have the conditions or patience for galax or shortia, consider our native Allegheny spurge (Pachpandra procumbens). It is a more refined plant than its Japanese relative, with larger, gray-green foliage that turns burgundy as autumn sets in. The change to red reveals silver mottling that was masked by the green, a pleasing foil for the bottlebrush spikes of cinnamon-scented blooms in early spring. As the flowers fade, the clumps erupt vvith a burst of fresh green leaves to carry them through the next year. Set them in two feet apart and they will form a solid carpet in two to three years.
Gardening beneath the trees has its challenges, but if you learn to work with them, carving out a bit of extra space for wildflow-ers to thrive, the results can be spectacular. There is really no other type of gardening where so many plants—from the smallest mosses and groundcovers to shrubs and trees—can all inhabit the same small patch of earth.
For sources of plants featured in this article, turn to page 122.
Stopping Wild Collectors
Commercial wild collecting of woodland wildflowers continues to be a serious threat to their long-term survival. When buying wildflowers, especially slow-growing woodlanders like bloodroot. hepaticas. trilliums, and lady’s slippers (Cypripedium pubescens, above), be suspicious of suppliers that sell inexpensive (less than $5-10 each) and/or bare-root plants. If you’re in doubt, ask the nursery about the source of the plants, and buy them only if you’re convinced they’re nursery-propagated. Your local native plant society is often a good source for lists of reputable vendors.—W.C.
For more on woodland gardening, visit www.hortmag.com/features.