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The Shady Border

The Challenge of Shaded Soils

by Charles & Martha Oliver

Shade gardening is both an opportunity and a challenge. Gardeners have the opportunity to create an overall effect of serenity and peace by emphasizing the beauty of leaf color, form, and texture—often in the absence of showy, colorful flowers. The challenge is to achieve this effect while working with the poor soils and other difficult conditions that can predominate in shady sites. What follows are some suggestions for working with the conditions you've inherited, and to help your plants grow as well as they can in those conditions.

Amending the soil

Most shade comes from trees, and trees have enormous root systems. When visualizing the root system of any tree, imagine a root mass at least the equal of the above-ground trunk and branches; some trees, especially those in dry areas, have root systems that are much larger than the canopy. These extensive root systems are fierce competitors for soil moisture and nutrients, and the shallow and limited root systems of small perennials can't hope to compete successfully.

Cycles of moisture and dryness

Shady borders for the Midwest

The region known as the Midwest stretches from the edge of the eastern deciduous forest west to the rain shadow of the Rocky Mountains, and from boreal Canada south to Oklahoma. Within this vast region, diversity—in vegetation, soils, and climate—is the rule.

Deciduous woodlands are dominated by oak (bur, white, and red), hickory, basswood, sugar maple, and ash, and the forest floor below is carpeted with wildflowers. The mineral soils are rich clay or silt loams, topped by a thin mantle of organic soil; many are influenced by limestone bedrock and thus are slightly acidic to slightly alkaline. Gardeners in this region have an embarrassment of riches, and except where subsurface clay is the only soil, gardening is easy. Massive amounts of organic matter and a strong back will eventually transform clay into soil for shade plants. Pine, oak, and hickory cover the poorer soils, where nutrients and water can be scarce, but with the addition of organic matter, drought-tolerant shade plants will thrive. In the coniferous forests, perennials with persistent leaves such as partridgeberry and cold-adapted plants such as beadlily combine to make glorious gardens.

Winter temperatures can drop to -30°F. for weeks on end, and summer temperatures may soar to the mid-90°s F. When moist Gulf air is pulled in, the atmosphere drips with humidity. Winters in the northern reaches of the region generally have the consistent snow cover that makes gardening with herbaceous plants easier than it is for more southern neighbors in zone 5. The vagaries of climate severely limit the woody vegetation in zones 3 and 4.

—C. Colston Burrell

Shady borders for the Northwest

Gardeners in the Pacific Northwest need not go far from home to experience nature's best shade gardens: A day's hike in the Cascades will teach the observant gardener much about design and planting. Firs and cedars tower overhead, shading alders, vine maples, and other native trees, while below, Oregon grape and huckleberry, maidenhair and sword ferns inhabit the forest floor. Wildflowers including trilliums, bunchberry, vanilla leaf, and goatsbeard light up this natural woodland garden.

Although gardeners here complain about the wet winters and often-dry summers, our generous rainfall assures lush growth and abundant, verdant shade gardens. While plants in natural settings have adapted to this unique climate, gardeners often fail to recognize that a newly planted shade garden will need regular watering until plants are well rooted and a leafy canopy develops; established gardens may also need watering in the dry months.

Soil conditions are extremely variable: Volcanic mountain ranges, alluvial deposits, and glacial movement sculpted the Northwest, leaving a rich mix of soils from silt to heavy clay over hard pan. Soils on the west side of the Cascade Range and on the coast are acidic. In general, pest and disease problems are few—but slugs seem to thrive in our heavy clay soil and on the lush vegetation that it supports, and are a constant challenge, especially in the shade garden.

—Lucy Hardiman

Shady borders for the Northeast and Mid-Atlantic

In the mid-Atlantic region (Delaware, the District of Columbia, and Maryland), moderate temperatures last from mid-April to mid-October, so gardeners can grow a wide range of plants. Search your garden for microclimates hospitable to more demanding candidates; in a protected niche you may be able to grow marginally tender plants like calanthes and camellias while many of the minor bulbs—Japanese arisaemas, hardy cyclamen, and woodland iris—like a corner with good drainage. Repeated spring freezes and thaws can heave plants with weak root systems, so plant and transplant in early fall. Water during inevitable periods of summer drought; your plants will cope better with heat and look better, too. Soils in the mid-Atlantic region are generally fertile, but often low in organic matter.

The farther north you go, the shorter the growing season and the colder the winter. Massachusetts, New Hampshire, Vermont, and Maine gardeners should pay close attention to hardiness ratings so as not to waste money on plants that can't possibly survive. Cooler summer temperatures may mean that less watering is required, while winter's snow cover provides valuable protection. In both the Northeast and the mid-Atlantic areas, high heat and summer humidity can cause mildew and other fungal problems, and various forms of crown and root rot; work to improve air circulation and provide good drainage. Northeastern soils are often thin and acidic. Rather than undertaking massive soil amendment, grow plants that are adapted to these soils. Your success rate will be much higher.

—Judy Springer

Shady borders for the Southeast

Most of the Southeast is blessed with a long growing season, a warm climate, and abundant rainfall, although rainfall patterns vary: Winter and spring are generally wetter than summer or fall. Humidity is high during the growing season, reducing the evaporation of water from leaves and soil and mitigating the stressful effects of dry shade.

Southeastern soils tend toward acidity, and vary from nutrient-rich red clay hill soils to anaerobic blue, yellow, or white clays, to infertile coastal sands, to shallow, stony mountain soils, to rich, black, well-drained Mississippi delta loess.

Warm night temperatures characteristic of the Southeast pose unique problems for shade gardeners. Increased nighttime respiration limits growth in many garden species and hot nights encourage diseases such as Southern blight (Sclerotium rolfsii).

Gardening in shade is the natural lot of the Southeastern gardener, because we garden on land that, if left untended, would revert to forest. In fact, local forests are excellent models for shade gardens, with a wealth of well-adapted native species, both ephemeral and persistent. Southeastern gardens also welcome many plants native to warm temperate and subtropical Asia.

—Edith Eddleman

Many herbaceous perennials have an elegant way of skirting the competition. When conditions become dry and shady, and air temperatures exceed 85° F., ephemeral plants can't draw moisture out of the ground fast enough to prevent wilting; maintaining a herbaceous top is too risky, so they slough off their stems and leaves. The plants do not die, but become temporarily dormant—which is why they're called ephemeral. Virginia bluebell (Mertensia virginica), for example, always discards its leaves by June, drawing moisture into the thick roots, which act as a reservoir for the plant.

A woodland garden's cycles of moisture and drought are very predictable, and plants that thrive there have adapted to these conditions. In spring, abundant rainfall is the signal for spring ephemerals to come up and bloom; often this coincides with the greatest light levels, as the leaves of deciduous trees are just emerging. The woodland plants rush to bloom and seed; many of the loveliest natives are in this early group. By the time the leafy canopy is spread over their heads, their season's work is done and they are giving way to other plants more able to cope with drier, shadier conditions. Not all woodland plants are ephemerals, however. Many plants, including certain asters, white snakeroot, and goldenrod, persist through the season and wait until autumn to bloom.

In summer, soils can get quite dry. Evapo-transpiration from tree leaves can pull hundreds of gallons of water from the ground every day. The most successful dry shade plants—ferns, mosses, Symphytum grandiflorum, epimediums, and hostas—have evolved large fleshy roots, leathery leaves, and other strategies to help them compete with thirsty trees and to cut down on moisture loss.

The surest way for gardeners to provide shade plants with an adequate supply of moisture is to amend the soil, as organic matter helps soils hold moisture. A good woodland soil should contain large quantities of organic matter in the form of humus (rotted leaves and twigs). In a soil heavy with clay or sand and with little organic matter, plants may find it difficult to thrive.

Enhance your soil's water-holding capacity by spreading several inches of leaf mold, compost, and well-rotted manure or wood chips over the surface of the soil and turning it in to the depth of a spading fork. If roots are thick, tangled, and fibrous, it may be impossible to dig in; if so, pile on the mulch and plant in that. For soil that has been "strip-mined" and then run over by heavy machinery, add liberal amounts of rotted wood chips and leaf mold; this type of soil needs lots of amending, so keep a heavy mulch of wood chips, shredded bark, or shredded leaves on the garden and renew it as it rots into humus at soil level.

Never use freshly ground wood chips or sawdust. As these materials rot they will deplete the soil of nitrogen; plants will yellow (the first danger signal) then die. If possible, buy loads of chips and allow them to sit for a year before use.

Soil pH

Your soil's pH—its acidity or alkalinity—may limit the types of plants you can grow, and it's far easier to choose plants suited to your soil type than to try to change it. (In soil buffered by a lot of organic matter, pH is less critical.) Rhododendrons and azaleas will grow in very acid soil, as will a whole range of other plants including clethra, tiarella, and shield and cinnamon ferns. But in neutral or slightly acid soil many more kinds of plants will grow; in fact, most woodland shade plants prefer this type of soil—among them trillium, great merrybells, baneberry, wild ginger, and wood and Christmas ferns. And then there are plants that prefer a neutral to limy soil (maidenhair ferns, primroses, and wood poppies, shooting stars, and twinleaf, for example), which are found in nature on limestone ledges. If you want to grow these plants and the soil is acidic, sprinkle ground limestone around them as you plant.

Shallow roots and other impediments

Some trees are better to garden under than others. Norway and silver maples, apples, and beech trees all have very shallow, almost surface, roots. Lawn grasses will not grow under these trees, so choose drought-tolerant groundcovers or other plants that will adapt well. Try Symphytum grandiflorum, a comfrey relative that produces hanging, pale yellow bells in April, and dark green leaves all season. Epimediums are also good choices for dry shade. Ferns for dry shade include marginal wood fern, male fern, and New York fern.

Some other good choices to grow under trees with shallow roots are Cyclamen coum or Cyclamen hederifolium; these need moisture in winter and early spring, but very dry conditions in summer. Plant cyclamen corms within the root zone of a beech tree, where conditions will be just right for these charming woodlanders, as beech roots slurp up any moisture from summer rains. Plant nursery-grown corms three inches deep, cutting a hole in the fibrous tree roots with a knife (if necessary) and cover the corm with a light mulch. The flowers and leaves will pierce the mulch from below with no trouble.

In the worst case—very acid soil under tangled maple roots—elevate the garden in raised beds or planters; keep the beds at least 6 feet from the base of the trunk so that you don't damage the tree. Frames of fieldstone, railroad ties, cement block, or brick can be filled with prepared soil mixes for woodland plants: Compost mixed with coarse sand and topsoil in equal proportions will create conditions so prime that the plants will be lush and enormous. Of course the tree roots will sense the bounty and find their way into the planters, too, unless you put a barrier across the bottom. Landscape fabric won't stop most tree roots; you may need a solid floor; several layers of 6-mil plastic sheeting should do the trick.

If you have just one large tree in your garden, you may still have a soil problem: Trees grown in the lawn or out in the open often have wide, shallow root systems. Only the shade cast by a building will not be accompanied by a root problem—but these gardens may have their own set of troubles: dense shade, or too much water if the slope or the edges of the building channel water there. To increase drainage, add coarse sand, fine gravel, or grit to the soil; if the soil is very moist, you will be limited to plants that tolerate soggy conditions.

When to plant

Once the soil is amended, it's time to think about planting. Bulbs are easy to plant: Dig a hole four times the depth of the bulb and place the rooted end of the dormant bulb firmly into the base of the hole. Cover with soil. Wait until spring, when the new growth emerges, to fertilize.

Plant perennials before or after active growth; plant primroses, for example, in early spring or after blooming, when they die back. Dig a hole large enough so that you can spread the roots out. If you are planting potted material, make sure the hole is deep enough so that you can place the plant slightly deeper than it was growing in the pot. If the soil is dry, fill the hole with water before planting, and wait for the water to drain away. Then water to settle the soil after planting. Mulch with well-rotted wood chips or leaf mold around, but not up, the plant's stem.

Never plant in soggy soil, because you can compact it and destroy tilth and structure while working it. Plant when the soil is drier than usual, during periods when rain isn't frequent, or temporarily cover the planting area with plastic to repel rain or roof water to keep the soil workable.

Maintaining your plants

Diphylleia cymosa (Umbrella Leaf) is a must for any garden with a flair for the dramatic.

Diphylleia cymosa (Umbrella Leaf) is a must for any garden with a flair for the dramatic. Photo © C. Colston Burrell

If you've chosen plants well-suited to the site—paying special attention to pH and moisture levels—you'll have fewer maintenance chores than you would in a sunny herbaceous class="border"r. You won't be dividing plants in spring and summer, because most woodland plants don't require the constant division that perennials for the sunny border do. Foamflowers and creeping phlox simply move to another site if they need additional space, using their own method of travel (stolons on top of the ground). These perennials will form large clumps or drifts of plants; should you wish to increase your holdings more swiftly, lift small clumps after flowering and move them to a new site.

You shouldn't have to water your shade garden often unless you've sited plants in the wrong spot. During periods of drought, though, watering can make a good deal of difference in your garden. Watering with a sprinkler is not ideal, as it will encourage plants to send feeder roots to the top inch of soil, making them more vulnerable to heat and dry conditions. Water thoroughly and deeply, soaking the ground with more water than you think the plants need. An underground soaker hose on a timer is ideal, and you will lose less water to evaporation than you would with a sprinkler system. Water at night to give plants and soil more time to draw up the moisture.

Weed your garden to keep out unwanted plants that try to seed in. Vigilance with weeds is the best strategy; pull them before they seed next year's crop. Discovering the source of weed seeds can be helpful. A large weedy field close to your woodland garden, for instance, will be a continual source of airborne seeds and aggravate your maintenance problem. Birds will also carry seeds into your garden in their guts; most of these will be tree seeds, so hand-pulling can usually keep the problem in check.

The shady border, overall, requires less maintenance than the sunny herbaceous border, the rock garden, or the vegetable garden, because it is modeled on a climax vegetation—a stable evolutionary system. The herbaceous plants of established forests change little over time, and a garden modeled on a forest will provide less trouble and more rewards than any other type of garden.

Charles and Martha Oliver are the owners of The Primrose Path, a mail-order company in southwestern Pennsylvania that specializes in primulas, heucheras, heucherellas, tiarellas and other native and exotic woodland plants.