Solve design problems with perennials

Herbaceous perennials have soared in popularity in recent years. They are no longer relegated solely to the "perennial garden," but rather have become common sights in the landscapes of gardeners and grounds-care professionals alike. Too often, however, people continue to think of perennials as embellishments rather than integral and functional parts of the landscape. This is unfortunate, because perennials offer so much more than beauty. They can be quite useful for solving specific landscape problems. Here are eight design challenges in the landscape that you can solve with perennials.

1. Dry shade. Among the most difficult sites for plants are shady areas that receive little water: * Areas under shade trees where the trees' roots compete for water * Areas north or east of buildings where overhangs, wind patterns or obstructions reduce expected rainfall * North-facing slopes.

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Fortunately, several excellent perennials thrive in dry shade. A perennial planting of pigsqueak (Bergenia cordifolia), columbine (Aquilegia x hybrida), wood lily (Lilium philadelphicum) and white wood aster (Aster divaricatus) provides a sequence of color through the season. A loose, mixed planting of male fern (Dryopteris filix-mas) and Goldie's Fern (Dryopteris goldiana) with a few clumps of bird's-foot violet (Viola pedata) for a spark of early spring color would add depth to an area under shade trees or limbed-up evergreens. Masses of barrenwort (Epimedium pinnatum) make a perfect low-maintenance groundcover along the north side of a house.

2. Moist-to-wet shade. Moister-than-average soils in shady locations are only slightly easier to plant than dry shade, but several outstanding perennials thrive in such sites. As low-growing groundcovers, bugleweed (Ajuga reptans), yellow archangel (Lamiastrum galeobdolon) and spotted deadnettle (Lamium maculatum) offer excellent color. The latter two have been the subjects of considerable breeding efforts, and several cultivars offer highly colored foliage that brightens dark corners of the landscape. Tall white-flowered perennials for moist shade sites include black snakeroot (Cimicifuga racemosa), gooseneck loosestrife (Lysimachia clethroides) and Solomon's seal (Polygonatum commutatum). Any of these can serve as the focal point of a garden. Canada lily, a native of moist woodlands, is becoming more common in landscapes with a woodland theme. Several other perennials thrive in wet shady sites: marsh marigold (Caltha palustris), turtlehead (Chelone lyonii), cardinal flower (Lobelia cardinalis), cinnamon fern (Osmunda cinnamomea, see photo, page 28) and interrupted fern (O. claytoniana).

3. Dry sun. Dry, sunny locations are problems in many landscapes. These locations include areas of sandy soil that retain little water, places where irrigation systems are not available, regions with naturally low summer rainfall, south-facing slopes, containers and raised beds in landscapes, and planting beds in parking lots. Drought-tolerant shrubs such as juniper, potentilla, broom and peashrub do well, but they are overused, often too large for the location and may not meet the design needs of the setting.

Many perennials thrive in poor soil in full-sun locations and should be considered for such sites. Traditional dry-sun perennials for flower gardens and borders include yarrow (Achillea millefolium and A. tomentosa), rock cress (Arabis spp.), wormwood (Artemisia spp.), milkweed (Asclepias tuberosa), false indigo (Baptisia spp.), tickseed (Coreopsis spp.), blazingstar (Liatris spp.), opuntia (Opuntia spp.), black-eyed susan (Rudbeckia spp.), stonecrop (Sedum spp.), thyme (Thymus spp.) and yucca (Yucca spp.). Some of these (opuntia, stonecrop and thyme) make excellent groundcovers. Additional perennials that thrive in dry sunny sites include pigsqueak (Bergenia cordifolia), draba (Draba spp.), myrtle euphorbia (Euphorbia myrsinites), blue oat grass (Helictotrichon sempervirens) and inula (Inula ensifolia).

4. Moist-to-wet sun. The majority of perennials common to flower gardens perform best in full sun with well-drained soil that retains moisture in summer. Most of these plants perform poorly in overly wet soils that do not drain in winter because alternate thawing and freezing in winter causes the plants' root systems to heave out of the ground. Subsequent exposure of those roots to dry, cold, winter air is often fatal.

Several perennials, however, perform well in overly wet soils. Large plants like queen of the meadow (Filipendula rubra) and rose mallow (Hibiscus moscheutos) do well in wet sunny sites. Beebalm (Monarda didyma), buttercups (Ranunculus spp.), globeflower (Trollius spp.) and spiderwort (Tradescantia virginiana) do better in wet soils than in dry soils. While most irises demand perfect drainage, yellow flag (Iris pseudacorus), blue flag (Iris versicolor) and Japanese iris (Iris ensata) are native to wetlands. Most bulbs require perfect drainage too, but Canada lily (Lilium canadense) does well in much wetter soils.

5. Groundcover. Several woody and herbaceous plants have been overused as landscape groundcovers. Plants such as pachysandra, vinca, juniper and ajuga are excellent groundcovers, but some sites call for something special.

To be an effective groundcover in most landscapes, a plant should be low-growing, have high-quality foliage that is both durable and evergreen, establish quickly from small propagules, mat quickly to control erosion and offer some visual interest.

In wooded or other shady locations, both North American and European wild gingers (Asarum canadense and A. europaeum, respectively) make elegant groundcovers (see photo, above). Other groundcover options for shady sites include spotted deadnettle (Lamium maculatum) and partridgeberry (Mitchella repens). If evergreen foliage is not required of the groundcover, the options are greater, including lily-of-the-valley (Convallaria majalis, see photo, at left) hay-scented fern (Dennstaedtia punctilobula), sweet woodruff (Galium odoratum) and woodland phlox (Phlox divaricata-evergreen in some locations). Some species and cultivars of hosta (Hosta spp.), barrenwort (Epimedium spp.) and foamflower (Tiarella spp.) make excellent groundcovers, but many newer hybrids within these genera are clump-formers and do not spread enough to perform satisfactorily as groundcovers.

For sunny sites, consider these perennials for use as groundcovers: dwarf Chinese astilbe (Astilbe chinensis pumila; not evergreen), pigsqueak (Bergenia cordifolia), daylily (Hemerocallis spp. and cultivars-most are not evergreen and some are not spreading) and spring phlox (Phlox subulata).

6. Focal points in the landscape. We often think of trees and shrubs, statuary and other hardscape items when choosing focal points. However, perennials make excellent focal points, and they can change the landscape subtly or dramatically, as desired, from one season to the next. Here are several examples of perennials that command attention when they are at their peak. They can serve as excellent focal points if massed in the right location and have high-quality durable foliage that ages gracefully as the season progresses.

* Clumps of daffodils interplanted with clumps of daylilies provide strong color in both spring and midsummer. The high-quality daylily foliage hides the deteriorating daffodil foliage as the summer progresses and maintenance of such a planting is minimal.

* A wide line of spring phlox planted between a lawn and a walkway or driveway can provide a strong focal point in spring. As the season wears on, the phlox foliage will blend in and look like an extension of the lawn, thereby "disappearing" into the landscape.

* Daylilies planted in boulevard planting beds or curbed planting beds in parking lots can provide excellent color in midsummer, yet tolerate salt and snow-plow blades in winter. Such a planting offers an additional benefit in parking lots: The daylilies won't obstruct views or provide concealment, thereby addressing a safety concern in public landscapes.

* Clumps of lilies planted among shrubs provide color in midsummer, long after most shrubs have flowered. Later in the season, clumps of surprise lily (Lycoris spp.) have the same impact.

7. Lines in the landscape. Lines in the landscape help direct the eye and also help direct traffic. Paths, walkways, walls, fences and planting beds can all serve as lines. However, perennials planted in masses also form good visual lines (see middle photo, at left). Masses of Siberian iris, for example, create a good focal point in the early summer landscape, but the foliage remains high quality throughout the season. In masses, Siberian irises make an excellent line along a pathway or at the back of a flower garden. Another perennial that offers good line is showy stonecrop (Sedum spectabile). You can use masses of it in landscapes for low-maintenance textural interest throughout the season. Then, as a bonus, stonecrops produce good flower color in late summer, excellent foliage color in fall and good landscape line all winter as the dried flower heads catch drifts of light snow.

8. Movement in the winter landscape. Creating interest in a summer landscape is easy. Maintaining that interest through the winter is a bit more difficult. Evergreens provide the backdrop for the landscape, but the fact that they don't change dramatically from one season to the next makes them somewhat static. Deciduous trees and shrubs provide good interest during the growing season, and some (for example, crabapples, winged euonymus, winterberry and other hollies) have excellent winter characteristics, primarily through their bark and fruit. But winter landscape movement is harder to come by. That's where ornamental grasses come into play. Some grasses, like blue fescue (Festuca ovina glauca) and blue oat grass (Helictotrichon sempervirens) offer mounds of bluish evergreen foliage that move in the winter breezes. Taller grasses, like the heavily fruited cultivars of eulalia grass (Miscanthus sinensis), pampas grass (Cortaderia spp.) and feather reed grass (Calamagrostis spp.) wave gracefully and reliably in the winter winds.

Lois Berg Stack is an extension ornamental-horticulture specialist at the University of Maine Cooperative Extension (Orono, Maine).

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