Native Groundcovers: Sustainable Choices for Sun and Shade
Groundcovers are important elements in the designed garden. They fill the blank spaces of our beds and borders, knit together trees and shrubs, provide a "picture frame" for sculpture and other structures, and unify our plantings. We all know the big three—Japanese pachysandra, English ivy, and periwinkle. These are the most commonly used groundcovers in the American garden, and they fit the bill well: They are easy to grow, rapidly spreading, inexpensive, and readily available, and they create dense, low carpets of attractive foliage that persist throughout the year and keep out most weeds.
However, there's a problem: Two of these plants are major invasive species. English ivy (Hedera helix) rapidly invades forests in the East and Northwest, creating "ivy deserts" that smother and displace native understory flora. Periwinkle (Vinca minor) is also a common and harmful invader of U.S. forests. At the Scott Arboretum in Swarthmore, Pennsylvania, where I work, we have "blacklisted" both these plants and no longer recommend them to our visitors. What we do recommend is that gardeners replace their invasive exotic groundcovers with one or more of our very fine native groundcovers. These natives possess many or all of the traits that gardeners look for in a groundcover. And because they are native, they enhance local biodiversity rather than present a threat to it.
I should say that by "native" I mean indigenous to specific geographical regions. The plants discussed here are native to the eastern U.S. and generally hardy from USDA Zones 4 to 8. But they shouldn't be considered native plants in the South or West. What's more, just because they are eastern natives doesn't mean they will perform well in all situations there. Soil, moisture, and light all play key parts in determining a plant's success in the garden, whether it's a native or not.
Following is a selection of my favorite native herbaceous groundcovers, organized according to light requirements. There are many others I would profile if space allowed, including Carex plantaginea and native Asarum species (wild gingers). And of course there are excellent native woody groundcovers worth consideration, such as the petite Paxistima species. But the point is to highlight the fact that many wonderful native groundcovers are out there, waiting for you to discover them.
A good plant to start with is Allegheny pachysandra (Pachysandra procumbens). Like its popular Japanese cousin (Pachysandra terminalis), this native produces rounded, coarsely toothed foliage on low fleshy stems, but it has a much greater flair for the dramatic, changing its look as the season progresses. In early spring, short spikes of small, fragrant white flowers appear amid the previous year's foliage and last for a week or two. Shortly after, new upright shoots the color of celery emerge. These grow about a foot tall and bear medium-green leaves that expand and darken over the course of the spring and summer.
In the late summer to early fall, silver-gray mottling appears on the dark green leaves, adding another dimension to the color palette. Over the winter, the stems and leaves sink to the ground but generally remain evergreen. (In Zones 6 and lower, the foliage may die back completely.) In the spring, the cycle repeats. Allegheny pachysandra is a clumping plant and thus spreads more slowly than the stoloniferous Japanese species. Nonetheless, it spreads well: Several clusters in my home garden have doubled or tripled in breadth in a handful of years to give good cover. This native is easy to propagate from root divisions. Grow it in dry soil and full or dappled shade.
The genus Phlox is quintessentially North American: All but one of its more than 60 species are native to this continent. Quite a number of these have been grown and cherished as flowering perennials and rock garden plants, but there are several eastern species that also fit our expectations for groundcovers very well. Phlox stolonifera, creeping phlox, comes immediately to mind.
This plant's somewhat lax, six-inch-tall stems spread rapidly by stolons, creating a mat of delicate, rounded, dime-size dark green leaves that remain evergreen in all but the harshest winters. In May, dense, eight-inch-tall spikes of pale to deep purple flowers appear atop the foliage, forming a patch of solid color. Because of the showy blossoms, many gardeners regard creeping phlox primarily as a spring wildflower, but it also makes an excellent groundcover. Small plugs of this phlox planted half a foot apart will provide effective, lasting cover within a growing season or two. The plant performs best in partial shade and humus-rich, moist but well-drained soil.
A number of cultivars are available, including 'Bruce's White', which has bright white flowers. To see the plant used on a grand scale and to great effect, visit Peirce's Woods, a section of Longwood Gardens, in Kennett Square, Pennsylvania, where carpets of P. stolonifera spread out over great distances between majestic old trees. When the plant is in bloom, the effect is pure garden magic.
A great groundcover companion for Phlox stolonifera is foam flower, Tiarella cordifolia. Foam flower blooms at the same time as this phlox species, producing an abundance of small, creamy-white flowers on upright stems that rise above the plant's maple-shaped leaves at the same height—about one foot—as the phlox. The combination is used to great effect in the same woods at Longwood Gardens: The cream flowers of the foam flower are woven together with pale blue phlox flowers in a beautiful abstract tapestry. Like the phlox, the foam flower is stoloniferous and forms a consistent expanse of leaves. The milder the winter, the more evergreen it remains. New selections of foam flower with colored leaves and showier flowers seem to be popping up each year, giving gardeners a broad palette to paint the ground with. However, don't buy the clump-forming selections, such as Tiarella cordifolia var. collina (T. wherryi), because they don't have a good covering habit.
Heuchera americana, or alum root, is adaptable to sun or shade, given sufficient moisture. While it's not a spreading plant, it does form strong rosettes of persistent leaves, which, if grown en masse about a foot apart, function effectively as a groundcover. In the last decade or so, scores of cultivars and hybrids have appeared, and gardeners now have the option of carpeting the ground with everything from silver-tinged dark emerald-green leaves ('Emerald Veil') to frilly purple leaves ('Purple Petticoats'). Alum root flowers are tiny and greenish and appear on tall, airy stems in early summer. Unless the light hits the flowers just right, though, the flowers can obscure the effect of the foliage without adding much in the bargain. I advise cutting the flower stalks off after they finish blooming to restore the full impact of the decorative layer of foliage beneath.
Two native phloxes that make great groundcovers in sunny situations are Phlox pilosa and P. subulata. Some gardeners label P. pilosa a garden thug, but I think that's because they mistakenly expect it to behave like a classic garden phlox. If you're looking for year-round cover for open areas, this vigorous plant will provide it and also throw in the bonus of dramatic pink flowers in late spring and summer. Phlox pilosa grows a little taller than P. stolonifera, about one foot, producing softly hairy stems and three-inch-long leaves. It prefers a medium to dry, well-drained soil.
A full-sun plant, it works well as a groundcover under shrubs. At Ashland Hollow, William and Nancy Frederick's Delaware garden, this phlox is planted under a border of mixed shrub roses. In early summer, the various mauve, pink, and red flowers of the roses are united by a billowing layer of pink phlox flowers as everything blooms together in great floral exuberance. In the winter, the foliage of the phlox persists, providing something to look at other than thorny rose canes. At the Mt. Cuba Center, in Greenville, Delaware, Phlox pilosa battles it out with hay-scented fern (Dennstaedtia punctilobula) at the edge of a meadow, providing a lovely transition groundcover planting for a woodland garden.
The other sun-loving phlox, Phlox subulata, or moss pink, produces dense six-inch-tall cushions of needlelike evergreen foliage. Easy to grow even in tough spots and readily available, it certainly qualifies as a good groundcover. But the plant's use between rocks on steep slopes has perhaps become a garden clichÃˆ. It also shouts when it blooms in late spring and early summer: The flowers are almost too vivid to enjoy, especially in the bright, midday sun. Restrict yourself to one color to avoid the patchwork effect that comes from mixing the dozen or so cultivars available (the spectrum runs from purple to pink to white), and the plant snobs may forgive you.
If the moss pink is not for you, then try Sibbaldiopsis tridentata (formerly Potentilla tridentata), three-toothed cinquefoil, which is a good candidate for moss-pink-type situations. With its three-part leaves and ground-hugging habit, it resembles a strawberry plant. The white, early-summer flowers are also similar to those of the strawberry. A few of the cinquefoil's shiny evergreen leaves turn orange or red in the fall and winter. Years ago I admired and watched this native growing in full sun on a steep south-facing slope in the rock garden at Mt. Cuba. It seemed to have all the virtues of a handsome groundcover plant, and I've wondered ever since why I don't encounter it more often in gardens. Grow it in moderately fertile, well-drained soil.
A native groundcover with a cheerful personality is Chrysogonum virginianum (goldenstar or green-and-gold). I say this because it features bright yellow daisy flowers that bloom over an extended period of time. I've seen a few flowers occur as early as March and as late as October, although peak bloom is May and June. There is considerable variation in the habit of this species: Some plants form mounds reaching a foot tall, while others form dense, spreading layers of leaves only inches above the earth. Before purchasing one, make sure to read the catalog description of the particular cultivar closely, or study it in its pot to make sure you are getting the habit you're after. Some years ago I visited a private garden in Connecticut that had an extensive hillside garden full of choice native plants, with vast expanses of Chrysogonum used as a groundcover. The plant was dotted with many yellow flowers and made a lasting impression.
Nursery Sources:Bluestone Perennials
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