Several different thymes blanket this difficult-to-fill space in a garden walkway.
Photograph by J. G. Strauch, Jr.
A diversity of plant material and the visual interest of a well-designed herb garden or border are appealing, and a well-groomed lawn can be an impressive sight, but in some parts of the landscape, the high maintenance of these plantings is a drawback. Ground covers—relatively low-growing plants that are aggressive enough to eat up some territory and keep weeds at bay—can be the solution. Ground covers may be used to carpet areas surrounding a formal garden; cover steep banks that would require a lawn mower to defy the laws of gravity; fill areas too shady, wet, or arid for other plants, or naturalize areas that would otherwise be difficult to maintain. Many perennial herbs are among the hundreds of plants that can serve as ground covers. Many of these are not only problem solvers but beautiful plants in their own right.
When I develop a vision for landscaping an area, I want to get my hands on some plants immediately, but I’ve learned to restrain myself and deal first with site preparation. Omitting this essential step can lead to a planting that’s overrun with grass and weeds. If the site is currently part of your lawn, the lawn has to go. If the turf is in good condition, you may want to rent a sod cutter to strip off the sod and then use the sod to fill in thin spots in another part of the yard. Then turn over the soil with a rototiller or by hand as you would for any herb garden. If the lawn is in poor condition, proceed directly with the tiller or shovel. If you’re willing to wait all summer, you can smother or cook the existing vegetation by covering the area with heavy-duty black or clear plastic for a few months. Pin down the edges with 6-inch U-shaped anchoring pins purchased at a garden center or improvised from pieces of coat hanger. You can also use a nonselective herbicide such as glyphosate (one brand name is Round-Up) to kill all existing vegetation. Follow label directions. Whether you use plastic or weed killer, you’ll still need to till the soil afterward.
Slopes can be a challenge to clear. Last spring, I attacked a steep bank covered with a scattering of grasses, assorted broadleaf weeds, and the semirotted stumps of small privet and hackberry trees that a previous owner had cut down. Chopping the stumps out with a mattock proved effective but arduous and eroded some places on the bank. When I went at the grass and weeds, first with a trowel, then a hoe, then my hands, the stolons of Bermuda grass and the taproots of dandelions broke off; I knew they’d be back. My ankles got cramped as I knelt on the slope with my toes dug in to keep me from sliding; my toes fell asleep. I finally resorted to my sprayer. I ended up with a lovely bank of ground cover and received lots of compliments (but I wouldn’t want to do it again anytime soon) .
Don’t take shortcuts with your soil preparation. You want your ground cover to get established quickly, spread out optimally, and be a long-term addition to your landscape. When you do it depends on your climate and your schedule, but early spring or fall is best if the weather is agreeable and the soil is not too wet to work.
A soil test can tell you whether you need to add lime or sulfur to adjust your soil’s pH and how much to use. The addition of compost and other organic materials will improve both clay and sandy soils and in fact is a good idea for any soil but muck. Choosing plants well suited to the kind of soil you already have is the best way to ensure a successful planting.
Incorporate the amendments by hand or with a tiller. On steep banks, mixing in the soil manually will help minimize erosion. It’s tempting to prepare only the area where a plant is to be placed rather than an entire bed, but this won’t work for plants that spread by creeping or rooting at the branch tips.
Now pause for a few weeks to allow residual weed seeds to germinate. Till again, hoe, or pull out the seedlings to reduce the future weed population.
When planning a bed of ground covers, consider the mature size of a plant and its growth habits. Will one plant eventually cover 5 square feet or only one? Does the plant spread by underground stolons, or do trailing stems root at the nodes as they touch the ground? Does it form symmetrical mounds of a certain diameter or grow unevenly in unpredictable directions? Is it a rapid grower or a slow spreader?
Dividing the number of square feet to be covered by the distance (in feet) between plants from center to center will give you the number of plants you need to fill that area. If you want quicker coverage and don’t mind the cost, buy more plants and space them closer, but don’t crowd them so much that they can’t develop the growth characteristics for which they were selected. If you’re trying to cover a large area on a budget (and if you’ve started your planning early enough), consider using plants that you can grow from seed under lights, or buy a few plants and root cuttings from each stem. Many of the herbs recommended below are easily propagated by division; the gift of a few shovelfuls of sweet woodruff, for example, could be chopped into enough small starts to plant 20 to 40 square feet of bed. Whether you space your plants evenly in rows or randomly is up to you and may not even be evident after the plants have filled in.
Having planted your ground-cover herbs, be prepared to water them frequently until they become established. Initially, the spaces between your plants offer places for weeds to gain a foothold, although eventually the ground cover will fill in enough to crowd or shade them out. Landscape fabrics, also called weed barriers, available in 3- to 12-foot-wide rolls, can reduce or eliminate weed growth. Unlike plastic, these durable fabrics let moisture, air, and nutrients through while excluding light. The fabrics are cut to fit the shape of the bed and pinned down with anchor pins, and an X-shaped slit or a circle is cut out for each plant.
Landscape fabrics are most suitable for plants that grow from a central crown with branches spreading out above or along the ground. They can hinder the growth of ground covers that spread by underground parts or root as they grow.
Mulch also can reduce weed growth to some extent and conserves moisture as well. Be careful, however, not to introduce new weed seeds with your mulches. Hay is full of weed seed, straw less so. Composted garden flowers and vegetables can contain viable seed unless heated well during decomposition. For herbs of Mediterranean origin such as thymes, oreganos, and germanders, which are prone to rot and need good air circulation around the foliage, rock dust or gravel is a good choice, especially in areas of high humidity. No matter what kind of mulch you choose, leave an inch or so between it and the stems of your plants.
Herbs to try
Herbal ground covers can be found to suit a wide range of landscape situations. Probably the greatest limitations to their use are winterhardiness and heat tolerance. Before you get your heart set on a particular plant, make sure that it is likely to survive your winters—or summers. You don’t want to make the investment in time and money, only to have the plants succumb to cold or fail to flourish in the heat and humidity. A ground-cover planting should provide years of beauty in the landscape.
The hardiness zones stated here are based on references or estimates from my experience. Winterhardiness zones on a map look well defined, but microclimates—local areas with different conditions—abound, especially in borderline areas. The best way I’ve found to find how a plant will perform is to make a small test planting and monitor its performance for a season. Knowing that another local herb grower has grown the same plant successfully is a good sign, but be aware that conditions at your place may be quite different.
Heat tolerance has not been well studied for herbs, but it’s obvious that 90°F in coastal Virginia is nothing like 90°F in Arizona. The difference is the humidity, and it’s the combination of heat and humidity that is lethal for so many classic herbs in the southeastern United States.
There are many possibilities for herbal ground covers. Here are some suggestions for covering the ground in different situations.
• Many kinds of thymes (Thymus) have established themselves as useful ground covers over years of use. Those discussed below all can be propagated by division and should be spaced 6 to 12 inches apart.
Woolly thyme (T. praecox subsp. arcticus ‘Lanuginosus’) is a widely used inch-high ground hugger with densely hairy leaves and stems that is hardy in Zones 5 through 8. In June, lavender-pink flowers are set off against the gray-green foliage. In some years, flower production is spotty, but the foliage effect alone is reason enough to grow this plant. Woolly thyme needs a warm, sunny location. It spreads happily over gravel walks and stepping stones and can tolerate light foot traffic. To keep it from rotting in wet or poorly drained soils, try using a gravel mulch.
Among the many other cultivars of T. p. subsp. arcticus, 3-inch-tall ‘Coccineus’—crimson thyme—is noteworthy for its vivid, almost fluorescent magenta flowers (the foliage is green). It’s sure to draw comments when in bloom, but too much of it can be overwhelming. Use it with discrimination. It’s hardy in Zones 4 through 8.
One of my favorite thymes is T. quinquecostatus, a native of Japan. The sweetly scented plants are about 4 inches tall and spread at a moderate rate. The species has medium green foliage and lavender-purple flowers while the white-flowered cultivar ‘Albus’ has yellow-green foliage. Both forms flower heavily in June and often have a lighter second flowering in late summer. Both are evergreen and reliably winter-hardy to Zone 5.
One notch taller, at 6 inches or a little more, is mother-of-thyme (T. pulegioides, often offered as T. serpyllum). This species is hardy in Zones 5 through 9 and is naturalized in the Northeast. It tolerates mowing, adapting its height to that of the mower, and spreads vigorously. Unlike most other thymes, which grow best in full sun, mother-of-thyme can take some shade. Cultivars with white, pink, lavender and magenta flowers are available.
• Roman chamomile (Chamaemelum nobile), hardy in Zones 3 through 10, is used as an aromatic ground cover and sometimes mowed as turf in areas where the ground does not freeze for long periods. In full sun and a dry, light soil, seedlings or divisions of Roman chamomile planted 6 to 12 inches apart will cover the ground quickly. It also tolerates partial shade. The 1-inch daisylike flowers, which appear sporadically all summer, are held above the bright green lacy foliage. The crushed foliage is pungent and sweetly fruity.
• Another pungently scented plant with fine, fernlike foliage is woolly yarrow (Achillea tomentosa), but this plant’s evergreen leaves are gray-green and hairy. Hardy in Zones 4 through 10, woolly yarrow spreads by short runners that hug the ground. In early summer, the flower stalks rise 6 to 12 inches above the foliage, bearing flat-topped clusters of 1/8-inch-wide yellow flowers. The woolly mat of foliage will remain attractive throughout the growing season if you cut off the flower stalks when the flowers fade. Woolly yarrow tolerates poor, dry soil and may die out in spots if the soil is too wet or fertile. Full sun is a must. Propagate it by division and space plants 6 to 12 inches apart.
• Several germanders (Teucrium spp.) are commonly grown in knot gardens, where they are clipped into neat hedges. T. chamaedrys ‘Prostratum’ (or ‘Nanum’) is a low-growing form that spreads rather leisurely by underground roots. Plants form 8-inch-tall mats with dark green leaves and 5/8-inch-long rosy pink flowers in July and August. They are hardy in Zones 5 through 8; good snow cover can protect plants from severe winter damage.
The related T. montanum is a low, spreading shrub with silvery stems and attractive cream or yellow flowers in summer. Hardy to Zone 6 and evergreen in warmer parts of Zone 7, it needs full sun and a well-drained, even dry soil. Propagate these germanders by cuttings or divisions and space plants 12 inches apart.
A little taller
• A larger-scale herb for ground-cover use is lamb’s-ears (Stachys byzantina). My daughter loves stroking the thick, soft, 5-inch-long silvery leaves covered with long, thick hairs. The dense mats of sprawling, 10-inch-tall stems spread irregularly but reliably to cover a large area in dry, sunny sites. Small pinkish purple flowers on 2-foot stalks in summer contribute little to the plants’ ornamental value. Some people cut off the flowers or grow the nonflowering cultivar ‘Silver Carpet’. Divide lamb’s-ears in spring or fall; space plants 12 to 18 inches apart.
• Hardy in Zones 8 through 11, rosemary (Rosmarinus officinalis) as a ground cover is a reality only for gardeners in California and the Southwest. The rest of us must look on in envy. On the West Coast, prostrate forms of this culinary herb are used effectively in shopping-mall landscapes as juniper is used here. Though not as dense as juniper, prostrate rosemary has a resinous scent and the bonus of blue flowers in winter. Evergreen, rooting at the nodes and spreading in informal shapes, it grows 2 to 4 feet tall and spills over the edges of raised beds. Propagate rosemary by stem cuttings in spring, and space small plants 12 to 18 inches apart.
• Oregano (Origanum vulgare) is a good selection as a ground cover and soil stabilizer for a sunny, dry slope in Zones 5 through 9. Although this species is not the culinary choice and its leaves are virtually flavorless, it is durable in the landscape and forms such dense mats over time that you can barely penetrate it with a shovel. If you ever decide you don’t want it, you can peel it off like turf.
Oregano plants hug the ground with dusky green leaves that usually show some purple undertones in winter. From late summer into early fall, 2- foot-tall flower stalks bear open clusters of pink flowers surrounded by purple bracts. Shear them off later in the fall to tidy up the plants. Space plants 12 to 18 inches apart.
For variety both in height and color, try the 6-inch-tall cultivar ‘Aureum’, whose bright yellow-green leaves contribute a conspicuous, cheery accent. The flowers, white with green bracts, are of less interest than the foliage. ‘Aureum’ is hardy only to Zone 6. Propagate both of these oreganos by division.
• Beach wormwood (Artemisia stelleriana) thrives in full sun, sandy soil, and salt air to Zone 3, languishing in fertile soils or humid inland climates. Its outstanding feature is its white felty, lobed leaves. Those of the cultivar ‘Silver Brocade’ are even whiter. Vegetative stems are prostrate with upturned ends, while the flower stalks bearing compact clusters of tiny yellowish flowers rise to 2 feet tall in June and July. Native to northeastern Asia, beach wormwood is naturalized along eastern coastal beaches. Propagate it by division or cuttings; space plants 12 inches apart.
• Although the species of St.-John’s-wort (Hypericum perforatum) known for its medicinal uses is too weedy to serve as a ground cover, several of its relatives make good ground covers under a wide range of conditions. Aaron’s-beard (H. calycinum), a semievergreen shrub 12 to 18 inches tall, quickly spreads by rhizomes to cover a large area. The 3-inch golden yellow flowers are borne on new growth in July and August. The plants tolerate drought and are hardy in Zones 6 to 10. They grow best in full sun or partial shade and fertile, loamy soil. Space them 21/2 to 3 feet apart.
Creeping St.-John’s-wort (H. reptans) grows only 4 to 6 inches tall. Its stems root as they spread at a moderate rate. Like those of most other hypericums, the 13/4-inch flowers are yellow. This species is hardy to Zone 7. Given a place in full sun and sandy soil, it grows well but dies back to the ground in colder areas. Space plants 12 inches apart. Both species are most easily increased by division, but they can also be propagated by seed or cuttings.
• The low-growing evergreen woody shrub bearberry (Arctostaphylos uva-ursi), whose 1-inch glossy leaves have been used as folk remedies to treat diseases of the bladder and kidneys, makes a splendid ground cover. Native to sandy areas across the northern United States and hardy in Zones 2 through 10, bearberry’s 6- to 12-inch-tall plants have 5-foot-long trailing stems forming thick masses of foliage that turns bronze in the fall. White 1/4-inch, bell-shaped flowers bloom in summer and are followed by 1/4-inch red berries that last all winter. Bearberry likes acid soil and full sun or part shade. It is hard to propagate. You’ll probably want to purchase plants. Space them 12 to 24 inches apart.
In the shade
• Sweet woodruff (Galium odoratum), which with wine and sugared strawberries is an essential ingredient of the traditional German Maibowle, makes a fine ground cover in naturalized areas. Plants spread quickly by rhizomes to form dense mats 6 to 8 inches tall and need at least partial shade, particularly at midday. Sweetly fragrant, small white flowers appear in loose clusters in early summer.
Although the flowers are pretty, it is the foliage that I find most appealing. The narrow, bright green leaves are arranged in whorls like spokes on a wheel, one neat tier above the next. I only wish that they were evergreen, but the tops die back to the ground in winter. The dried stalks smell like new-mown hay or vanilla due to the presence of coumarin.
• Patches of wild ginger (Asarum canadense) can be found in rich woods from New Brunswick to the Appalachians and into eastern Oklahoma. Several other species are native to the southeastern states and the West Coast. The rhizome smells and tastes somewhat like the spice ginger (which comes from the tropical plant Zingiber officinale), and it has been used as a seasoning. The plants are 6 to 7 inches tall with stems that creep along the ground. The heart-shaped leaves are up to 71/2 inches wide, light green, iridescent but not glossy. The reddish brown cup-shaped flowers hide beneath the leaves just above the ground. Wild ginger prefers a shady location and neutral to acid soil that is high in organic matter. Propagate by dividing plants in spring or fall (much the easiest), rooting cuttings, or sowing seed. Fresh seed sown outdoors in summer should germinate the following spring, but seedlings will need another year to reach transplanting size. This species is hardy in Zones 3 to 7.
Wild ginger has a European cousin that I loved the first time I saw it. The evergreen leaves of A. europaeum are slightly smaller than those of A. canadense and are a shiny dark green. It is hardy in Zones 4 through 8 and is easily propagated by division. Both species spread relatively slowly and are best suited for shady areas of limited size. Space plants a foot apart.
• Another native to eastern woods is wintergreen (Gaultheria procumbens), whose small, glossy evergreen leaves are the original source of the oil of wintergreen used as a flavoring and topical liniment. Hardy in Zones 3 through 8, wintergreen spreads by creeping stems but does not form dense colonies. The stems are 3 to 6 inches tall with 2-inch elliptical leaves borne in clusters near the top of the branches. The bareness of the lower part of the branches contributes to the open appearance of a planting. The flowers and fruit resemble those of bearberry. Wintergreen prefers a moist, acidic soil high in organic matter and dappled to full shade. Propagate it by cuttings and space plants 8 to 12 inches apart.
Other good herbal ground covers for shade include lungworts (Pulmonaria), dead nettles (Lamium), false dead nettles (Lamiastrum), and violets (Viola).
True, ground covers are supposed to cover ground, but some advance with the aggressiveness of Sherman’s army. Take the mints (a common plea of people whose mints have taken over their garden), or the bugleweeds (Ajuga spp.), or moneywort (Lysimachia nummularia), or goutweed (Aegopodium podagraria). The same is true of Roman wormwood (Artemisia pontica), as well as the ‘Silver King’ and ‘Silver Queen’ artemisias (A. ludoviciana), beloved of wreath makers. All these herbs can be attractive in the landscape; the problem is keeping them within bounds. Confining them in bottomless pots or barriers sunk into the ground never works for me. The wiry stolons eventually find a way over or under the barrier, and the confined plants don’t seem to grow as vigorously as free-ranging plants. Nowadays, I just naturalize an invader in an area I’m not using for anything else and let it go where it will: a moist spot for mints, bugleweeds, or moneywort, a dry one for artemisias. Goutweed will grow anywhere.
Harriet Phillips lives and gardens in Goochland, Virginia, and breeds essential-oil plants for R. J. Reynolds Tobacco Company in Merry Hills, North Carolina.
Most of the herbs mentioned here are readily available at nurseries. The following mail-order sources have a wide selection.
Arrowhead Alpines, PO Box 857, Fowlerville, MI 48836. Plant list $2.
Forestfarm, 990 Tetherow Rd., Williams, OR 97544-9599. Catalog $4.
Garden Place, 6780 Heisley Rd., Mentor, OH 44061-0388. Catalog $1.
Logee’s Greenhouses, 141 North St., Danielson, CT 06239. Catalog $3.
Richters, Goodwood, ON, Canada L0C 1A0. Catalog free.
Wayside Gardens, Hodges, SC 29695-0001. Catalog free.