Clay soil presents a problem for gardeners since it is heavy, poorly draining rather than easily amended. When wet, it is sticky and pliable; sturdy clay soil develops breaks and becomes hard. On the plus side, clay holds moisture and nutrients. Many plants won’t grow in clay soil, but quite a few species, like U.S. indigenous plants, thrive in clay that’s in a full-sun site. The best ground covers for such an area are those that provide visual interest for multiple seasons and don’t need frequent maintenance.
Clay soil exposed to winter flood presents particular challenges, and several ground-cover plants have the ability to withstand these problems. 1 candidate is brown-headed creeping rush (Juncus phaeocephalus phaeocephalus), a low-growing indigenous plant suitable for coastal or low-lying areas. The plant is all about 1/2 to 1 inch tall and is perennial in U.S. Department of Agriculture plant hardiness zones 7 through 10. Yet another offender is California goldenrod (Solidago californica), which grows best in areas that are wet in winter but dry in summer. It reaches 15 to 36 inches tall, bears bright-yellow blossoms in summer and is hardy in USDA zones 6 through 10.
Some ground-cover plants that tolerate clay soil also take dry conditions and are suitable for low-water areas. Among these plants that offers evergreen shade is the shrubby “Pigeon Point” cultivar of dwarf coyote brush (Baccharis pilularis pilularis “Pigeon Point”), that grows 12 inches tall and up to 10 feet wide. Once established, it is very drought-tolerant. It is hardy in USDA zones 7 through 10. Yet another low-growing, evergreen shrub that tolerates clay dirt and dry conditions is “Carmel Sur” manzanita (Arctostaphylos edmundsii “Carmel Sur”), that bears urn-shaped, pink blossoms in spring. Hugging the ground at 4 inches tall, this plant may spread 18 feet broad. Suitable for coastal regions, “Carmel Sur” is perennial in USDA zones 8 through 10.
Enjoy bright flowers in addition to ground coverage from full-sun, clay-tolerant plants such as threadleaf coreopsis (Coreopsis verticillata) and daylily (Hemerocallis spp.) . Threadleaf coreopsis is actually the best coreopsis for clay soil, according to the August 2010 “Garden Beet” newsletter by the Jackson County, Oregon, Master Gardener Association. Profuse, yellow, daisylike flowers cover the 18- to 30-inch-tall plant from early to late summer. It is hardy in USDA zones 3 through 9. Mass plantings of daylilies make effective ground covers in flowerbeds and on slopes, with the plants’ slender leaves forming clumps that protect the dirt. Daylily flowers look on stalks held over the leaves and therefore are in a wide range of colors, from yellow, red and orange to cream and purple, and some blooms have multiple colours. Daylily grows 1 to 3 feet tall and 1 to 2 feet broad; it is hardy in USDA zones 3 through 10.
Sometimes you might want ground covers with leaf colors other than green to contrast with background plantings or as foils for intense colors in clay, full-sun gardens. 1 option is the soft-textured, silvery gray leaf of “Canyon Gray” sagebrush (Artemisia californica “Canyon Gray”), that spills over planter edges and rock garden terraces. That aromatic ground cover is hardy in USDA zones 7 through 9. A ground cover with gray-green leaf is California fuchsia (Epilobium canum subspecies latifolia), which also has brilliant orange-red, tubular blooms in summer and autumn. It grows 4 to 20 inches tall, is perennial in USDA zones 8 through 10 and tolerates full sun to partial shade and clay to sandy soil.