Clay soil can challenge even experienced gardeners. This soil type holds excessive amounts of moisture, which prevents air from reaching the roots. According to Dr. Leonard Perry of the University of Vermont, most plants have a difficult time thriving in clay soils, and nursery-grown plants often cannot adjust at all. If you want to grow perennials in a clay bed, amend the soil with large amounts of organic compost, mulch and gypsum, till the soil down to 20 inches deep and choose clay-tolerant perennial species.
Goatsbeard (Aruncus dioicus) can thrive in clay soil; it grows up to 5 feet tall and produces showy, cream-coloured flowers in spring. A North American native, goatsbeard grows in large clumps and prefers partial shade. This perennial is hardy in U.S. Department of Agriculture Plant Hardiness Zones 3 to 8. Maidengrass (Miscanthus sinensis), an ornamental grass, produces large, white, feathery plumes in late summer that last through autumn. This attractive grass tolerates sun or shade conditions, heat, drought and clay soil. Maidengrass comes in a variety of cultivars that range from 3-foot-tall dwarf varieties to large, 15-foot-tall grasses that spread up to 6 feet wide. Most species are hardy in zones 5 to 9.
Golden Marguerite (Anthemis tinctoria) blooms with yellow, daisy-like blossoms from mid- to late summer. This perennial grows up to 2 feet tall and has fragrant foliage. Hardy in zones 3 to 7, Golden Marguerite can grow in clay soil but may have a shorter lifespan in this soil type. Leopard's bane (Doronicun spp.) tolerates both clay soil and a variety of light exposures, from full sun to full shade. Leopard's bane is hardy in zones 4 to 7 and produces daisy-like, yellow blooms in spring. This plant grows 12 to 24 inches tall.
Joe Pye weed (Eupatorium maculatum) thives in partial to full sun and tolerates moist, clay soil. This perennial attracts butterflies with its pale pink, fuzzy flowers. Joe Pye weed blooms in late summer though autumn and is cold-hardy down to zone 4. The Visions cultivar of false spiraea (Astilbe chinensis) blossoms in early to midsummer with large, deep-pink plumes. This astilbe can grow in full sun to partial shade and moist soils. Astilbes grow up to 4 feet tall and are hardy in zones 4 to 8.
Russian sage (Perovskia atriplicifolia) thrives in full sun and can tolerate clay soils. This aromatic perennial has silver foliage and produces long-lasting, purple to blue flower spikes in mid- to late summer. Russian sage grows up to 5 feet tall and is hardy in zones 3 through 9. Aster (Aster x frickartii) flowers are also purple to blue. These perennials produce their daisy-shaped, yellow-centred flowers in late summer through fall. Asters are hardy between zones 4 and 8, tolerate clay soil and grow best in full sun. Asters grow 12 to 48 inches tall.
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- University of Vermont Extension; Landscape Gardening on Clay; Leonard Perry Ph.D.; 2003
- University of Minnesota Extension, "Yard and Garden Line News"; Perennials for Clay Soils; Anne Gachuhi; July 2004
- North Carolina State University Cooperative Extension; Perennial Flowers; Alice B. Russell and Erv Evans
- Ohio State University: Miscanthus Sinensis
- Michigan State University Extension; Anthemis Tinctoria; November 1999