Wet clay soil is a hostile environment for many garden plants. The microscopically small particles in clay swell into a nearly solid mass when wet. Water squeezes the air from between them, depriving plants rooted in the soil of oxygen. Designing your garden with ornamental plants and trees that flourish in wet soil, and improving your clay's drainage with organic material before planting, can help overcome the challenges of wet clay.
Butterflies and hummingbirds are regular visitors to the summer to midautumn bloom of swamp milkweed (Asclepias incarnata). This 2- to 4-foot, multiple-branched perennial has upright stems of narrow, lancelike green leaves. Flat, showy clusters of deep pink to purplish-pink flowers crown its stems, making swamp milkweed a standout in wet clay. Lizard's tail (Saururus cernuus) brings cooling, deep green, heart-shaped foliage and curving spikes of fuzzy, fragrant white summer flowers to wet clay soil. This water-loving, 3- to 4-1/2-foot perennial survives in as much as 4 inches of standing water. Both these plants do best in full sun to partial shade.
Coastal sweet pepperbush's (Clethra alnifolia) sweetly fragrant, 3- to 8-inch flower clusters unfurl at the ends of its upright stems between mid and late summer. The 6-to 12-foot, clumping green-leaved shrub brings hummingbirds, butterflies and autumn shades of orange and yellow to the garden. Coastal sweet pepperbush grows best in full sun to partial shade. It flourishes in acidic soils, including wet clay. Common buttonbush (Cephalanthus occidentalis), another up-to-12-foot shrub, has twisting branches of glossy, tapering green leaves. Protruding stamens cover its spherical, white June-to-September blooms. Ducks and other water birds feed on its round, brown seedpods. Fragrant buttonbush loves full sun and wet soil.
Texas red oak (Quercus texana), at 50 to 80 feet high with an up to 65-foot spread, has multiple, grey- or black-barked trunks and deeply lobed, dark green leaves. Its modest, yellow-green flowers appear in April. Red fall foliage gives this tree its major ornamental value. The wet clay-tolerant oak performs best in full sun. Brown dogwood is a small -- no taller than 15 feet -- Pacific Northwest tree happiest in partial shade and moist clay soil. Wild plants always grow in or along streams. Its silvery-green, spring and summer leaves become bright red in fall. Birds consume the pale blue berries that follow its clusters of white, early-spring flowers.
Bushy bluestem (Andropogon glomeratus) adds summer-to-winter colour, texture and form to the landscape. Its 2- to 5-foot stems' bluish-green summer leaves progress to coppery orange in fall and hold their colour through the winter. They contrast strikingly with the sheen of its fluffy, silvery blue metallic flower heads. Bushy bluestem's seeds are important winter food for a number of bird species. This ornamental, sun-loving grass thrives in high-saline soils as well as wet clay.