The size of a soil's particles determines if it is sand, loam or clay, according to the Colorado State University Master Gardening Program. Large particles make sand gritty. Small particles make clay sticky. Even soils containing only 20 percent small particles and 80 percent sand behave like clay. Clay soil is heavy, with compacted particles that compromise its airflow, and retains water. Some plants, nonetheless, have root systems strong enough to penetrate clay and thrive.
False Blue Indigo
False blue indigo (Baptisia australis) is a woody pea family perennial standing 2 to 4 feet tall and up to 3 feet wide. Growing wild from New Hampshire, south to Texas on prairies and woodland edges, it needs soils with the organisms that can produce nitrogen in its root system nodules. False blue indigo has dense, 4- to 16-inch spikes of blue violet flowers between April and July. Its bark becomes silver in autumn, when wind may snap the plants off at their trunks. Plant it in full sun and moist, acidic or neutral (pH 7.0 or below) well-drained clay.
Beebalm (Monarda fistulosa), also known as bergamot, belongs to the mint family. A popular ornamental perennial, it has aromatic leaves used in tea and for perfume. The 2- to 5-foot, open-branched plants grow wild in meadows and woods and along roadsides from the eastern United States to the Rocky Mountains. Beebalm has edible 2- to 4-inch pompom-like blooms in white, lavender or pink from May to September. They bring hummingbirds and butterflies to the garden. Plant mildew-susceptible beebalm where it will get good air circulation. It likes full sun to partial shade and can handle any soil type from acidic sand to alkaline clay.
Also called spotted geranium, wild cranesbill (Geranium maculatum) is a clay-tolerant, spreading perennial standing 1 to 3 feet high. Its small, five-lobed aromatic green leaves have red and yellow splotches in autumn. Wild cranesbill grows throughout the eastern United States as far west as Minnesota. From March to July, it produces delicate clusters of 2 to 5 pale lavender flowers that are 1 inch each. Remove the spent flowers to extend the blooming season. Plants spread slowly by rhizomes but are non-invasive. Wild cranesbill likes partly shady to shady locations with moist, acidic soil. It grows in sand, loam and combination soils as well as clay.