Clay soils remain saturated, which can rot plant roots and lower stems. Plant roots often have trouble drawing oxygen from clay soils. Amending the soil with organic matter can help, but adding insufficient quantities can turn the clay to baked brick under the hot summer sun. Fortunately, several types of flowers will thrive in clay soil, helping to break up the clay soil and drawing in water and nutrients as they bloom.
Spring bulbs usually require well-drained soil to prevent their underground nutrient storage systems from rotting in early-spring rains and snow melts. However, crocus (Crocus imperati), grape hyacinth (Muscari) hyacinths (Hyacinth orientalis), daylilies (Hemerocallis), and irises (Iris) all do well in clay soils.
These spring bulb types also bloom in succession, creating a floral display that will last from earliest spring to mid-summer and return each spring with minimal care. Adding a clump of sedum will finish off the clay-soil garden blooming season with late summer and autumn color.
Another advantage of spring bulbs and sedum in clay soil is that they do not need to be divided often, perhaps every four years, so the soil experiences minimal disturbance and compaction.
Roses love clay soil. Their hardy roots break through the clay structure, drawing nutrients and water from deep in the ground, while the clay helps anchor the plants against wind, provides nutrients necessary for rose blooms, and deters rodent damage.
For the heaviest clay soils, try more rambling and wild rose varieties such as Rosa rugosa, which also has the advantage of producing large beautiful and edible rose hips in the fall, or Rosa palustris, the swamp rose which thrives in wet soils. Both have large five-petaled pink flowers.
The Rosa multiflora, which blooms in sprays of white roses, is a wild import from Asia which grows in clay or poor rocky soils, but may have to be contained to preclude it from taking over your yard--or your neighborhood.
Herbs are notoriously adaptive, thriving in poor soils with little care. Tea herbs like purple coneflower (Echinacea), St. John's Wort (Hypericum), and Yarrow (Achillea millefolium) present showy displays of late-summer blooms, and thrive in difficult clay soils.
Many ornamental and edible varieties of sage (Salvia) are superb clay-soil flower choices, as is betony (Stachys) and wild thyme (Thymus serpillum), and in warmer climates, bay (Laurus nobilus). Plant a separate clay-soil herb garden, or intersperse flowering herbs with clay-tolerant spring bulbs for a full-season flowering perennial bed.