Clay soil does not contain a lot of organic matter. This makes it easy for the soil to become compacted and form clods, a condition that most plants cannot survive in. Before putting in any plants, break up the clods and mix in some good organic fertilizer. Even plants that do well in clay soils will benefit from the added help.
Hollyhock prefers a well drained soil, but does well in a clay soil as well. They need to have full sun. Hollyhocks flower from June up to the end of August and can grow from 2 to 9 feet high. The flowers can be white, pink, peach, red, wine, lavender, purple and almost black . The flowers grow first at the bottom of the stem and work their way up to the top, which blooms last. Hollyhocks can be grown in most of the continental United States, from zone 3 to zone 8.
Moneywort or Creeping Jenny is a perennial ground cover. It does well in shade where grass will not grow and can stand up to a moderate amount of foot traffic. It will only grow to a height of about 2 inches, but can spread out as far as it is allowed to. It produces leaves about the size of a penny that grow on very thin stems. When planted in moist but well drained soil, the plant will spread rapidly. The heavier clay soils will slow its growth. It belongs to the primrose family and produces yellow flowers in the late spring, but the leaves are its best feature.
Thalictrum, also known as giant meadow-rue, produces a deep-green fern-like foliage and flowers that can be lavender or mauve in color that grow in clusters on top of deep purple stems. Both the leaves and the flowers are used in cut flower arrangements. Thalictrum likes partial shade and can take normal moist and well drained sandy or clay soil. It can grow to a height of 50 to 70 inches and might need to be staked. The plant can also spread out from 18 to 23 inches.
Kniphofia moench, also known as red-hot pokers, can be seen in temperate regions all over the world. The flowers can be red, orange, yellow, lime green or cream. Most of the varieties are evergreen and produce tall spikes of small tubular flowers that do not have stems. The leaves are long and arching and grow in clumps around the base of the plant.