Clay-based soil holds onto water, containing it around plant roots. Many plants prefer a well-drained soil, without too much clay or too much sand, so the water reaches the roots, but doesn't puddle around them. Too much water at the roots can cause root rot. If your soil is clay-based, you can either amend your soil with compost or choose plants that flourish in, or at least will tolerate, a clay soil.
The potentilla (Potentilla fruticosa) is a shrub suited to many different conditions, one of them is clay soil (or bog conditions). Interestingly, it also does well in drought conditions. Potentilla can be found in varieties with pink, white or yellow flowers, which bloom from June to the first frost in your location. It is an easy-care plant that requires little attention after you plant it. Let it grow tall and wild or trim it down to the size you desire. But, save the trimming for late fall or winter, so you don't risk cutting the new growth and flower buds.
Be aware that potentilla bushes do not do as well when they are grown in containers; it seems the roots prefer to be free growing. If you do pot your potentilla bush, be sure it is given adequate water, especially in the dry, hot days of mid summer.
You might mistake the black-eyed Susan (Rudbeckia) flower for daisies if they were white with yellow centers. However, these bold blooms have golden yellow petals and deep brown centers. The most common varieties grow to 24 inches. A moist soil is preferable, the reason they do so well in clay soil. These flowers are sun lovers, so make sure you plant them in a location where they will receive sun for at least 6 hours of the day.
The Michaelmas daisy (Aster) is a common daisy-like flower, which grows in bright colors of blue, pink, purple, lavender, white and yellow. It is actually a member of the sunflower family. This flower blooms in the autumn season, normally. It may begin blooming, however, in mid to late summer in warmer climates. If you'd like this perennial to mature into a bushy plant, trim it in the early spring, down to about 12 inches. If they are planted under too much shade, mildew may develop. So, it's best to find a full-sun location in which to plant them. Asters thrive in containers on the deck or patio.