Clay soils are not always bad. They hold more water than sandy soils and are often high in nutrients plants need. But clay soils can become so waterlogged that they deprive plant roots of oxygen, or so dry that they become too hard to dig in. To determine how much clay is in your soil, feel the soil. Run a pinch of moist soil between your fingers. The smoother it feels, the more clay it contains. If you can roll moist soil into a ball that hangs together, it contains more clay. Fortunately, you can find plants that thrive in clay soils and soil amendments that will help you grow a wider range of plants.
Often plants with taproots will grow well in clay and their roots help break up tough clay soils. Butterfly milkweed, bluestars, and Silphiums have deep tap roots that penetrate clay and that make the plants drought tolerant. Bluestars bloom in spring and have great fall color. Silphiums grow very tall and have yellow flowers. Butterfly milkweed is short to medium height with bright orange flowers blooming all summer. Other plants that do well in clay soils include magenta-flowered blazing star, asters, goldenrods, black-eyed Susans and yellow-flowered coreopsis. Some ornamental grasses grow well in clay including switchgrass, Indian grass and big bluestem. Most of these plants are plants that grow in meadows and prairies and they prefer full sun. Some shade-tolerant plants for clay include ferns, wild ginger, and wood aster.
You are not limited to perennials in clay soils either. Fragrant bayberries thrive in clay as do showy winterberries. Fall flowering witch hazel and spring flowering dogwoods and viburnums also grow well in clay.
Even plants that love clay will appreciate a little help. Adding compost to the planting hole or amending the clay soil with Miracle-Gro® Organic Choice® Garden Soil will give new plants a good start. The organic matter will loosen the soil and improve drainage. For trees and shrubs, smaller plants and bare root plants may establish better than large plants because their roots will adapt better to the clay soils around them. Lime or gypsum will also help improve clay soils. Avoid adding sand because it will mix with the clay and form cement-like soil. Another way to improve soils is to plant a green manure - a plant that will grow during the fall or winter that you then till into the soil to add organic matter. Winter rye can be planted as a green manure in fall and turned under in early spring.
Article by Sylvan Kaufman. Dr. Kaufman is a writer of popular scientific and gardening articles. She is also an ecological consultant.