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Got Clay?

by Cindy Gilberg

Recent lack of rain has turned most soils into a hard surface with fissures and cracks, thirsty for a good autumn rain. Missouri, well known for its clay, was once famous for bricks and pottery made from that same sticky clay. Clay, an important component of soils, is electrically charged and attracts molecules (necessary plant growth nutrients such as potassium, calcium and nitrogen) to its surface. In his book, “Dirt—the Ecstatic Skin of the Earth”, Logan states that “from the locked closet of the mineral world, clay becomes an open shelf, where roots of plants may shop for what they need” to maintain healthy growth. While clay is a necessary component of healthy soils, it doesn’t take much and a high clay content in the soil will send most gardeners into a panic!

If you can make a pinch pot out of your soil, reminiscent of Ceramics 101, then your soil has a high amount of clay. Many homes have had their topsoil removed during construction and are left with red, rocky subsoils not intended for growing plants. Keep in mind that construction rubble or stripped subsoils are not considered clayey garden soils and will need some amending with topsoil and compost. Much of the poor soils in the St. Louis region are not so much high clay as they are compacted silt soils with very little organic matter (compost). When designing and planting native plants, beware of using too much compost as most sun-loving native plants are not used to overly rich, organic soils. The best approach is to follow the “don’t fight the site” guideline by using plants that are tolerant of more clayey soils. It is, in addition, a good situation for using our resilient and beautiful native plants. They are better adapted to soil and climate conditions here and will contribute to good habitat so that your view will be filled not just with plants but also birds and butterflies.

Design the landscape to include a diversity of plants referred to by many as “claybusters”. Some native plants, such as deeply rooted prairie plants, are well-adapted to thrive in conditions that make other plants curl up and die. Their roots not only tolerate clayey soil but function to break up and improve its texture as they grow. These problem-solving plants can be used in any design style, from formal to naturalistic. Add them into your existing landscape or use them to replace plants that are struggling or that have died.

One of the most difficult sites is one that has a soil with both a high clay content and that is wet. Many sedges (Carex annectans; C. muskingumensis) and rushes (such as Juncus effuses) are well adapted for these conditions. Other wetland species like marsh milkweed (Asclepias incarnata), shining bluestar (Amsonia illustris), rose mallow (Hibiscus lasiocarpus) and copper iris (Iris fulva) are good, sun-loving choices for planting in this situation.

Another idea for sunny, dry locations is to plant a “prairie garden”, a more natural style garden using a plant palette made up of prairie species. Use a widely spaced planting of shorter grasses, for example, prairie dropseed, little bluestem and shorter sedges such as Carex praegracilis or C. pennsylvanica. Interplant these with groupings of forbs, such as false indigo, blazingstar, butterfly milkweed, showy goldenrod and aster, which bloom above the grasses’ fine texture. For a more formal look, design with naturalistic groupings of plants, weaving them together while paying attention to contrasting textures.
The following list includes just a few of the attractive, sun-loving, clay-tolerant native perennials with garden merit for use in dry sites.

Flowering perennials:
Agastache foeniculum—Anise hyssop
Asclepias tuberosa— butterfly milkweed
Baptisia—false indigo
Dalea purpurea—purple prairie clover
Echinacea pallida— pale coneflower
Eryngium yuccifolium—rattlesnake master
Heliopsis helianthoides—false sunflower
Liatris spp.— blazingstar
Monarda— beebalm/bergamot
Penstemon digitalis—foxglove beardtongue
Rudbeckia fulgida—orange coneflower
Solidago spp. —goldenrod
Symphiotrichum oblongifolius —Aromatic aster
Short prairie grasses:
Bouteloua curtipendula—side oats grama
Schizachryrium scoparium—little bluestem
Sporobolis heterolepis—prairie dropseed

Cindy Gilberg is a Missouri native and horticulturist whose work includes design and consulting, teaching and writing. Much of her work focuses on native plants, habitat gardens and rain gardens. Cindy’s projects include work at Shaw Nature Reserve and its Native Plant School, the Shaw Profes-sional Landscape Series and the Deer Creek Watershed Alliance. You can ontact Cindy at 314-630-1004 or cindy.gilberg@gmail.com.

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