Ways to Improve Clay Soil in the Flower Garden

Person with gloves on planting a pot of marigolds.

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When you see standing water in your garden in the spring, but the same area transforms into a web of cracks in the summer, you have clay soil. There are several ways to improve clay soil in the flower garden, but as they say, when you have lemons, make lemonade: you should also plant flowers that are tolerant of heavy soils.

What Is Clay Soil?

As a component of soil, clay is inert; it is a mineral rather than the product of decayed organic material. Two features of clay contribute to its behavior in the garden: the size and shape of the particles. Whereas a grain of sand may be roughly 2 millimeters in size, a particle of clay is only about .002 millimeters in size. Furthermore, clay particles fit together in a sheet-like structure, giving soils with a high clay content the ability to retain water. The very qualities that make clay a great substance for creating pots and jars can cause big problems in the garden when rainwater can't drain away.

Do You Have Clay Soil?

Clay soil is made of fine particles that cause it to stick together, preventing normal drainage processes that keep plant roots healthy. If a gardener squeezes moist clay soil into a ball, the soil will retain that shape. Gardeners who regularly fight chicory, sorrel, and buttercup weeds are probably dealing with the clay soil that attracts these weeds.

Amend the Soil

Some gardeners make the mistake of adding sand to clay soil, thinking that adding matter from the opposite soil type will moderate the problem. However, sand and clay make the soil more akin to concrete than the rich loam all gardeners desire. The best soil conditioner for clay is organic matter and lots of it. Gardeners should add organic compost, leaf mold, and green manures from cover crops to improve soil structure and tilth. Although you'll get quicker results by mixing the amendments into the native soil, you can let earthworms do the work for you: they'll burrow up to feed on the organic matter, pulling it into their tunnels, which themselves aerate and improve the soil.

Garden centers commonly sell gypsum as a soil amendment for clay conditions. However, unlike organic matter, gypsum does nothing to improve the fertility of the soil. Furthermore, according to the Puyallup Research and Extension Center at Washington State University, gypsum negatively affects the mycorrhizae that enhance root health and development.

Try Double Digging

Double digging is highly laborious, but it can be an effective way to remedy extremely heavy soils in small areas. The double-digging method involves digging a 2-foot deep trench in the garden, filling it with a compost and soil mixture, and returning the native soil to the trench and mixing it. The toil involved in this project makes double digging a four-letter word for many gardeners. If you go this route, plant flowers that won’t take your efforts for granted such as roses.

Install Raised Beds

Creating raised beds is an easy way to subvert many soil problems, whether sandy or clay-based. Gardeners can choose the type of raised beds they want, exploring lasagna gardening options, raised garden bed kits made of composite or wood, or even straw bale gardening for the back of the flower border. Raised beds have the bonus of warming up quickly in the spring for earlier planting.

Prevent Soil Compaction

Clay soil is naturally dense and tends to waterlog easily, and these characteristics become exaggerated in the spring. Gardeners must take care not to walk on clay soil or work it with a tiller when it’s cold and waterlogged, or it will become nearly impervious to water and oxygen. Work clay soil gently with a pitchfork when it’s as moist as a wrung-out sponge, as this garden tool maintains the integrity of the soil strata.

Choose Plants for Clay Soil

Plants that thrive in clay soil are those that can cope with the extremes of sogginess and dry cracked earth that clay can bring. Gardeners with sunny spots can choose daylilies, coreopsis, and asters. Gardeners with shady clay soils can plant forget-me-not and ajuga. Plants such as Joe Pye weed tolerate clay soils, but gardeners must remember that the "weed" part of a plant’s name is there for a reason, and these plants can be invasive. Plants to avoid include all that thrive in rock gardens such as sea thrift, campion, anemones, and dianthus. These plants require sharp drainage, which means the constant movement of water through soil and away from roots.