It seems that conversations with gardeners, if they go on long enough, will turn to the topic of clay soils: how difficult they are, how to fix them, or how to live with them.
Native plants are true multi-tasking tools in the landscape: they reduce pollution, provide habitat for wildlife, and supply gardeners deal with solutions for difficult site conditions – including clay soil.
So, what are good native plants for clay soil?
This will, of course, depend on where – precisely – you live. Generally speaking, however, you are looking for plants that can tolerate two particular conditions.
One is that true clay soil can sometimes contain very little oxygen relative to other soil types. Clay, by definition, is a soil with very small particle sizes and these small particle sizes form a tight soil with few pores between.
Another is that clay soil provides challenging moisture conditions. On the one hand, they don’t drain especially well and so they can be very wet. On the other hand, in drought conditions they clay particles bind very tightly with water molecules leaving little moisture available for plants.
As a general rule, both of these conditions are similar to ones found in poorly drained flood plains, river bottoms, or marshlands. Look for species that naturally grow well in areas that alternate between drought and flood.
Here are five species native to Maryland that I think work very well in clay soil.
Asclepias incarnata, or pink milkweed. This perennial is also known as swamp milkweed, which should be a clue to it’s suitability. It produces attractive pink blossoms in summer which are attractive to pollinators, and is a host plant for Monarch butterflies.
Iris versicolor, or blue flag iris. A native iris with extensive roots, this plant is drought tolerant once established but also very happy in excessively wet soils. Because of its massive root systems, it is also very effective at filtering pesticides from storm water.
Clethra alnifolia, or sweet pepperbush. Sweet pepperbush has upright clusters of fragrant white flowers and dark green leaves that turn yellow in fall. This shrub blooms in early summer, when few other plants are blooming, and a large stand of clethra is a veritable magnet for pollinators.
Ilex verticillata, or winterberry. This is the time of year that winterberry really shines: bright red berries on the female plants of this deciduous holly brighten the garden in fall and winter and provide an excellent food source for migrating or over-wintering birds.
Carpinus caroliniana, or American hornbeam. This small tree is a great choice for dense soils. The foliage is very attractive in fall, and is a food source for swallowtail butterfly larvae. Seldom exceeding fifty feet in height, this is a valuable understory tree.
Given that soils take thousands of years to form, it is usually unrealistic to expect that homeowners can amend clay soil sufficiently. If you want to try, incorporate organic matter (like compost or wood chips) NOT sand.
Best, though, is to simply ask yourself “what native plants LIKE clay soil” and build your garden around those plants. Native plants that like clay soil will be less work for you, and better habitat for wildlife.
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