University of Vermont Extension
Department of Plant and Soil Science
GARDENING ON CLAY SOILS
Dr. Leonard Perry, Extension
University of Vermont
Many gardeners are stuck with clay
soils that are hard to cultivate, and in which many plants don't
How do you know if your soil is clay? How can you improve clay
What plants grow best in clay?
If your soil dries like a brick with
cracks when it's dry, or in clods, and is like putty when wet sticking
and tools, you have clay soil. It is
hard for most roots to penetrate such soil. Take some soil, add a
and form a ball in your hand. Then
squeeze the ball into a flat ribbon. If
the ribbon reaches two inches long or more before breaking, this is a
have clay soil.
Soil particles come in different
sizes, which contribute to soil texture.
Sand particles are the largest, clay particles the smallest.
Being so small, clay particles pack together
not allowing the necessary spaces between them for air and water to
especially air that most roots need to function.
Contributing to this is the fact that clay
particles are plate-like, stacked like a deck of playing cards, with
space between them. This traps water for long periods. One positive
clay is that its particles hold onto nutrients, making them more
So how do you increase the space
between these particles? Some recommend
adding sand, but unless it is coarse sand, and you add about 3 parts to
part clay soil, the soil structure will likely just get worse.
When planting, some recommend putting gravel
in the bottom of the planting hole. This
is bad as it just raises the water level, creating what is termed a
water table". Another
recommendation is to add gypsum, similar to lime, but this may increase
and pH levels too high (clay soils tend to be alkaline).
Best is to add organic matter,
particularly compost. Be sure and add
when your clay soil is dry, as working in wet clay is not only quite
will compact it even further. Peat moss is sometimes recommended as an
addition, but this breaks down quickly in wet and clay soils, and can
undesirable (for most plants) bog.
Compost is good as it not only helps with the soil structure, but a
compound (glomalin) the microorganisms (mychorrhizal fungi) in compost
binds the small clay particles together into aggregates with a waxy
thus creating more space between them for air and water to flow. As an
glomalin also benefits the soil, and earth in general, by storing
You can't really overdo the amount
of compost added to clay soils. For a
lawn and landscape that is a quarter acre, just increasing the soil
matter from 2 to 3 percent would take 5000 pounds of an
amendment! Don't get discouraged by this, as adding some
is better than none. You can add organic
matter over time, and you can deal with small areas or beds at a time.
to 4 inches at minimum is recommended.
If you have a local compost facility, check to see if you can have a
bulk load delivered. This is cheaper
overall and avoids having dozens of plastic bags to dispose
Best is to work organic matter
deeply into the soil as roots will eventually end up there, and do so
planting. You can till as deep as a
tiller will go. Or, dig sections of a
bed at a time to a foot deep, work in compost, then replace the soil
and do the
next section. In future years, just work
the surface, as organic matter you've worked in deeply will decompose
when brought up to the surface.
Another method to increase organic
matter in clay soils is with cover crops.
These are crops
planted as you prepare beds, for a season or year prior, or in fallow
between annual crops such as flowers and vegetables. They consist
of small grains and grasses like
buckwheat, ryegrass, and oats. Legumes,
such as clover, also benefit by "fixing" nitrogen from the air for
use in the soil. Cover crops have
additional benefits such as suppression of many weeds.
If all this sounds like too much
trouble, till or break up with a spading fork the area you'll
plant. Then build a berm, or raised bed with sides 6
to 12 inches high or more, over it and fill with a good loam topsoil.
use drainage tiles, or pipes, on the bottom if a wet area.
If you're planting an individual
tree or shrub, it is especially important to chose ones that will
soils. Their root systems are so
extensive that you can't amend the soil well enough over a large enough
especially for trees. Dig the hole only
as deep as the pot or rootball.
Otherwise it will sink over time, causing the plants to end up too
low. This is a major cause of woody
plant demise after a few years.
Don't make flat sides to the planting hole
that wont let water drain. You'll only
be creating a bathtub for the roots, without a drain. And only
amend the backfill soil with no more
than one third of a better soil as the plants were growing
If planting large plants, break up an area
around where you'll plant (out to as wide as the plants will eventually
with a long spade or fork. Add organic
matter on top of this area which, over time, will work into these
For low, groundcover perennials that
tolerate heavy clay soils, consider the carpet bugle or dead-nettle (Lamium),
both of which can spread vigorously. Low
to medium height perennials, mainly attractive for their foliage and
massing, include the lady's mantle, sea thrift, pigsqueak, daylily,
iris, Japanese painted fern, Ostrich and Cinnamon ferns, and the
flowering globeflower. Most spring-flowering bulbs need well-drained
checquered lily (Fritillaria meleagris)
will tolerate clay and periodic wet soils.
Taller perennials for clay soils
include aster, Helen's flower (Helenium),
foxglove, coneflower (Echinacea),
false sunflower (Heliopsis),
blazing star, black-eyed daisy (Rudbeckia),
meadow rue, Joe-pye, and the
compass plant (Silphium).
have a contained area you might also consider these aggressive
spreaders: white or yellow loosestrife, plume poppy, and
bee balm. Several grasses will live in
clay soils including the switchgrasses and eulalia grass.
There are actually many shrubs that
will tolerate clay soils, some better suited to them. These
include the chokeberry, Siberian
peashrub, shrub dogwoods, forsythia, common ninebark, potentilla,
shrub willows, and viburnums.
Best choices for smaller trees
include European alder, river birch, hawthorns, crabapples, and
pear. For large trees consider
hickories, hackberries (a native plant and vase-shaped replacement for
black and green ash, common honeylocust, Kentucky coffeetree,
amur corktree, cottonwood or aspen, oaks, willow, linden, and elm.