Planting in Clay Soil

Credit: Barbara Rayment

If you are trying to garden in heavy clay soil, and wondering if you should just trade in your gardening gloves for a potter’s wheel, don’t despair.

You have several options, and a gorgeous garden is indeed possible. The bright side is that these heavy soils hold water well and usually have good mineral content. On the other hand, clay barely drains at all and is extremely difficult for plant roots – or shovels – to penetrate. It is sticky gumbo when wet, and concrete when dry. There are several different ways to deal with clay, and I have used all of them in the gardens here, depending on the type of plants I want to use and how much work I am prepared to put into it.

The easiest solution, especially for those of us who prefer low-maintenance gardening, is to stick to plants that are well adapted to heavy soils. There are a surprising number of them, so our palette isn’t limited by anything other than our imagination. If you want a broader range of plants – roses, say, or a productive vegetable garden – it’s necessary to either improve the existing soil or to avoid the matter entirely by installing raised beds. Each of these has its pros and cons. Improving clay soil requires the addition of both gypsum and organic matter. Gypsum is a naturally occurring substance that bonds with the clay molecules to open it up so that it drains more freely and plant roots can get down into it.

In the garden, a 22-kg (50-lb.) bag will treat approximately 18 square m (200 sq. ft.). Top-dressed or tilled in, this is a one-time application that just has to be watered into the clay for it to start working, although the visible results are not instantaneous. The second step is to amend the soil with large quantities of organic matter, which builds up humus in the soil. Tilled in and/or used as a mulch, annual additions of aged wood chips, finished compost, mushroom manure, coco fibre or leaf mould will eventually add up to a deep rich soil.

Use different amendments every year, and test your soil every few years to make sure you’re not adding too much of any specific nutrient. (It’s easy to do with composts or manures.) When working to improve clay soil, it’s important to address the entire garden and not just the planting hole, as this results in a “bathtub” effect as the hole fills with water and it doesn’t move to the surrounding clay soil. It’s also best not to work the soil when it is wet as this compresses it. Sidestepping the problem of clay soil by building raised beds is often the first choice of busy or impatient gardeners. A foot or two of imported topsoil, the most that can practically be contained in a raised bed, allows enough root run for most annual vegetable crops and shallow-rooted fruits such as strawberries. It is surprising how deeply rooted many plants are, however, and these plants will very quickly encounter the clay that still exists under the raised bed. The clay layer will interfere with drainage and root growth, leading to perennials and shrubs that do well for the first year and then go downhill. A combination of the two methods – a layer of gypsum on the existing clay and a raised bed built over it – can offer the best of both. The gypsum will have done its work by the time the roots get down that far, and plant growth can continue uninterrupted. Beautiful gardens are entirely possible on clay soils, with the added bonus of less frequent watering than if you have fast-draining sandy soil.

Cold-tolerant plants for clay soils:

The following plants are hardy to the zone number indicated: • Acer rubrum ‘Northwood’, A. saccharinium, A. tataricum, A.tataricum subsp. ginnala (maple) – zone 3 • Achillea (yarrow) – zone 3 • Aster alpinus, ericoides, macrophyllus and northern native asters – zone 3 • Baptisia australis (false indigo) – zone 3 • Campanula glomerata and C. punctata species and cultivars, C. takesimiana (avoid aggressive C. rapunculoides) – zone 3 • Cornus alba (redtwig dogwood) – zone 2 • C. stolonifera (red osier dogwood) – zone 2 • Eupatorium maculatum, E. purpureum species and cultivars (Joe Pye weed) – zone 3 • Filipendula rubra, F. vulgaris, F. hexapetala species and cultivars (meadowsweet) – zone 3 • Helenium hoopesii (sneezeweed) – zone 3 • Hemerocallis species and cultivars (daylily) – zone 3 • Iris setosa (Arctic iris) – zone 3 • I. sibirica (Siberian iris) – zone 3 • Larix laricina (tamarack, eastern, American or Alaska larch) – zone 2 • Leucanthemum hybrids (Shasta daisy) – ‘Alaska’ is the hardiest at zone 2, many other good cultivars, ‘Silver Princess’, ‘Aglaia’, ‘Becky’, ‘Marconi’ are zone 3. The fancier (more frilled and double) ones tend to be less hardy and need better soil. Liatris aspera, L. pycnostachys, L. spicata species and cultivars (blazing star, gayfeather) – zone 4 • Lupinus polyphyllus (lupine) – zone 3 • Quercus bicolor (swamp white oak) – zone 4 • Q. macrocarpa (bur oak) – zone 3 • Q. palustris (pin oak) – zone 4 • Persicaria bistorta ‘Superba’, P. affine ‘Dimity’, ‘Darjeeling Red’, other cultivars (fleeceflower, may also be sold as Polygonum) – zone 3 • Rudbeckia fulgida var. sullivantii ‘Goldsturm’, R. laciniata, R. maxima, R. triloba (coneflower) – zone 3 • Sanguisorba menziesii (Alaska burnet) – zone 3 • Solidago canadensis ‘Baby Gold’ (goldenrod) – zone 4 • Tilia cordata (littleleaf linden) – zone 4 • Trollius europaeus, T. chinensis, T. x cultorum (globeflower) – zone 3 Barbara Rayment operates Birch Creek Nursery, in Prince George, where she grows and experiments with a wide variety of hardy plants.