Learn to Love ... Your Clay Soil

You dream of creating a dynamic mixed perennial border. You buy lots of fantastic plants. Then you start digging - into your heavy clay soil... You start looking for a different spot to plant your dream garden. What's an enthusiastic gardener to do with heavy clay soil? There are three ways you can garden with clay soil: (1) amend the soil with organic matter; (2) raise or berm the soil; or (3) choose the right plants and learn to love your clay soil.

1. To improve clay soil you can .... amend the soil.

Let's start by looking at the structure of soil. Soil is made up of: rock particles; water; air; and organic matter. The ratio of the particles (clay, sand and silt) determines your soil type. Clay soil is forty percent or more clay particles. Clay particles are the smallest rock particles. Individual clay particles bind tightly together and create horizontal sheets. These sheets stack up to create layers. This compact structure limits the ability for water, air or plant roots to penetrate the soil. Dig a hole, and up comes slabs or clods of clay soil. This soil does not break down into workable "crumbs". It resists breaking down. Water drainage is an obvious issue. Clay soil is often water-logged in winter and spring. In summer, the surface of clay will harden, crack and/or fissure in the heat. Water will run off the hard surface. Thus, erosion can be an issue. If you add top soil, on top of a hill of clay, it will wash away after a heavy rain. If water penetrates the soil, it will hold onto that water - so clay soil is relatively drought resistant. Clay particles do attract minerals such as potassium, magnesium and calcium. So on the positive side, clay soil can be a nutrient-rich soil.

You can try and change the structure of clay soil by heavily amending it with organic matter such as composted manure, bark or leaf mulch. This can help to "open" up the soil. The humus coats the clay particles and helps to prevent these particles from sticking together. Humus breaks down over time. You will need to amend the soil every year. To begin with, add a ten cm layer of organic material to the soil and try and work it into the soil. Avoid working the soil when wet. Working the soil, either by digging or rototilling, will bring up huge clods of clay that are difficult to break up. Walking on the wet soil will compact the soil further. If you want to manually move the organic matter into the soil, it is best to do this work in the fall and then let the winter weather break down the clods. Every fall after the initial amendment, add an additional five to ten cm of organic matter to the top of the soil and let the earthworms work it into the soil. You can also use cover crops (eg. buckwheat, grasses & grains, legumes, clover) as green manures. Sow crops with dense fibrous root structures (eg. oats) in late summer/fall, if the winter cold does not kill the crop cut back in spring.

What about adding sand to open up the soil? Simple answer - don't. You will need to add a tremendous amount of very coarse sand to the clay soil at a ratio of half and half. If you add sand that is too fine, like playground sand, you will end up with concrete. So the safest bet is to stick with adding lots of organic matter. Your soil will improve, albeit slowly, over the years. I have been adding organic matter to the clay soil in my vegetable garden and presto, twenty years later, I have a 20 cm layer of good workable soil. Dig any deeper ... and you will find dense clay.

2. To improve the drainage of clay soil... raise the level of the plant's root zone.

Photo: A berm in my garden with thyme, lavender, russian sage, santolina and catmint.

While amending the soil can help improve the structure of the soil it may not improve poor water drainage. Raising the root zone of the plant above the "ground" level of the soil will help with drainage. Raised beds are often used for vegetable gardening and are usually filled with a good garden soil mix. A typical raised garden bed has a flat surface and a rectangular shape. It will be raised 15 - 30 cm above ground level. On the other hand, a berm is a rounded mound of soil. It will need to be at least 30 cm above the surrounding grade. For every 30 cm in height it will need to be 1.8 m in width. A berm needs to have a gentle slope. Berms are often made up of a bottom layers of fill, intermediate layer of clay (to hold the shape of the berm), and then a layer of top soil. A berm may be ideal for a herbaceous border. Heavy equipment is often needed to move the soil. Of course, the process of creating the berm will result in more compaction of the soil in the surrounding area.

3. OR choose the right plants and learn to love your clay soil!

Photo: What do these plants have in common? They love clay soil! Top left to bottom right - Amsonia tabernaemontana, Ratibida pinnata, Iris sibirica, Veronia novaboracensis, Eryngium yuccifolium, Asclepias incarnata. Images from Gardenia.net

Choose plants that cope, and better yet, thrive in clay soil. Forget amending. Forget moving soil around. Many plants that are native to prairie meadows thrive in clay soil. Here is a partial list of perennial plants that can be used to create a naturalistic meadow type planting in clay soil. Of course there are grasses, trees and shrubs that also thrive in clay - but that's a topic for another post!

• Amsonia tabernaemontana (Bluestar)

• Asclepias incarnata (Swamp milkweed)

• Aster ericoides (Heath aster)

• Baptisia australis (Blue false indigo)

• Camassia quamash (Common camas)

• Convallaria majalis (Lily of the valley)

• Coreopsis tripteris (Tall tickseed)

• Crambe cordifolia (Sea kale)

• Dodecatheon (Shooting star)

• Echinacea purpurea (Purple coneflower)

• Eryngium yuccifolium (Rattlesnake master)

• Eupatorium maculatum (Joe Pye weed)

• Filipendula rubra (Queen of the prairie)

• Geranium sanguineum (Hardy geranium)

• Heliopsis helianthoides (Oxeye)

• Helenium autumnale (Sneezeweed)

• Helianthus salisifolium (Willow-leaved sunflower)

• Hemerocallis fulva (Day lily)

• Iris sibirica

• Liatris pycnostachya (Prairie blazing star)

• Liatiris spicata (Blazing star)

• Ratibida pinnata (Grey-headed coneflower)

• Solidago lepida (Canada goldenrod)

• Veronia novaboracensis (Ironweed)

Still not convinced that you can create beautiful garden with clay soil? Looking for inspiration?

Check out James Golden's Federal Twist. He describes his soil "like wet plastic"...

https://federaltwistdesign.org/ Garden Masterclass interview accessible from here

https://www.youtube.com/watch?v=4m1pmm7mB00

For past posts visit the Blog page: https://www.leslieanngardendesign.ca/blog