One of the most common complaints we hear at the nursery is, “I have terrible clay soil.” While clay soils have a bad reputation, it’s not necessarily bad soil. It can be rich in nutrients and excellent at holding moisture. It’s also a relatively easy problem to tackle in the garden.
The trick is to stop fighting it.
One of our favourite solutions is to plant into broad, gently raised mounds, creating a well-drained, healthy growing zone where plants can thrive. By raising the root zone of plants, you improve drainage, give roots access to well-structured soil and create a growing environment that’s much easier to manage.
Why clay soils get a bad rap
Clay soil can be literally hard to work with. When it’s dry, it can become rock-hard and difficult to dig and, when wet, it can become heavy and waterlogged. Compacted clay restricts root growth and reduces the air spaces roots need to spread, while waterlogged soil can lead to root rot. Working clay when it’s too wet can also make compaction worse.
But clay isn’t all bad. Its fine particles can hold nutrients and moisture extremely well, making it potentially very fertile soil. The challenge is creating the right structure and drainage for plants to thrive.
Why planting in mounds work
One of the best ways to work with clay soil, rather than against it, is to create mounded garden beds. By raising the planting area above the existing clay, you give plants a deeper layer of well-structured soil with better drainage and plenty of room for roots to grow.
After planting, roots have time to establish in loose, amended soil rather than immediately hitting dense clay. By the time they reach the underlying clay, they have a well-established root system and access to the moisture and nutrients held within it.
Don’t add sand to clay! |
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It might sound instinctive, but adding sand to clay can create problems. Small amounts of sand mixed into heavy clay can actually produce a dense, concrete-like texture rather than fixing drainage. |
How to create a mound
The basic idea behind planting on a mound is simple: build up layers above your existing soil to create a healthy growing zone that you can plant directly into. The approach is similar to hugelkultur, lasagne gardening and no-dig gardening, but you don’t need to build a huge mound from logs and branches. For a garden bed, a simple combination of cardboard, quality soil, compost and mulch is often all you need.
Map out your garden bed with cardboard or several layers of newspaper, making sure to remove any non-biodegradable materials such as tape and staples. Thoroughly wet this layer before adding your growing medium, building it up with a combination of quality soil, compost and other suitable soil conditioners. Feather the edges of the mound into the existing soil so it looks like a natural part of the garden, rather than a pile of dirt dropped on top.
And that’s it! You can plant directly into your newly created growing medium. Finish with a layer of mulch to help retain moisture and protect the soil, taking care to keep it away from plant stems and tree trunks.
Should I mix the medium into the clay soil?
It can be tempting to fix problematic clay soil by simply digging it out and replacing it, but this can create more problems down the track. When you dig into heavy clay, particularly when it’s wet, you can create a bowl that traps water rather than allowing it to drain away. Essentially, you’ve built a pond. And no amount of organic matter is going to turn a pond into a well-drained garden bed.
By creating mounds, you’re not only giving plants a loose, well-structured growing medium, but also gradually improving the clay soil beneath. As organic matter breaks down, it provides food for beneficial microbes and earthworms, helping to improve soil structure and create more air spaces within the clay over time.
Clay soil doesn’t mean you can’t have a beautiful garden.
In many cases, the easiest solution isn’t to spend years trying to transform the soil beneath your feet. Build the right growing conditions above it.
A broad, gently sloping mound filled with good-quality soil can give your plants the drainage, air and root space they need to thrive, while the clay underneath continues doing what clay does best: holding onto moisture and nutrients.
