Plants and Tips for Clay Soils

Check out all the plant varieties that we have tested in our local soil here.

Within Horfield, Filton, Bishopston, Ashley Down, Lockleaze (apart from the very north end of Landseer after the turn off for Hogarth Walk), Southmead (to the east of Coleford Road and to the north of Greystoke Avenue), and Henleaze (to the east of Henleaze Rd/Linden Rd) we have lime-rich loamy and clayey soils with impeded drainage (source: the Land Information System www.landis.org.uk/soilscapes).

That means our soil falls between a clay and a loam, and between poor drainage and well-drained. It is also reasonably fertile, and it is alkaline (the opposite of acidic).

This applies to the places within the sandy brown

area of the map.

Our soils can be considered 'moist but well-drained'; they have some drought resistance but aren't heavy clay. They don't need a lot of added fertiliser (depending on what you are growing). Plants that like chalk should do well provided they don't need a very well-drained soil, whereas plants that need acid soils won't thrive without the addition of ericaceous compost.

Below, you can find two simple tests to find out i) what kind of soil you have and ii) what your drainage is like, as well as how to address compacted soils, how to dig a planting hole in clay soil and what you can do with a waterlogged soil.

WIthin the area shown on the map, there will be some variation. If you are unsure what type of soil you have, take a conker-sized amount of damp soil and roll it into a ball. Next, roll it into a long sausage shape. Finally, bend the long sausage round into a ring. If you can't get beyond the ball stage then you have a sandy soil. If you can roll the soil into a long sausage but can't bend it into a ring then you have a loamy soil. If you can form a ring then you have a clayey soil.

Additionally your garden may have a clay-to-loam soil with reasonable drainage, but also patches of ground that get waterlogged. It is worth looking out for these because most plants will die in waterlogged soils. If you spot water pooling on the surface this can be because 1) the underlying drainage is poor, 2) the soil has become compacted* or 3) the water table is high after lots of heavy rain. To find out which of these is the cause of the water pooling and to help you decide what to do next, all you have to do is dig a hole.

When the weather is going to be dry for a day or two, dig a hole that is 60-100cm deep. If the hole fills with water as you dig it then the water table is currently high and you will need to wait for it to drop to test how well your soil drains. Once you have an empty hole and some free time, fill the hole with water. Check back every 15 minutes or so and record how long it takes the water to drain out of the hole. If it takes over six hours then this is classified as poor drainage. When we tested a wet part of our Horfield garden the water drained in 4.5 hours so the drainage was on the slow side (impeded) but not classified as poor and is good for plants that like a 'moist but well-drained soil'.

*Clay soils will become compacted if people (or vehicles) walk over them in wet weather. If there is an area of your lawn that you regularly walk across you will probably notice that the grass doesn't do as well and water pools on the surface in winter. This is bad news for worms and bad news for plants because their roots are starved of oxygen when the soil is waterlogged. If you have a clay soil then the good news is that it's not a good idea to do a lot of gardening in wet weather!

To counteract a compact soil, you can use an aerator or simply use a garden fork. Drive the fork about 15 cm / 6 inches into the ground, tilt the fork to gently loosen the soil, return to upright, lift the fork and repeat roughly every 15 cm / 6 inches.

If you have a clay soil, you should avoid digging a planting hole that is deeper than the root ball of the plant you are putting in. It does help to loosen the soil around the hole, but if you dig a deeper hole in a (heavy) clay soil and back-fill it with compost then you risk creating a sump. Essentially, rain water will run into the freely-draining compost and be held there by the surrounding clay, meaning the root ball is left sitting in water and may suffocate. If you want to use compost, you can mix it in with the soil around and above the root ball. Many ornamental plants and wildflowers don't require any extra nutrition and compost is not essential or a mulch on top of the soil is plenty.

Finally, if you have a waterlogged area of your garden you could go with the (lack of) flow and plant a bog garden. Check out my blog on creating a bog garden to find out more and build a wildlife haven.