Perils and Plans for Gardening in Heavy Clay Soil

Dry cracked clay soil

Heavy clay soil can make life in the garden difficult, but with the right plans, effort, amendments, and time, gardens can still survive and thrive. Come and join us for a virtual visit to our new garden and we’ll tell you our cay soil woes. Let’s talk about the good and the bad of clay, and tips for working with heavy clay soil in the garden.

Our New Home's Heavy Clay Garden Challenges

Our new garden is – unexpectedly and disappointingly – located in a pocket of heavy clay soil in an otherwise very fertile growing region. We and our new neighbours throughout the development are discovering that growing here will not be as simple or smooth as planned.  

As the garden develops, we will share our successes (hopefully) and failures (hopefully fewer) in establishing and maintaining a garden in our clay soil. The first casualty was our garden plan. Additional drainage has been added, plantings changed, amendment strategies implemented, and hardscaping altered. I’ll share detailed posts on different aspects as we progress, but first an introduction to heavy clay woes for fellow gardeners. Cue the sad music…

Muddy black gumboots

Assessing Garden Soil Composition

If you aren’t sure about your soil composition, there are several quick DIY soil tests that you can do at home. You can also send a soil sample for lab analysis for a more precise assessment of composition. 

Heavy clay soils are often readily visible by their fine silky mud when wet, and hard cracking when dry. On areas of lawn and garden surfaces, pooling water that doesn’t soak in after manual watering or heavy rain is another signal of soil problems lurking below.

There is no need for a squeeze test or settling test here. Our clay is obvious in texture and behaviour when wet or dry. When wet, it forms a gluggy mire that coats my hard-working gumboots and anything else it contacts. When dry, it opens up in cracks and sets like cement. The degree of clay varies greatly between areas of the property, but it is all clay dominant. 

As former farmland pre-development, it also has several buried layers of hardpan from past lives and varied uses. Although it was weed-covered before levelling, there is little visible life in the soil, such as earthworms and insects. We have a lot of work to do!

Cracked muddy clay soil

Common Garden Issues with Heavy Clay Soil

Compaction and Heaving

Clay soil is prone to compaction and heaving. Both of these make it a struggle for plants to grow and maintain roots.

Compaction can be reduced by limiting traffic and working of the soil when wet. Unfortunately, with how long our wet season is here, complete avoidance is almost impossible. It’s particularly tricky for us on the lawn, especially with all the work of building the new garden, foot traffic, and dog traffic. We use periodic core aeration to help.

Heaving and shifting with changing weather conditions can damage roots, hardscaping, and more. Heaving comes from the expansion and contraction of the clay, which is driven by moisture content. Addressing underlying moisture issues can help reduce heaving as conditions and seasons change. 

Density

Even if babied to reduce compaction (a difficult task), clay soil is still extremely dense. Set into hard clay, amended planting holes can simulate in-ground pots. As roots reach the edges of the amended soil and encounter hard clay, it acts like a container. The plants are similarly prone to root-binding and impeded growth and health.  See the planting tips below for tips on reducing the container effect.

Poor Drainage

Clay soil is notoriously slow to drain. Surface water can flow over harder clay areas to pool in the softer soils of garden beds and planting holes. The container effect applies here as well. Amended planting holes can act like sump holes or buckets, gathering water from above as well as below ground, risking water-logging roots and drowning plants. Even with semi-functional drainage, wet soggy soil can leave plants vulnerable to rot and fungal disease. 

In wet weather, as can often be expected here in New Zealand, our soil simply does not drain well. This persists even with the addition of expensive professionally-installed drainage. 

The picture below is a post hole, not a test hole (see more on draining testing here), but the effect is much the same. We’ve dug test holes in different areas of the garden with surprisingly different results. Some are terrible and other are great, even within close proximity. In some cases, we’ve switched plant selections or changed plant positions, but we’re space limited so in other cases we are going up with a combination of mounding and raised beds.

Hole filled with water in soggy clay soil

Nutrients and Microorganisms 

Clay soils are often nutrient rich, but the structure of the soil makes it difficult to take advantage of this in supporting plant life without making significant long-term amendments to the soil. The nature of clay soil’s seasonal muck or dry concrete-like conditions also means that the soil is often lacking life – e.g. beneficial insects, microbes, and fungi. Soil inoculations may be beneficial during amendment. Really heavy soils may be so compacted and/or water-logged that they are lacking air, becoming anaerobic and unable to support normal in-soil life or plants.

Amending and Improving Heavy Clay Soil

Adding Soil Amendments

Adding clay-specific amendments like gypsum (along with checking pH and nutrient levels) and lots of organic matter, and working to open and aerate the soil over time can slowly improve the structure and make it viable for planting. Slowly being a key word here. It can take years of patient work.

Adding organic matter and allowing it time is vital. There are no short-cuts with gravel or sand, unless you want to make clumps of clay concrete. Eek! Adding organics like manure and compost will help to physically separate some of those fine and densely packed clay particles, loosen the soil, improve drainage, add nutrients, and restore life to the soil.

Clay soil mixed with compost and gypsum

Using Cover Crops and Other Temporary Plantings

Cover crops and sacrificial plantings can also help, if circumstances allow. Deep rooted covers can be used to try and loosen dense or compacted soils and then either harvested (or taken for compost) above ground with the roots left to rot naturally back into the soil (no-till gardening), or fully tilled back into the soil as green manure.  

Tolerant Longer Term Planting Selections

Water-loving plants can be used as a short-term garden or a long-term strategy. Trees that thrive in wet and heavy soils can help to soak up some of that water and grow roots to help loosen the soil. Beware that winter’s soggy clay can be summer’s crackled dessert, so plants that can tolerate both extremes are more likely to thrive in the long haul.

Unfortunately, these may not be options if (like us) you are bound to timelines and planting restrictions due to covenants. It might also delay or completely derail your preferred garden planting plans.  You may need to opt for a Plan B using elevated plantings to protect vulnerable specimens.

Protecting and Supporting Plants

Planting in Ground

Digging a straight sided hole in heavy clay soil is similar to making a pot. Plants won’t thrive in the long term. In addition to binding the root development, the hole can act like bucket and drown or rot the plant. I’ve also heard recommendations for trying to break through the clay, but with a high winter water table, hydraulic pressure might mean you’ve created a mini well, not a well-draining hole. 

We have a lot of unhappy neighbours who’ve had quick and easy post-hole planting, later followed by dead hedging and trees. Very sad. Sadder still that many were recommended to plant this way by the nurseries selling the plants!

My mantra for planting in clay is bowl not hole.  It’s been so important for our planting that it deserves some bold text.  As most of our big plants include root bagging or raising, I don’t have any great pics to illustrate, but never fear. This youtube clip on planting trees in heavy clay is an oldie, but is shows the bowl method very clearly. With heavy clay, the bowl should be shallow and wide, with a jagged bottom and sides. Smooth patches of clay act like fragments of the pot, but irregular edges will help encourage root development. It can be helpful for drainage, too.  

Don’t be too rich with the added soil. It can exacerbate the pot effect by holding water while is slowly transits into less receptive clay and discouraging root exploration into the tougher surrounding clay.

Planting in Raised Beds and Containers

Raised beds and containers will lift the roots (or a portion of the roots) above the soggy soil.  Even just a little mound can be helpful if soils are particularly poorly drained and/or the plants dislike wet roots. 

We’ve used a combination of raised beds, slightly elevated beds, and mounding. A few sensitive plants in problem areas needed additional lifting, but have now settled in well and are growing well. Fingers crossed!

Watering and Feeding

Heavy clay soil tends to hold water if absorbed, so less frequent watering is typically needed. Beware that, because absorption tends to be poor, water may run or pool instead of infiltrating deeply, so go slow and let it soak in.  Shallow watering encourages shallow roots, no matter what the soil conditions, which leads to more vulnerable needy plants.

Heavy clay is often rich and it holds nutrients, if it can get them. A feeding program with light feedings, foliage feeds, top dressings, and/or using slow-release is often best. Because clay soils absorb slowly, fertiliser run-off instead of absorption is a risk. Not only is this a waste, but it can make its way into drainage systems and move on to waterways. It can also collect and concentrate in areas, leaving some parts of the garden underfed and some overfed.  

Adding Beneficial Microorganisms

Because of our soggy soil situation and how common root rot (especially phytophthora) is here our wet fungus friendly climate, we’ve started a regime with initial planting boosters and ongoing beneficial fungus application.

I also use compost top dressings, seaweed solution, and the outputs from our worm farm. We initially though some areas of the garden would need even more assistance with restoring and increasing life, but with time, patience, and growing, things seems to be finding their own harmony. I used to get excited at earthworm sightings, but now they’re all over the garden. Yay!

Powder for mixing beneficial fungus to apply in garden

The soil amendment activity here in the new garden (aka – my new wheelbarrow-and-spade based fitness program) is ongoing and will be for quite some time.  I’ll share more as we progress, but if you are suffering clay woes of your own and are working on a plan, this article from Fine Gardening on Improving Clay Soils is a particularly good read with technical detail combined with good experience-based strategies for food for thought and inspiration. Good luck!

Perils and Plans for Gardening in Heavy Clay Soil


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