Clay soil always raises questions for gardeners. It’s difficult to dig, cracks when dry, and when wet it holds onto so much water that plant roots can rot. But it also has the nutrients to grow great produce if you can work organic matter into it. It’s entirely possible to use hugelkultur practices with clay soil as long as you keep it moist and mulch, mulch, mulch.
My Hugelkultur Story
In spring 2020, faced with establishing a new garden in hard clay soil, I wondered if hugelkultur might be a good strategy. The technique, which involves burying logs, branches, and mulch in a steep-sided mound, would certainly bring organic matter into my garden. But would they decompose properly when surrounded by clay?
I have used hugelkultur before as the coordinator of a communal permaculture garden in Toronto, Ontario, and it was extremely successful for us there. I knew it would be the right fit for my new property as well, since the previous resident left a large pile of fallen trees in various states of decomposition. I even have some experience with growing vegetables in this region’s clay soil—but this would be my first time putting the two together. I did as much research on the matter as I could, and then tried it out for myself. Here’s what I learned.
Common Questions about Hugelkultur and Clay
Do I have to dig a trench?
No, you can build your mound directly on top of the soil if digging is too difficult. But if you do want to dig, wait until the day after a good rain, or soak the area down 12 to 24 hours before you start. It takes time for clay to soak up water, but once it does it is much easier to work with.
Do I have to bury my mound in clay?
If you have options, you don’t have to bury your mound in your native clay. While most of the questions below are going to deal with the scenario of a mound that has had clay soil added on top of it, you can simply mulch over a hugelkultur mound and wait for it to decompose enough for planting. You can also add soil in small pockets where you plant seeds, or transplant seedlings directly into deep mulch.
If my mound is covered in clay, won’t the clay dry up and crack?
It will if you leave it exposed to the air. The best advice for any garden is to keep your soil covered in mulch, which retains moisture and shelters the soil ecosystem from sun damage. This is triply important with clay. With the mulch protecting your soil, you’ll give worms, insects, microbes, and fungi the environment they need to thrive, and they will do the work of integrating organic materials from your mulch and wood into the clay layer.
I heard fresh wood will rob nitrogen from the soil when it starts decomposing. Do I have to worry about that?
Don’t worry too much. If you add compost, manure, or green matter like grass clippings your nitrogen levels will be fine. You can also plant legumes (like beans and peas) that convert nitrogen from the air into a form available to plants. And after the first season, as long as you have kept your mound mulched to kick-start the soil ecosystem, there should be enough nutrient cycling going on that nitrogen immobilization will not be a problem. If you want to dig more into this topic, check out this blog post all about it.
If my mound is sealed in clay, do I have to worry about it keeping out oxygen?
Some gardeners with an understanding of compost bring up this concern. With a compost heap, if you don’t turn it and let some oxygen in, then it can go anaerobic—that is, the aerobic bacteria that need oxygen to survive (and that create nice, fresh-smelling compost) will die off, and the anaerobic ones that prefer oxygen-poor environments will take over, turning your compost into a smelly sludge. In every forum that I visited during my research, the experienced clay hugelkultur builders brushed off this concern. A hugelkultur mound is too porous overall for oxygen starvation to be a problem.
In my own yard, I have pulled up old timbers that were sunk into the permanently-damp clay along a drainage ditch. The wet suction sound that accompanied their removal testified to how well sealed they were, but their state of decomposition looked similar to that of wood buried in any other kind of soil. It’s possible they took longer to reach that state, but, as I mentioned, your mound will be more porous.
Some gardeners use a variation on the hugelkultur technique and bury wood and other organic materials without creating a raised mound. If you are going to use this technique, then clay’s ability to seal things might be more of an issue. Make sure you have plenty of smaller organic matter in your trench and mulch on top. That way you should still have a well-aerated soil ecosystem within that patch.
Will the clay make it hard for the mound to absorb water?
Again, mulch is your friend here. By keeping the surface of your clay moist, it will help it stay ready to absorb more water. The mulch itself will also hold moisture, as will the wood in your mound. As with any soil, a covering of plants will also improve water absorption—just compare a grassy part of your own clay yard to a bare one and you’ll see the difference in the hard-packed channels where water runs along the plantless section.
Some gardeners recommend adding gypsum to clay soil (such as dust from untreated drywall), since it may be a lack of calcium that makes clay impermeable. I haven’t tried the strategy myself, but adding a diversity of rock minerals to soil is generally a good move. Boosting a trace mineral for a plant whose environment lacks it is like taking vitamin tablets yourself if your diet is too low in a certain nutrient.
Should I mix my clay with other soils before I add it to the mound?
If you have the means, yes, this is a great step to take. Mixing in sand, compost, and/or manure will improve the clay’s texture and mitigate the challenges discussed above. However, if you don’t have the strength or the materials, don’t let that stop you from working with what you have.
My Experience Building a Hugelkultur Mound With Clay
I’ve been particularly interested in whether I can grow vegetables this season in a fresh hugelklutur mound coated mostly in clay, since I have just a small amount of compost to play with. Here’s what I have learned from my experience:
- Digging after a few days of rain was almost a pleasant experience, although tiring. Digging dry clay was not even worth the attempt.
- I piled up the clay dug from my trench, and during the days it took me to stack my wood, it dried out on the outside of the pile but stayed damp and crumbly inside. I picked up the big chunks of dried outer clay, rested them against the bottom of my mound, and kicked them into submission. Then I was able to shovel the rest on easily.
- Mulch really is absolutely essential.
- Using clay on the base of the mound and focusing my compost near the top, which is the most liable to dry out, appears to be an effective strategy so far.
- For this first year, I plan to grow a lot of squash on my mound. I have had very happy squashes in first-year hugelkultur mounds before, and I’ll be able to plant them in the rich soil at the top while their broad leaves will ramble down the sides and act as an extra layer of living mulch on the clay. I also know that strawberries feel right at home both in fresh hugelkultur mounds and in clay (wild ones grow EVERYWHERE in the grass here), so I can transplant some of the strawberries that are going to be smothered when the landlords finish the lawn they are planning up closer to the house.
I’ll update this post as time progresses and my mound begins its decomposition. Feel free to comment with any experiences of your own!
2021 Update
I sadly lost most of my photos and videos from 2020 and the first half of 2021, but I did make a few TikTok videos that survived. If I’d made one more in the autumn, you’d be able to see how the greenery covered the mound so thickly that the mulch was no longer visible.
I ended up getting a small amount of topsoil, which I focused at the top of the mound where I planted five butternut squash. At the base on the south-eastern side, I planted cherry tomatoes. The squash and tomatoes rambled all over the mound and over the trellis I’d built into it.
On the cold side of the mound I planted dinosaur kale, which did well, and some lettuces, which struggled—I think because they kept their roots bound in the small pocket of soil they were transplanted with. I also planted pole beans that reached up to the trellis. These did well as long as they survived their seedling stage; my all-important mulch was sadly a great home for slugs.
On the sunniest side I planted peppers. They didn’t thrive, but it’s hard to say if that’s because they didn’t have enough soil or because they ended up being shaded by the leaves of the squash and tomatoes. I also lost a patty-pan squash to cucumber beetles, and I think that’s because it was on the short southern end of the mound where it was in full sun all day and quite exposed. It was under too much stress to resist the beetles. They attacked all of my cucurbits (along with those of every other Canadian in 2020, seemingly), but I was able to reduce the stress on the butternut squashes by stretching floating row cover from the top of my trellis to some posts nearer the ground. With this partial shade it dried out less quickly and didn’t wilt in the heat of the day. The beetles kept eating the flowers but didn’t make much headway on the leaves.
In the fall I mulched the whole mound heavily with shredded leaves and straw, leaving most of the greenery to rot in place.
In spring 2021, the mound’s second spring, my neighbour’s dog dug part of it up trying to catch a chipmunk. This was annoying, but a good opportunity to examine the soil. This is something I did film, but the footage was sadly lost. I hope you’ll take my word for it when I say that even low on the mound, where I hadn’t really added any topsoil at all, the soil structure was vastly improved over last year. When I added the clay, it was in chunks that I had to crumble and kick into place. Now, with last year’s leaf mulch decomposed and blended into it, it’s a dark and fairly loose soil.
I didn’t want to plant squashes on the mound this year in case of beetle larvae hatching in the soil after last year’s infestation. Odd jumps between cold and hot weather this spring also scuppered my plans for growing lots of early greens, so the mound is unfortunately more bare than it should be in summer 2021. However, it does have some very happy ground cherries and a crew of peppers that are looking much better than they did last year. At its base where I had tomatoes the first year I now have little beds of cheerful nasturtiums. And in desperation, when I found my cold crops failing and my tomato seedlings thriving beyond what I had room for, I planted the excess along the cold side of the mound. They were planted late and in the wrong microclimate, so their growth is nothing to write home about, but they’re doing fine. I still plan to cover the bare patches in greens once the weather cools off a bit.