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Saturday, February 15, 2014

Hugelkultur Goes Underground

In November 2012, when the year's harvest was done, we began transforming the garden into a massive rotting wood battery. Why wood? The goal is conservation and availability of both water and heat, as well as creation of rich soil in the long term.

It is not a job for a shovel, to replace the first 5-6 feet of compacted soil and rocks with rotting wood and manure. 

My husband, Bob, has a front end loader with a backhoe attachment. We've been tackling the garden area in sections, digging one pit at a time. On average, the pits have been about five feet deep, and around 12x12 feet across. Some pits were 6 feet deep, and a couple were shallower; they were dug side by side, essentially joining on the edges. 

The process has been to remove the soil and rocks from a section, piling it off to the side. Bob removes the large boulders, and I hand-sort the cobble and small boulders from the soil, setting interesting rocks aside for future above-ground projects. Once the majority of the rocks are removed, the soil is available to put back in the pit, layered in among large pieces of rotting wood, manure, leaves, and whatever other organic matter I can get my hands on. 

Big thanks to Bob for all the digging, and for teaching our daughter to operate the backhoe in the process too!

The above-ground hugelkultur piles built during 2012 were a great source of woody organic matter for the new beds. One by one, they were pulled into the pits with the backhoe. The above-ground piles also had pieces of biochar in them -- partially burned wood that has a strong ability to hold water, helps reduce leaching of nutrients, and fosters the uptake of nutrients by plants.

The first few pits were filled with aspen, thanks to our friend, Lee Johnson, and our neighbor, Betty. I had read that aspen is a great species of wood to use for hugelkultur, so I sought it out. Aspen groves have a propensity to thrive and die in cycles, and around here, it is easy to find landowners who would like it out of the way after it dies. I was able to make real progress on this project because Lee decided to help by bringing wood by the truck- and trailer-load -- one of the most amazing gifts ever! Thank you, Lee!

The Willy's truck is loaded with manure from our neighbors, Chuck and Kathy, who own alpacas. The manure from their alpacas was amazingly rich and supported a lot of robust veggies the following growing season! This pit started off with long sections of aspen, which were arranged in rows, with alpaca manure...

...and leaves! I was able to use a lot of leaves thanks to the generosity of local people like Lenore and Paul Bouchard, Brianne Rowe Smith, and others. 

Here is the same shot facing East instead of West.

The second layer was made up of shorter lengths of aspen, being nudged into place here with the backhoe. Soil was also spread over each layer.

Into the pit goes one of my best producing above-ground hugelkultur beds. It was quite small and would not have produced well for as long as this new, larger bed will. 

Dark, rich, rotten wood, going in... The advantage to using existing hugelkultur beds in these pits is that they had already started breaking down, and their functionality was ready to peak.

Spreading soil over the wood, manure and leaves ~ looking West, November 2012

Looking South across the first two pits, September 2013

Harvesting fava beans from the first underground wood-for-food bed (Sept. 2013)

Big, beautiful onions from the aspen & alpaca manure pits (October 2013)

Don't have a backhoe? No problem!
It is not necessary to dig deep pits in order to reap the benefits of rotting wood for growing food. Our design came into place because of the fact that we have a backhoe. If you are working with a shovel, you can dig a trench only as deep as you feel like digging, and build your hugelkultur mainly above ground -- or don't dig at all and grow everything on the surface. 

There are pros and cons to every approach, but you can be successful regardless of what angle you take. I am excited about the pit system we are implementing, because it allows me to experiment with rainwater collection with the surface contours, and because of the mass that can be developed for water retention and for generating heat to extend the growing season. 

Benefits to above ground beds include quicker solar heating in the spring, and the ability to take advantage of solar exposure (South vs. North facing sides). Also, you can create microclimates depending on the shape and arrangement of the beds. 

The sky is the limit! Eventually, I hope to have a combination of below and above ground wood-for-food beds in our garden. 


  1. Funny what kind of gifts are appreciated by gardeners. Who knew poop and dry leaves can make one so happy...

    1. No doubt, it's the simple things that make life rich! :-)

  2. Julie I have a question about doing underground hugelbed's which perhaps you could help me with. How are your underground bed's performing nowadays? Do you think it's possible to dig them too deep? I'm thinking of going only 1m, but even that, I'm concerned that all the water from my garden will run to the bottom of the bed and just sit there, without the top benefiting much. I understand that it works like a sponge, but I don’t see how the water at the bottom would rise all the way to the top. If you get lots of rain, then I can see how the whole lot of wood from top to bottom can get saturated. But I'm in a warm temperate, somewhat semi desert environment, with low rainfall.
    Would you be able to tell me what your climate is, or rainfall?

    Also, could anaerobic activity become a problem being below ground?

    Thank you,
    Dieter Aschenbrenner

  3. Hello Dieter,
    Thanks for your comments and questions. I also live in a semi-desert environment with low rainfall. The way I think about moisture accumulation in underground hugelkultur is via the wood soaking up the water as the snow melts and as rainwater penetrates from the top down. You are right, if the water source was coming from the bottom, a very deep pit wouldn't be likely to "fill up" to the top. However, the water is coming from above in my case (and most people's), and so you can visualize the wood as the interceptor. As moisture slowly percolates down, it is absorbed by the wood at the top first. Thus, a lot of the moisture won't find it's way to the bottom, but instead will become trapped near the top, unless the whole top layer becomes saturated first. In our climate, this is one reason to water each layer very heavily while building the bed, if possible -- it may be the only chance to really saturate the bottom of the pit. I really don't believe we get enough rainfall for the top layer to supersaturate and start dripping down to the very bottom. Does that make sense? If you had a high pressure hose shooting water down, then yes, it would find a path directly to the bottom with gravity. But sprinklers, rain and snowmelt are incremental amounts at once, and more easily absorbed.

    Having a deeper pit is advantageous in a dry climate because it is less prone to drying out, with the sheer mass of moisture. A shallower pit will be more likely to bake and dry out. Underground is also beneficial in a dry climate as opposed to above ground, for the same reason -- less likely to dry out. I wouldn't worry about 1 m being too deep; anything less and the water may be more likely to evaporate during the heat of summer. Plus anything less won't have the same benefit for decomposition -- along the same lines of larger compost piles being more effective. I've done mini hugelkultur beds before and they are okay but tend to try out more easily.

    My beds are doing great. I'm really happy with the way they compress at the center, as it creates a beautiful rain collection shape.

    I don't foresee anaerobic activity being an issue because there is a ton of air space between the branches and wood pieces, and then also during decomposition, air pockets are created. If it did become anaerobic, I would be okay with with that too, because it would be most likely to happen at the very bottom, which shouldn't pose a problem for plants growing near the surface. I hope this helps and please feel free to continue the discussion! Let me know how it goes. :-)

  4. Thank you, Julie. I can see how what you say makes sense. Can I ask what type of soil you have? I was thinking that the water may travel down the sides of the bed to the bottom. You know how when two surfaces meet and one is harder than another, (or different) water will travel along this more easily. Like if you compact a pile of soil on top of rock, water will find its way between where the soil makes contact with the rock. But, maybe not.

    We have quite sandy soil here for about 600mm (I've heard), then clay. I've been told that during the winter here water goes down into the ground, and then rises up and evaporates during the summer... but not sure how deep.

    I thought that since a hugelbed is quite aerated, rainwater would travel down this more easily. But I suppose the hugelbed will compact over time which will slow the water from travelling down.

    I'm really only concerned that if it doesn't work out, we've disturbed the subsoil, which could mean if we ever plant trees there instead, they won't have good substructure to assist with collecting and evaporating water for the trees tap roots (like when you plant a tree, they say not to dig or disturb the soil beneath the depth of the roots for this reason).

    1. Hello Dieter,

      Our soil is very fine textured. I can see what you mean about the water choosing the path of least resistance. I try to include a lot of organic matter in my hugel beds (all throughout), including hay, leaves, and grass, to help absorb water. That way, the water can soak in more evenly and not just run along the surface to find an easy way down to the bottom of the pit. I think the volume of rain is a key factor, that it rains a little bit here and there as opposed to a monsoon type climate where the water really gets running. It's like irrigating with a mister instead of a hose -- there is more time for the moisture to be absorbed by the wood, leaves and hay in the bed, instead of sinking all the way down. I don't have any way to track the path of the water, but this is how it appears to me. Some of the beds I have built slowly over time, so I've had the chance to observe them when it rains at various stages, e.g. half-way full, etc. That helps give me a sense of what might be happening inside the bed, though I can't say for sure. I can tell you that in general my hugel beds have out-performed the regular soil beds quite dramatically in terms of veggie production (see the end of this post:

      If you have sandy soil, the water should be less prone to running across the surface toward an easier path at the edge. This is to your benefit, especially if you mix in a lot of finer organic matter, and a variety of wood diameter sizes, to help slow the water down and soak it up. You can also help guide the water in how you contour the surface of the bed. A very slight bowl shape is good for collecting water and not running off like a hill contour would.

      Personally I think that if you disturb the soil in order to add a lot of wood, even the trees will thank you. After some time goes by, the mycorrhizal community will be well established and the trees will be benefit from that, and will be able to access more water on account of the wood.

      I wish you the best with your endeavors!

  5. We dug the hole today, at 1m. Do you have any guidelines on how thick to make each layer, and what's in each layer? Like the log's at the bottom, how thick those are, the thickness of layers above that etc. Thank you

    1. How exciting! There are examples of layering techniques throughout my blog (I've tried several different approaches, e.g., but in general I try to put my largest diameter pieces at the bottom, and gradually work toward smaller diameter near the surface. I use the largest diameter wood I can get my hands on for the base layer. By the top layer, I'm using mostly branches with some smallish logs mixed in.

      One interesting approach is to place wood in the pit vertically instead of horizontally, to help wick moisture to the root zone. There is a discussion of this topic in the comments of the above link, which you might find interesting. It depends on what your wood source is like, whether this might be an option. It involves a lot more cutting, to generate lots of short pieces that can be stood on end.

      There are many recipes that can work well for how to layer, and a lot of it will be dictated by what materials you have available. One thing is, I have found it impossible to use too much manure. Some beds I added absolutely no soil in between layers (e.g., and other beds I put quite a lot of soil in between ( Our soil is alkaline so that is why I left it out of the blueberry bed. In other cases I did not use much soil because the ground had been so rocky that I didn't end up with much soil after digging the pit. For me the thing to do is relax and get creative and remember that there is no wrong way to do it. Have fun and good luck!

  6. Also, from the soil and clay that comes out, do you put any back in, between the layers or anything like that? Or is it all organic matter? (apart from the topsoil on the top of the bed)

    1. I went ahead and addressed these questions also in my above reply. Let me know how it goes!

  7. Finished the bed today, went well.


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