Composting allows us to recycle plant materials so they can be re-used in the garden, but the materials need to be broken up into small pieces for the composting process to work efficiently. Traditional composting works fine for for soft green plant matter, twigs and small branches, but what do we do with thick branches or even huge heavy tree trunks?
Long before the invention of chainsaws, chippers and mulchers, farmers found a way to recycle fallen trees to return the nutrients contained in huge logs back into the soil, and they have been doing this for hundreds of years. Of course, to keep things in perspective, Nature has been doing this for hundreds of millions of years – old trees fall to the ground, where they get covered in organic matter and slowly rot down back into the soil. Nature has evolved systems for recycling EVERYTHING!
In Permaculture we look to Nature for solutions and we utilise existing natural processes to make our work easier, more energy efficient and sustainable. We also aim to recycle materials and energy by capturing, storing and using them on site. These ideas are outlined in the Permaculture Design Principle – 5. Using Biological Resources and 6. Energy Cycling respectively.
What is Hügelkultur ?
Hügelkultur, meaning “hill culture” or “mound culture” in German, is an old European system of composting logs, branches and other woody material by covering them with compostable plant materials and then covering the whole lot with a thin layer of soil to create a mound which can be planted up as a form of mounded raised garden bed.
The way this system works is quite simple. It’s basically a slow composting of woody material. When the buried wood rots down, it slowly releases the nutrients contained within it to the plants growing on or near the hügelkultur bed. In addition, warmth generated by the composting process assists plant growth.
The Hügelkultur technique can be used to create a fertile garden bed where the soil is compacted or very poor, and it can also be used to improve soil quality and water retention in reasonably good soils too.
How does it do this? All composting processes improve the soil by creating humus. Humus is decomposed organic matter, whose presence in the soil increases soil fertility and water retention. When woody material breaks down in the soil, it creates a very stable humus because carbon compounds called lignins in the wood resist breakdown, providing the benefits to the soil for a very long time.
This technique of hügelkultur, which is hundreds of years old was popularised and developed further by Sepp Holzer, an Austrian permaculturist, leading to renewed interest in hügelkultur amongst Permaculture practitioners.
How to Construct a Hügelkultur Garden Bed
A hügelkultur bed is really just a compost pile with lots of wood in it, capped with soil and planted up with plants.
There are many ways to build hügelkultur beds, and they can either be built on flat ground or in a trench dug into the ground.
Hügelkultur beds can vary in size, typically beds are approximately 2m long x 1m wide (6’ x 3’) and 1m (3’) high. This size makes for a fairly easily manageable garden bed. Some prefer to construct Hügelkultur beds that are shorter (1.2m or 4”), longer (2.4m or 8’) or even continuous!
Here is a basic hügelkultur bed construction technique sourced form Toby Hemenway’s excellent book “Gaia’s Garden; A Guide To Home-Scale Permaculture”:
“To create a hugelkultur, pile up branches or brush a foot or two deep in a mound 4 to 8 feet long. Stomp on the pile to compact it a bit.Then toss compostable materials—grass clippings, sod, straw into the pile. Sprinkle some compost on the mound, and top with an inch or so of soil.Then plant the hugelkultur with seeds or starts.”
In the same book he states that potatoes grow very well in hügelkultur beds, and “Squashes, melons, and other vines do well here too”.
The hügelkultur bed construction technique is shown in the diagram below.
The basic steps to construct a hügelkultur bed are as follows:
- Lay a pile of branches, twigs, brushy prunings and other woody material to a height of 30-60cm (1’-2’) either on the ground or in a trench dug into the ground.
- Compact the pile of woody material a little, either by using the end of a spade or jumping on it (depending on the size of the pile and the materials used)
- Cover the woody material with plant materials that compost fairly quickly, such as mulched green prunings, grass clippings, sod or straw.
If you’re using sod (basically grass with roots attached and soil clinging to the roots, lay it upside-down so the grass is at the bottom, with the soil/roots at the top.
- Sprinkle a thin layer of compost over the pile to inoculate it with composting microorganisms to get the whole composting process started quicker.
- Cover the whole mound with a thin layer of soil, approximately 2.5cm (1”) is sufficient to cap it off.
- Plant up the hügelkultur bed, and cover with mulch as you would any other garden bed.
Choosing Your Materials
It is important to use then right woody materials when building a hügelkultur bed. Some trees are valued for their timber which naturally resists decay and rotting, such as Cedar. Obviously such a timber would be a bad choice. Another type of wood to avoid is one that contains compounds which stop other plants growing, such as black walnut which contains the allelopathic (plant growth inhibiting) compound juglone.
You see these warnings in almost every hügelkultur article, but seriously though, cedar and walnut are considered fine furniture-making timbers that are highly sought after so the likelihood of such wood being composted is rather remote and unlikely. If the branches of these trees are too thin to use for woodworking, leave them under the parent tree asa natural mulch!
One thing to keep in mind about using any gardening technique is the appropriateness to your site/location. A good permaculture designer thinks through their solutions. Building a hügelkultur bed because it it the latest trend or fad in Permaculture is the wrong reason for constructing one. Site analysis is critical in Permaculture design, and the outcome of the site analysis will determine which techniques are to be used. Every design element is chosen for a very good reason.
If we go back to horticultural first principles, a hügelkultur bed is a raised bed, and the soil in any raised bed drains faster than the soil in the ground unless you’re gardening in sand! If we look at the type of climate where hügelkultur originated, it is a part of the world with reasonable rainfall, in other words, water shortages are not a problem. Raising the bed increases drainage, but not enough to create problems in a climate where water is in ample supply in terms of rainfall.
Now consider how a hügelkultur bed would perform in a region with droughts and water shortages – the mound will be drier than the ground. Mounding is a common technique used in horticulture to create drier conditions for plants and trees that dislike ‘wet feet’ – high soil moisture levels that lead to root rot.
Digging a trench under the hügelkultur bed can help capture more water in the same way that a swale (contour trench) does, but any plant growing on the mound is going to be elevated above the moist soil. The plants growing on the ground beside the mounded bed will access the captured water.
It’s important to consider that if we adopt techniques from a different climatic region to our own, they may not work as intended. If we simply copy what others are doing in other parts of the world without understanding how the techniques are supposed to work, we can’t adapt the techniques to our local conditions. Unless you’re in Central Europe or live in part of the world that has similar conditions, you need to think through what you’re doing! Hügelkultur may work for you, it may not, maybe it will work with some necessary changes.
A Matter of Scale
If we revisit Permaculture’s foundational design design principles, we see that “In Permaculture, we design and build small scale systems because they can be managed with less resources, which makes them very energy efficient. We also construct these systems as intensive systems to obtain the maximum productivity from these smaller manageable spaces.” – the seventh Permaculture Design principle ‘Small Scale Intensive Systems’
To recycle woody material and return it back into the soil, we don’t have to laboriously dig huge trenches, use giant logs or move cubic metres of soil, possibly with earth moving equipment. The modern version of traditional Hügelkultur is great if you have a rural property and the resources to work on such a large scale, but where does that leave everyone else?
A New No-Dig Style Micro-Scale Hugelkultur
The basic principles by which Nature recycles woody materials high in lignins to create highly stable soil humus can be carried out on any scale. Nature uses the same principles to return a giant tree into the soil as it does for a small twig, the only difference being scale and time.
Since most people live in urban areas, and gardening space in many urban areas may be quite limited, I’ve designed a new small-scale backyard hügelkultur system that can be used to dispose of heavy prunings, branches and small logs in a small garden bed.
This new small-scale hügelkultur process I have designed complies well with Permaculture design principles because it is quick and easy to construct, takes very little energy and effort, and makes hügelkultur accessible to a lot more gardeners.
Each 1m x 1m (3’ x 3’) bed bed should take approximately 5 minutes to construct and is capable of processing up to a 7m (20’) length of 10cm (4”) thick branches or more thinner branches.
The basic process is illustrated in the diagram below and explained in detail in the step-by-step instructions that follow.
Step-by Step Small-Scale Backyard Hügelkultur Construction
- Select your site.
We will build our Micro-scale Hugelkultur system in this no-dig garden bed which has layer of mulch (dry organic matter) covering the soil.
- Clear the mulch (in a no-dig or mulched garden bed) to the sides to expose the soil surface.
This should be a very easy process, here an area of 1m x1m (3’ x 3’) was cleared with a hand trowel in seconds.
(In case you’re wondering, in the bottom half of this photo the brown plastic drip irrigation pipe that sits below the mulch layer is partly exposed near the soil surface)
- Gather your materials.
In this example we’re using:
– heavy branches and small tree trunks that chopped up with a pruning saw when they were still soft during pruning time
– a bucket of homemade compost
– a bucket of mulched green garden material
– some blood & bone fertiliser (or any other organic fertiliser such as manure)
- Sprinkle a thin layer of organic fertiliser over the soil.
In this example we’re using blood & bone fertiliser such as manure will work just as well.
The addition of fertiliser where the soil touches the woody material is important to prevent nitrogen draw-down. This phenomenon occurs when soil bacteria draw nitrogen away from the surface soil (and plants) to use it to break down the wood which is high in carbon. Remember from our composting theory that for effective composting we ideally need a carbon:nitrogen (C:N) ratio of 25-30:1, so the fertilizer provides some extra nitrogen to help the composting process along.
- Lay down a layer of heavy branch and small tree trunks or any other woody material that is difficult to dispose of any other way.
- Cover the wood with compost.
This will ‘inoculate’ the system by providing a lot of soil organisms which will start off the composting process. If your compost was made using woody material then all the better because it will be richer in fungi, as fungi play an important role in breaking down woody material in the soil.
- Cover the compost layer with mulched green garden material.
This layer will be rich in nitrogen and break down quickly.
- Pile on more compostable organic matter if you have it available!
When you think you’ve added enough mulched green garden material or organic matter, add more, pile it high!
In this example the heap was piled to around 45cm (1.5’) high. If it seems too high, it isn’t, it the pile of organic matter will drop in size rapidly.
- Completed, composting Begins!
After a few days, the pile has dropped in height and started changing colour. Composting is underway and all of that wood that you couldn’t get rid of will slowly work its way back into the soil!
Planting It Up
So how do we plant up this system?
For starters, you only need to plant it up if you want to, it’s not necessary to do anything else, it will deliver its benefits to the soil as it is.
When we’re ready to plant it up, we simply use the no-dig gardening planting process, which is comprised of four simple steps:
- Make shallow holes in mound approximately 10-15cm (4-6”) wide, and equally deep.
- Fill with compost.
- Plant seeds, seedlings or plants.
- Using a watering can or hose, water in well.
With either a traditional large scale hügelkultur system or my new small-scale hügelkultur system we can recycle tree trunks, branches and other woody material and create a slow nutrient-release system that will improve soil fertility and moisture retention in your soil.
Every gardening technique has its appropriate place and purpose, and if we understand the natural processes that make hügelkultur system works, we can modify our design to suit our climatic conditions and endure our success!