Hugelkultur: Composting Whole Trees With Ease
What is it?
Hugelkultur is a composting method that uses large pieces of rotting wood as the centerpiece for long term humus building decomposition. The decomposition process takes place below the ground, while at the same time allowing you to cultivate the raised, or sunken, hugelkultur bed. This allows the plants to take advantage of nutrients released during decomposition. Hugelkultur, in its infinite variations, has been developed and practiced by key permaculture proponents such as Sepp Holzer and Masanobu Fukuoka for decades.
Building… above ground
Building a hugelkultur bed can be as simple or as involved as you want. You can make small, 60cm mounds of rotting wood right on the ground. Place the wood like a puzzle, allowing as few gaps as possible. Add grass clippings and other finer nitrogen-rich organic matter to fill the gaps left between the logs. Lastly cover the whole heap with 5cm of top soil and plant on and around it. These smaller micro hugelkultur beds can be built if you just need to dispose of some smaller woody material. A pile of sticks of any size can be covered over with grass clippings and a layer of soil. Add a little bit of urine here and there to provide some nitrogen to stimulate the sticks breaking down and the plants growing; after one season, digging into the pile, the wood should be unrecognizable. The size limits of hugelkultur beds are up to you, but beds up to nearly 2 meters tall and/or wide are not uncommon.
What’s going on in there?
The most obvious fact is the rotting wood acts as a carbon source and the grass clippings are a nitrogen source, alerting you to the fact that some decomposition is going to take place. By using carbon-rich fresh wood, you may decrease the overall nitrogen content of the pile in the short term, because the wood will initially rob the surrounding matter of nitrogen. This is counteracted by using rotting logs that have absorbed much or all of their total nitrogen holding capacity, and by adding strong sources of nitrogen, like grass clippings or manure, to the hugelkultur as it is built. Adding urine to the hugelkultur bed periodically to feed the nitrogen need of the wood is also highly effective. After fresh wood absorbs nitrogen to its maximum capacity, it will start to break down faster and start giving nitrogen back in the process. If you use wood chips, or many smaller twigs and branches, you have a greater surface area to deal with in regards to nitrogen absorption and should add the appropriate amount of “greens”.
As the wood breaks down further, it will create air pockets that bacteria and mycelium can invade, further hastening the decomposition. These pockets will eventually collapse, shrinking the size of the hugelkultur over the first year or two. This is normal settling and can be counteracted a bit by adding a fresh layer of chop and drop green manure or compost to the surface of the hugelkultur bed each year. It can also be accounted for by starting out with a bed slightly larger than you intend it to be once settled.
Building… below ground
Hugelkultur beds can also be installed completely below ground, or even with the top layer a bit sunken in arid environments. Simply dig a ditch, fill it with the logs and some nitrogen-rich organic matter, and bury the lot of it under a layer of top soil. The deeper you bury your woody material the better; as roots grow into the rotting core of the hugelkultur, they are going to be treated to a nutrient rich, reasonably constant water source. When buried, hugelkultur beds will retain even more water than raised beds.
A pile of wood has a lot of energy in it — imagine burning it in a bonfire. This release of energy is equal to the energy produced during the decomposition, but consolidated into a short period of time, while the decomposition process takes months or years. As the hugelkultur bed decomposes this energy will generate heat to help stimulate root growth as well as extend your growing season. This energy also takes the form of fuel for all the life in the soil: mushrooms, worms, termites, beetles… everything! If things like termites end up in abundance to the point of being overwhelming, simply run some chickens over your hugelkultur bed to turn a nuisance into an asset.
To get the most benefits out of a hugelkultur bed, build it to an initial height of 2.1m! This will shrink to 1.8m or less within the first few months to a year. An often overlooked benefit to having larger raised beds in the form of mounds is an increase in surface area available to plant.
One of the main benefits to using a hugelkultur bed is that they can retain water better than humus alone. Think about a rotting log in the forest, if you break off the decomposing top layer, you will likely find a moist environment full of small life right under the surface. This moisture-containing life will remain constant in all but the most scorching of droughts. Our buried wood serves an almost identical function as a sponge buried in the ground. After a rain, any nutrients in the top soil are washed deeper into the ground, but in a hugelkultur bed this water and nutrients are captured by the rotting wood. In dry times, the plant’s roots can lick at the moisture contained within the rotting logs. A 60cm tall, above ground, hugelkultur bed can maintain a usable level of moisture for about 3 weeks after it is saturated. Larger beds of 1.8m can hold enough water for an entire growing season. All of this extra moisture retention in areas that receive moderate rainfall can often lead to riparian species growing where you would not otherwise find them. As the bed decomposes, the moisture retaining wood will become humus that can not hold water quite as well as the rotting wood, but makes up for this by being able to hold onto other useful nutrients and oxygen better than the wood alone.
A comment from a farmer in North Carolina noted that hugelkultur is a useful way to dispose of unwanted woody organic matter. By simply pushing their brush into a pile, burying it under some grass clippings and some excess poor quality soil, they solve the problem of the getting rid of all their “waste”. The side effect to this activity was to create a fertile bed that provided a good potato yield the first year.
If you are harvesting the woody matter locally, by installing a hugelkultur bed you can save a lot of energy that would otherwise be used just in disposing of it. Often times municipalities have green waste programs, such as tree clearing around power lines or keeping the sides of the roads mowed, that can offer you a free local source of waste material to build a hugelkultur bed in an area with no available brush, such as in a suburban neighborhood. Also look toward local businesses such as arborists or landscapers to acquire larger logs. All of these activities can help reduce the load of transporting these materials to a waste site while at the same time fueling a fertile plot of productive land. Since the logs can be used whole in hugelkultur you don’t even need to spend energy chipping them.
Soil life… fungus:bacteria
The content of a hugelkultur bed being what it is, predominately rotting wood, there will be more saprophytic fungus — fungus that feed on dead things — than bacteria, which prefers an environment more akin to the humus we will eventually create. This leaves a few interesting opportunities to the would-be hugelkultur bed designer. One opportunity you have when first building your hugelkultur bed is to choose to inoculate the wood yourself with a desired species of fungus. To ensure successful inoculation, you can cover the ground with a few layers of cardboard before building the hugelkultur bed. This acts as a temporary barrier to other fungi and becomes food later in the process, after your inoculation has taken hold. The higher initial ratio of fungus to bacteria will promote tree growth. The fungus interacts with the roots of the trees, trading nutrients for starches. The mycelium also aid in the distribution of the water stored in the rotting wood to the surrounding areas. .
The next opportunity comes from the soil changing dramatically over the first 3-5 years in the hugelkultur bed. It will start off with all that dense, rough, woody material that things like squash, melons, or other vining plants will love. Trees planted on or around the hugelkultur bed will see improvements with the extra fungal life in the soil. Masanobu Fukuoka noted that planting trees beside coarse organic matter, such as in hugelkultur, improved their growth. As time passes your hugelkultur will consist of more and more humus — eventually you will be left with a mound of deep rich soil. As the trees reach maturity the fungus of the rotting wood will subside to reveal a more vibrant bacterial life more suited for smaller plants.
As well as advocating burying coarse organic matter, Fukuoka suggests that a hugelkultur style bed offers “gradual soil improvement by planting deep-rooted trees, grasses and herbs” and that “channeling and capturing nutrient-rich water from upland forests above the property… is an important part of sustainable, fertilizer-free agriculture.”
Mature hugelkultur raised bed.
Used by permission from paul wheaton’s hugelkultur article.
Some of the more desirable woods are:
Less desirable woods include:
- Cedar – lasts long because of natural anti microbial properties
- Black walnut – contains the toxin juglone
- Black locus – resistant to rotting, may be more useful in humid areas
- Pine and fur – both contain tannins
A few interesting resources I found while researching for this article can be found below:
- A family installing a hugelkultur bed
- Hugelkultur plus keyhole garden
- Lots of pictures and videos
- Pros and cons of hugelkultur
- Hugelkultur compared
Comparison: traditional bed at left, hugelkultur bed at right