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A tree bog is a form of outhouse which has willows, nettles and other nutrient-hungry plants planted around it. The faeces are held in a chamber open to the air which allows it to decompose rapidly, feeding the trees around it. Unlike a conventional compost toilet, a tree bog should never need emptying. Effectively, it is a system for converting human faeces to biomass.



The tree bog was invented by Jay Abrahams of Biologic Design in the UK after he observed that the trees around the place where faeces were deposited were particularly vigorous. Tree bogs can be considered examples of permaculture design.

The tree bog is a simple method of composting wastes, and since its introduction by Biologic Design in 1995 over 500 tree bogs have been built in Britain. They have been on sites ranging from fruit farms and pick-your-own enterprises, campsites and an angling lake to annual festival sites, remote/low impact dwellings holiday cottages and churches[1].

A tree bog can be created if one is on the mains but wishes to lower the environmental impact of sending wastewater to the sewage works.

The tree bog has attracted the attention of NGOs and aid workers who hope to develop its potential for shanty towns or refugee camps - anywhere that water is scarce and the population pressure on resources is high. Most regions have vigorous and useful plants which, if willow is not available, can be used (e.g. Gliricidia sepium or ‘Quickstick’ in the subtropics).


A tree bog is simply an uncontrolled compost heap whose function has been enhanced by use of moisture/nutrient-hungry trees. Tree bogs use no water, purify waste as they create willow as a biomass resource, and also contain the organic waste, thus preventing the spread of disease - all whilst creating soil. A seating platform/cubicle is mounted at least one meter high. The area beneath the seating platform should be enclosed by a double-layer of chicken wire - this acts as an effective rodent/small animal and child proof barrier, but allows air to circulate through the compost heap.

The space between the wire should be stuffed not too densely with straw which acts as a wick to help sop up excess urine preventing the likelihood of odour problems due to incomplete biological absorption of the nitrogen from the urine. The straw filled wire also enables the pile to be well-aerated whilst acting as a visual screen for the first year’s use. The structure is then surrounded by two closely planted rows of osier or biomass willow cuttings; this living wall of willow can then be woven into a hurdle-like structure and its annual growth can be harvested.

Tree bogs can also be sited on the edge of existing stands of trees, woodlands or hedges: the mature tree roots will soon find the additional source of nutrients, so that the willow may be unnecessary, - or indeed, in the middle of a mature woodland, pretty well impossible due to shading.

The tree bog combines resource production and waste disposal. The tree bog functions by transforming faeces and urine into growing trees and organic soil. The roots of the trees are host to soil micro-organisms which decompose and mineralise the materials in the heap, making the nutrients available to the tree roots. Thus the willow are able to grow more vigorously as these added nutrients become available.

Inputs, costs and maintenance

There are few costs apart from initial construction and planting. Under heavy usage it is advisable, once a week, to add a fine layer of non-chemically treated wood sawdust/wood chips, shredded newspaper or straw. Half a cup of dried soil and/or wood ash helps prevent odour if added every other day. It is also advisable to occasionally level the heap with a pole.


Benefits include:

  • Disposal of waste without using water.
  • Soil is both generated & regenerated.
  • Earthworms proliferate within and around the compost contained within the tree bog.
  • Leaf mould from willow leaves.
  • Comfrey bed on leach field for nitrate/phosphate absorption.
  • Leaf matter and twigs for stock fodder.
  • No odour or fly problems.
  • Willow wands for baskets, fuel or structural use.

The tree bog is a simple means of taking responsibility for wastes produced in everyday life. It involves no secondary handling (except the poking with a pole) of waste matter. Management is minimal, being an annual winter cutting and weaving of the willow. The wands can be used as polewood, for basket or hurdle making, or chipped for use for animal bedding or cut and bundled as fagots to use as fuelwood.

Guidelines based on the most frequently asked questions for building a tree bog

A seating platform/cubicle is mounted at least 1m over the heap - the rest is up to the builder. There is no one method; the chosen structure is often dependent on the materials and budget available; each builder is free to interpret the basic tree bog principles in their own way. For example, a summer camp tree bog will differ from an all year round domestic version.



Tree bogs are sited where the surrounding willow and other planted species will have plenty of sunlight, otherwise photosynthesis will be limited and the plants will not thrive. To comply with the Environment Agency tree bogs are required to be situated more than 10m from any water courses or springs (consult your local authorities). Tree bogs are unsuitable where the site is liable to flooding, and if placed close to a badger sett the tree bog should be badger proof or the contents will be snaffled away by these inquisitive creatures. Fencing may also be useful to prevent livestock from eating the willows and other planted species.

Willow planting

Although not required for an effective tree bog, planting the willow wands at least one growing season prior to the tree bog being used has advantages. This way the willow roots are mature and the wands have grown up to a reasonable height before the platform is constructed - as the platform shadow can hinder the growth of the planted willow sets on the shade side.

The best time to plant out the willow for a tree bog (in the northern hemisphere) is from October to March, as then the willow has a chance to get established before the onset of summer. However it is possible to construct and use a tree bog at any time and then plant it out at the appropriate time of year. The straw acts as the visual screen for the compost pile until the willow is in place and growing.

Vigorous varieties of unrooted willow sets/cuttings work best, and if they are planted out after the middle of April they should be well mulched and watered every day for the first month or they will not take (i.e. the roots will not develop and they will wither away). If possible use named osier or biomass types since these will grow vigorously and can tolerate the annual coppicing as well as being suited for use in basketry and hurdle making. If you are gathering local wild osier or other willow to plant your tree bog, use willow that has been coppiced or pollarded recently, because young wood makes the best cuttings.

Willow management

The willow can be coppiced annually or left to grow for more than one year, though you will find it grows quite large. Cut the willow between November and early March and don’t be afraid to cut it right down to the ground or to the top of your living woven willow hurdle - as long as it is not smothered with weeds and has sufficient sunlight it will regrow vigorously, using the nutrients and moisture within the tree bog.


Sawdust is useful to aid the correct carbon-nitrogen ratio, but too much can dilute the mixture too much. Use untreated sawdust only, in small quantities, or add a daily or weekly layer to the compost pile. Wood ash is helpful in keeping the smell down. In normal use the urine drains away from the compost pile by gravity and soaks into the soil where it may contaminate the water table, and if large quantities of urine are entering your tree bog, you should consider separating the inputs - composting liquids and solids separately - e.g. a simple straw urinal, for liquids only.

The ‘gravity powered’ separation of solids and liquids should prevent the smells caused by the compost pile going anaerobic. The through-flow of air below the platform is also important as a boxed-in pile with no ventilation may well go anaerobic and start to stink.

Some treeboggers have installed in their tree bog a urine separating system like a conventional compost toilet, this enables pipework to take the urine directly to the planted willow and surrounding straw mulch and so the liquids and solids do not mix to any great degree - it is not clear if this is advantageous in that the fecal mass may then become too dry.

It is worth noting that any design used should prevent splashes of misdirected urine from soaking into wooden seating or seat support structures as this may cause a smell over time, a metal splash plate or plastic sheet pinned into place can prevent this from occurring.

Official requirements (in UK)

Planning - if your tree bog is a temporary structure and has no permanent foundations, planning regulations do not usually apply, but be sure to check locally.

Local Environmental Health Officers may have no objections if access to the compost pile is restricted from children and pets (hence the chicken wire or similar barrier) and hand washing facilities are available nearby. The full gamut of Environmental Health regulations for composting toilets may be applied; do not assume a tree bog is exempt from these regulations.


Bog here is a British English slang word for toilet, not to be confused with its other meaning of swampland.

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