Compost Wood and Woodchip
Wood may not loom high in the life of the average composter who may only be faced with the decision as to whether to compost woody prunings from shrubs and the occasional branch from a small tree or to avoid the effort by putting them in the "green" wheelie bin.
However in most gardens there will be the need to dispose of the occasional rotten fence post or wood from a garden building which might result in a trip to the recycling centre.
There are gardeneers and composters who may have more wood waste as a result of hobbies such as wood carving or turning resulting in the need to deal with small off-cuts of more exotic woods and quanties of sawdust and shavings.
Most pallets are made from untreated wood (See below) and can therefore be home composted.
The allotment composter and those with large gardens are likely to use whole pallets to make compost bins and those that are broken could either be be “chipped for home composting if the equipment was available or taken to the recycling centre.
Composting Wood from Garden Trees
Shredded or chipped untreated wood can be home composted, ideally in a separate compost bin, or heap as it will be a slower process than composting normal garden and kitchen waste. The material should be kept moist and aerated. Small quantities of woody hedge trimmings or thin prunings can be cut using secateurs and added to “normal” compost bin or heap. The smaller the pieces the quicker they will compost. It is good to start cutting the twiggy wood into 1– 2“ lengths accepting that the standard might slip after a while and the pieces being cut longer. In the case of larger quantities or thicker wood, it is better to use a shredder. Most domestic shredders can take wood up to 3-4cm (1¼-1½in) in diameter. Where the branches are thicker, it may be necessary to hire a bigger machine or employ a garden maintenance contractor or tree surgeon with a bigger shredder or wood chipper
In general, wood from poisonous plants, such as yew, can be hot composted, as the most of the toxic substances will be broken down with time or during the composting process composting. To be extra safe the wood can be stored in a woodpile for a few months before shredding and finished the compost kept six months or until the following season to mature
As wood shreddings added to the compost bin are carbon rich browns there will be a need to provide additional Greens as a nitrogen source, e.g. lawn mowings or comfrey, to provide the necessary C: N ratio for effective aerobic composting. It may take 3-4 years for woody chippings to decompose to form good compost. When hot-composting food waste in some of the modern commercial hot composters wood chip is used as a bulking agent so shredding your own wood may reduce the need to purchase a bulking agent. ( Composting Food )
The use of composted wood chip and sawdust as a bulking agent in food composters is quite common and wood chip may be used as a source of carbon in conventional compost bins as well as being composted after use as chicken or pet bedding.
Fresh wood chip produced by landscape gardeners from woodland maintenance and tree surgery may also be available to allotment societies to make paths and any surplus could be used as mulch or to make compost.
In addition to chippings from trees, branches (including leaves) wood chip is a term used to describe a range of wood products including sawmill residues and sawdust.
However, concerns are often raised by home composters as to whether woodchip or wood-based compost is safe to use because of the risk of nitrogen depletion
It is true that wood chip consists mainly of carbon compounds (Browns) and the lignin and cellulose may take considerable timeto be brocken down by the composting fungi and bacteria. Wood chip only contains a small proportion of nitrogen and if fresh wood chip were added directly to the soil, the composting microbes would take up nitrogen from the soil depriving growing plants of soil nitrogen resulting in the risk of nitrogen deficiency.
However, the answer is simple do not add fresh wood chip compost it first. It can then be used as soil improver or mulch. It is reported in some sources that nitrogen depletion is less significant where the wood chip is used as surface mulch.
If the wood from which the wood chip was made included leaves, it will contain more nitrogen but should still be composted before use.
The initial C: N ratio of a wood chip heap may be in the region of 150:1 but as composting progresses this falls to about 40:1 as the decomposition releases carbon dioxide to the atmosphere from the breakdown of carbon, so just leaving it in a heap has a positive effect. (Compost C: N Ratio ).
Webber and Gee describe a technique for the composting of large quantities of wood chip that may be appropriate for community composting sites (https://www.trees.org.uk/Trees.org.uk/files/a0/a0b752e9-19e2-4f02-8530-dbb592074388.pdf)
Under this technique, the wood chip is arranged in windrows up to 2m wide and up to 1.5m high; this size allows air to percolate through the heap, the top of the windrow cupped to help retain water. The wood chip is moistened by adding 30litres of water per cubic meter of woodchip to give a moisture content of 50-70% (Compost Moisture)
The rate at which the chip decomposes can be increased significantly by the addition of a nitrogen source. If large quantities of wood chip are being composted this could be by the addition of Ammonium Nitrate, Ammonium sulphate or Sodium nitrate when building the windrows rather than adding “Greens” which we would recommend if home composting or using conventional bins on an allotment.
If using windrows the temperature should be monitored (temperature ) and the pile turned when the temperature declines below 50-55°C. Turning may be necessary at about two weekly intervals and should continue until turning does not result in an increase in temperature. Once the active stage is completed, it has been suggested that the immature compost is left for between 3 and 12 months to mature. However, to be on the safe side it might be better to leave it for one to two years and then leach it well before incorporating into the soil.
One of the advantages of wood chips is that in smaller quantities it will compost aerobically on their own without the need for turning or aerating, providing the moisture level kept at a sufficient level throughout the pile. This can be difficult as while the freshly chipped branches start with an adequate moisture content this will be reduced during the initial heating process. Hosing the pile often results in the water running off and soaking the surrounding ground.
In fact for wood chip to be composted effectively, and in a reasonable time in a home compost bin, it needs to be mixed with a good supply of “Green” material, e.g. lawn cuttings, kitchen waste, chicken manure should result in the material reaching the immature compost stage within a year. Some composters overcome this problem by using a hot composting system, layering the woodchip with freshly cut grass clippings or manure to help retain moisture and provide a source of nitrogen rich Greens and turning it regularly during the initial stages. I would certainly recommend this variation in composting woodchip in a New Zealand or a plastic bin at home or on the allotment. It should then be allowed to mature after which it is dug into the soil.
Home Composting wood chip in a dedicated bin
If there is sufficient woodchip available to warrant the use of a dedicated “woodchip” compost bin, I would suggest using a double width pallet bin made using six large pallets where the rear, and front, of the bin are made from two pallets. In the six-pallet model, the sides consist of a single pallet each side but it is better to use eight pallets with sides, the front and back consisting of two pallets. In the case of wood chip, composting size does matte.
As a alterative to pallets a wire netting bin, similar to those used to make leafmould could be used again or even an old builders bag as used to deliver sand (these are non-returnable so you will be doing neigboroughs a favour by taking the ).
The woodchip will need to be soaked regularly so it is best to use a concrete or slab base or alternatively a thick plastic sheet so that the water can be drained by a gutter in the ground into a storage container sunk also sunk into the ground enabling it to be reused.
If making a pile solely of wood chips do so in layers, soaking each layer as it is added. When almost at the top of the pallet bin added wet autumn leaves from one of your “leaf mould” sacks. The leaves should be soaked with water once added. Cover with a tarpaulin to help retain the moisture. The bin will need soaking as the wood chip decomposes to maintain the moisture level. The moisture level can be monitored by inserting the probe from a moisture meter through sides of the pallet bin.
An alterative method of making woodchip compost is to use build it using layers of woodchip, providing a source carbon rich Browns, and layers of Grass mowings as a source of Greens.
Some report that the wood chip will decompose in a few months and while this may be true in the centre of the heap, it might be better to allow up to three years depending on the conditions.
Composting note for Wood Workers & Hobbyists
The question of using waste from the wood workshop (sanding dust, sawdust, wood scraps, shavings, etc.) in compost has featured in many forums around the web. Including http://www.woodworkersjournal.com. and
Sawdust from most woods, such as, apple, ash, maple, peach, pine, poplar, oak, and sapele are can be home composted or used a mulch or animal bedding. . Larger shavings and material from the lathe or power planer can also be composted or used for garden paths.
Sawdust from woods such as Black Walnut can also be composted if the right techniques are used.
Sawdust can be Home composted if it is from untreated wood. In addition to providing a source of Browns, sawdust will also help absorb excess moisture in kitchen food waste. It is advisable to soak the sawdust directly either before or when adding it to the bin, compost or comfrey tea is very useful as a means of adding moisture to sawdust. The wet sawdust should be mixed well once in the bin to avoid it forming a wet mat excluding air and encouraging anaerobic fermentation.
Sawdust or wood shavings used as animal bedding for vegetarian animals or chicken can also be added In the case of chicken bedding the manure and feathers are a useful bonus. .
Composting Sawdust with manure as a source of Nitrogen
Sawdust being high in carbon will benefit from the addition of a source of nitrogen during composting such as grass clippings. An alternative on the allotment or vegetable garden would be farmyard or stable manure.
A trial using pig manure and sawdust using different manure: sawdust ratios was reported by Troy, S.M., Nolan, T., Kwapinski, W., Leahy, J.J., Healy, M.G., Lawlor, P. (2012) 'Effect of sawdust addition on composting of separated raw and anaerobically digested pig manure'. Journal of Environmental Management, 111:70-77.
This work was actually looking at the composting of pig manure using sawdust as bulking agent and source of carbon. This recommended ratio was 3:2. And the hope was that the amount of sawdust could be reduced to save costs
Two composting experiments were conducted to determine the impact of varying the proportion of sawdust using raw, or anaerobically digested pig manure using manure only, manure/sawdust mix 4:1, and manure/sawdust mixed 3:2. The mixtures were composted with regular turning in tumblers for 56 days.
It was found that the proportion of manure-to-sawdust ratio of 4:1 was effective compared to the previously recommended ratio of 3:2.
Horse or cattle manure could be used instead of pig manure but the ratio of sawdust to manure will differ. The Carbon: Nitrogen ratio of the mix should be between 25:1 and 30:1. Horse manure is about 27:1 so there will be a need to adjust the amount of manure added to the sawdust when manure from other animals is used.
The table below gives examples of the C:N ratios of different manures
Type of manure
( dry weight)
Manure with straw
27 : 1
With wood shavings
Broiler breeder layer
Source: Manure Composting Manual
Walnut Trees & Juglone
Juglone is a chemical that produces yellowing leaves, wilting and eventual death in susceptible plants. It is produced in the roots, wood, leaves, and fruit of trees in the walnut family (Juglandaceae) which includes, in addition to the black walnut which has the highest juglone concentration, butternut, hickories and pecan. Persian (English) walnuts also produce juglone but in much smaller quantities than the Black Walnut.
Fortunately, not all plants are susceptive to the chemical. Most trees, vines, shrubs, annuals, perennials, corn, beans, onions, beets and carrots are tolerant of juglone but if you grow any of these trees, it is worth checking the plant list http://www.extension.iastate.edu/news/2005/jul/070701.htm
Composting Walnut Leaves and Sawdust
Juglone is found in Walnut leaves and during the spring can be concentrated in the actively growing leaves but despite this they can be composted as the juglone toxin breaks down when exposed to air, water and bacteria. The toxic effect from leaves can be degraded in as little as two to four weeks. (Toxic Invasive Plant ) However, some sources still advise against using material from black walnut see below.
Walnut mulch and woodchips
Mulch or woodchips from black walnut are not recommended for plants sensitive to juglone. However, composting the woodchips for a minimum of six months allows the chemical to break down to a safe level even for plants sensitive to juglone.
Location of the Compost heap
If using an open bin, heap or pile it is better to locate it well away from any walnut trees vegetation so that leaves and trigs do not fall into the compost and that . rain does not drip down from the tree into the bin.
Sawdust from Walnut Wood
As the Juglone exuding from the Black walnut can affect other plants many sources on the internet advise that sawdust from the wood should not be composted. For example, Barb Larson Horticulture Educator for Kenosha County University of Wisconsin Extension wrote in 2014 “Take care not to use black walnut leaves as mulch or in compost”. Other reliable sources report that the juglone breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks in leaves. In the soil, breakdown may take up to two months after the living walnut tree has been removed. This means that sawdust from Walnut can be safely composted using the right techniques.
C Haynes, Department of Horticulture, Iowa State University wrote in 2912 that Black Walnut “Leaves and sawdust can be composted if allowed sufficient time to break down the juglone. Decomposition generally takes 2 to 6 months at which time composted material can be safely used around sensitive plants”. https://hortnews.extension.iastate.edu/2002/6-14-2002/juglone.html
A six-month composting period is suggested by Ohio State University. While Ontario Ministry of Agriculture recommended that the leaves, twigs and husks should be composted for one year prior to spreading into gardens or used as mulch around sensitive plants. http://www.omafra.gov.on.ca/english/crops/facts/info_walnut_toxicity.htm
To be on the safe side I would suggest that after the active stage of composting has been completed it should be allowed to mature until the next season or for six months and that, a safety test should be carried out by planting tomato seedlings in a sample before using the compost on the garden. Tomatoes are recommended as the test plant as they are very sensitive to juglone. To be on the safe side the composted material can then be used with plants that are not susceptible to juglone damage.
Although hot composting should break down the juglone more quickly, it might be best not to use it on susceptible plants without testing it first to ensure that the juglone has been broken down completely.
Treated woods present problems. Wood used in fencing, other outdoor structures or building work is often pressure treated with a range of preservatives such fungicides and pesticides, to prolong its life, and with other chemicals such as fire retardants. Such treatments may have desirable functions while the wood is in use, such as resisting fungi, insects and fire but when trying to get the wood to decompose resistance to attack by fungi and insects ceases to be a desirable property. The treatment is likely to contain a range of chemicals such as copper, chromium and arsenic that may be harmful to health and would contaminate other organic material if mixed with it when composting.
Home composting of treated wood is not recommended. It should not be put in the Council Green bin kerbside collection system or added to the Green waste at the local Recycling Centre. If treated wood and its residues e.g. sawdust , were placed in a green organics wheelie bin, there is a risk that the harmful residue could contaminate a whole truck load of organic material and eventually enter the food chain, Similarly a residue surviving home composting could in theory contaminate plants grown using the compost.
Councils may permit householders to put small pieces of treated wood building material scraps, processed (either treated or machined) wood, off-cuts and residues such as sawdust in their “landfill” kerbside waste stream while larger pieces should be taken to the Recycling Centre where there will probably be a separate bay for wood.
Community Wood Recycling website provides background information and details of the grading system operated by the Wood Recyclers Association, which shows the end uses of different grades of wood. This information on grades is provided to encourage householders to separate their wood for disposal. (http://www.communitywoodrecycling.org.uk/learn-more/waste-wood/) http://www.woodrecyclers.org/
Grade A: “Clean” recycled wood – material produced from pallets and secondary manufacture etc and suitable for producing animal bedding and mulches. Commercially recycled woodchip may be used as garden and landscaping mulch and it is added to green waste compost to adjust the C: N ratio.
Grade B: Industrial feedstock grade – including grade A, but mixed with construction and demolition waste; this may be used to make panel board.
Grade C: Fuel grade – this is made from all of the above material plus that from municipal collections and civic amenity sites and can be used for biomass fuel.
Grade D: Hazardous waste – this includes all grades of wood including treated material such as fencing and and requires disposal at special facilities.
Commercial Composting of treated wood waste
Commercial Composting of shredded waste wood requires a Standard Rules permit (SR 2010 number 14.) and treated wood must not to be used for any form of composting or land spreading, without a further permit
Treated wood, including screened wood fines, must not be taken to composting sites after shredding. Compost produced using such materials does not meet PAS100 and the CQP and would be classified as a waste.
However the Environment Agency would in principle allow it to be mixed with other allowed wastes to be biologically treated to produce a Compost Like Output (CLO) or Separated Organic Material (SOM) at sites with the relevant waste permit so if taking treated wood waste or shreddings to the recycling centre please check the position.
An alternative to pressure treating wood, acetylation is chosen by some woodworkers, where available, as it results in resistance to decay and insect damage as well as improved stability during moisture changes while apparently making it easier to machine and finish. Unlike pressure-treated wood, acetylated lumber does not contain any copper or biocides.
In a paper Acetylated Wood The Science Behind the Material by Dr. Callum ( Renewable Materials School of the Environment and Natural Resources University of Wales. www.accoya.com/wp-content/uploads/2011/05/Acetylated-wood.pdf) it states that “Due to its nature, it could be ground down to a form suitable for composting, or incinerated without release of toxic materials into the environment. A more appropriate strategy would be to develop a materials cascade approach. Acetylated wood would be used as a feedstock for particleboard, MDF or other reconstituted wood products to make more dimensionally stable products”.
However this is more expensive than pressure treated wood and is not widely available, it is included here for information purposesonly in case it does become more widely used. For the present it should be disposed off as treated wood.
In summary it is not permissible for treated wood to put in the garden waste bin or green waste container at the Recycling centre as this could result in it being used for composting. Untreated wood may be beneficially spread directly to agricultural and non-agricultural land but only under an exemption or a Standard Rules Permit