Acetylated wood, wood waste for composting

Q. We have recently been asked about using some acetylated wood. (One trade name is Accoya.) What can you tell me about it?

A. The process of adding acetic acid (vinegar) to wood has been studied for nearly a century. However, the cost of adding chemicals has limited its commercial adoption until recently.

By adding this chemical to wood in a process called acetylation, the wood is transformed into a wood that is still easy to work and finish, but wood that also has enhanced stability when the moisture changes. Further, the wood is highly resistant to decay and insect damage; when used above ground, it is expected to last at least 50 years.

The treatment, which is applied to 100 percent of the wood in a piece, is permanent; it will not leach out. There is no strength loss either. Machining is reported to be improved as well. In other words, for exterior use (windows, doors, siding, outdoor furniture), this acetylized wood offers, at an increase in price compared to untreated wood, excellent exterior performance without noxious chemicals. With longer life of acetylized wood products, it also means that we will need to harvest fewer trees. Presently, radiata pine from Australia, New Zealand and Chile is used; usually the timber is from an FSC source, which also adds to its desirability for some people.. As time goes on, we can expect other species to be treated.


Q. Probably like many operations, we have some dry wood scraps and sawdust that we would like to dispose of properly and profitably. We have been approached by a local garden club to supply wood compost. What can you tell me about this possibility?


A. First, this could be a great idea. However, there are four key issues that you need to address.

First, never include any walnut or butternut waste in the wood you want to compost. These two species have an herbicide that will kill plants that it contacts.

Next, composting of wood requires fairly small particles. This means that you will have to have a small chipper to reduce the wood scraps to pencil size. Sawdust needs to be mixed well with larger particles.

Next, you have to decide if you will do the composting or if the purchaser will. In many cases, you will find it is easier and more profitable if you do it.

Finally, with wood there is a nitrogen starvation issue, discussed below.

It is important that you understand a few basic concepts about using wood in gardens; many gardeners do not know much about this and so might be disappointed.



Mulch is a protective covering (made of sawdust, compost material, or paper) spread or left on the top of the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, prevent crusting, or keep fruit (as strawberries) clean. This layer of composted wood should be 2 to 6 inches thick.

Sawdust alone does not make a good mulch due to the high surface area to volume. Plus sawdust can easily compact to a dense layer and water holding is poor. So, it is best to mix the fines in with a lot of larger pieces. Nature, in order to build richer soil, relies heavily on wood mulches--fallen limbs, leaves, cones, seeds and, eventually, the tree stem itself.


Soil Amendment--Definition

A wood soil amendment uses composted wood mulch that is covered and mixed with the soil. Using composted wood mulch as soil-building material (soil amendment) is a strategy that promises huge, immediate and long-term returns for the gardener. Although wood does have trace elements, a better soil amendment will include other organic material with the wood. A good report is at



Wood is made of hydrogen, oxygen and carbon. There is no nitrogen. However, for wood to decompose, the fungi require nitrogen and will use up any nitrogen they can find. This means that the plants around the fresh wood will turn yellow and will not thrive due to the lack of nitrogen; the technical expression for the nitrogen-deprived plants is that the plants have the “yellows.”

So, when wood will be used as mulch or a soil amendment, the wood first needs to be composted with extra nitrogen added during composting. This composting before using as mulch or as an amendment means that the wood will no longer absorb nitrogen (time frame is about three to six warm months). In fact, when the composted wood is used as a mulch or soil amendment, the wood will, over coming months and years, release the nitrogen it has tied up. The plants will love it. If well composted, the most of the wood will no longer be recognizable as wood pieces or chips, but will look similar to coarse soil.

If you sell the wood to the gardeners for composting or if you only partially compost it, you need to make sure that the gardeners know about the nitrogen situation, or they may be very disappointed.

How to Compost. To compost wood make a separate compost pile that is at least four feet from any desirable plants. Follow normal composting procedures (including moist but not soaking wet). Aerate, mix or stir often (every few days) as we do not want anaerobic decomposition (end result is pH = 3; byproducts are bad). Repeat: Aerate the entire pile often. Add nitrogen so that the composting occurs at a good rate. In the cooler months, you might also add heat if you want the process to continue in the cold weather.

Adding nitrogen to the compost. Add a few pounds of blood meal (which is essentially pure nitrogen, 12-0-0) or commercial ammonium nitrate should be added--1 pound of ammonium nitrate per 20 pounds of wood. (A gardener would possibly add fresh grass clippings weekly; try for 50 percent brown and 50 percent green.) Chicken manure is fine too.

Using Mulch. Note that prior composting is not suggested when the wood will be used as a pathway or in other areas where plants are not desired. The “raw” wood compost in this case would be applied in a layer about 4 to 6 inches thick.

Also note that the fungi that decay wood can also decay the bark of a tree, so mulch around a tree should not look like a volcano, but should be about 2 inches thick and not touch the stem itself. You will find that your county agent can provide information on using mulch properly. Consider having copies of this information available for anyone that picks up a load of your “Garden Enhancing, Organic Compost.”


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.