Good gardeners know that a generous layer of mulch will help garden plants in many ways. Some of the benefits include reduced weeding, cooler soil in the summer, water conservation (need less frequent irrigation), and the slow addition of organic matter as organic mulches break down — all of which leads to healthier plant growth.
There are many materials that can be used as mulch. I am often asked about using fresh woods chips (sometimes called arborist chips) for the landscape or garden. Fresh wood chips are often available at municipal landfills, compost sites and following tree removals at residences.
Before getting into some details, let's be clear about our terminology. Mulch refers to a layer of some type of material on the surface of the soil. It is usually coarse in texture, and provides the benefits mentioned above. This is opposed to compost, which is used as a soil amendment — something that is worked in to the soil to improve its texture and other benefits. A good compost is in a highly advanced state of decomposition, and you shouldn't be able to tell what it was made of.
Folks often have various concerns about using arborist wood chips for mulching. One concern that some have is that diseases on the wood might be transferred to ornamental plants. Plant pathologists indicate that it is very unlikely that the right combination of factors could come together to spread diseases in this way. Again, wood chips are used on the surface of the soil, not incorporated where plant roots occur. So, don't worry about diseases.
Another concern is related to naturally occurring allelopathic chemicals that theoretically could be leached into the soil and affect nearby plants. Allelopathy is the release of chemicals produced by a plant that discourages or suppresses the growth of other plants nearby (mainly germinating seeds and young seedlings). Research over the years has not demonstrated any detrimental effect of wood chips on established plants. Walnut is one tree with demonstrated allelopathy, but it is unlikely you'll encounter wood chips with a high content of walnut.
A common misconception is that fresh wood chips tie up nitrogen during their decomposition. For sure, nitrogen depletion will be a temporary problem when fresh wood chips are incorporated into the soil, which is why we should only use fresh chips as a surface mulch. In this case, nitrogen depletion would only be right at the soil surface, which may be one reason wood chip mulches are efficient at suppressing seed germination. Because of this, and the general coarseness of wood chips, they probably are best not used around vegetables and in annual flower beds.
But, they are a perfect choice for shrub beds, natural areas and around trees. Several research studies have shown there is no nitrogen depletion problem for established wood plants using fresh wood chips. If you are still concerned, you can let them age before using, but it is not necessary.
Because wood chips from tree services are usually a combination of bark, sapwood, hardwood and leaves (during the growing season, or from evergreen plants), as they break down, they slowly provide small amounts of nutrients. Also, as they break down, they increase the organic matter of the soil. This organic matter gets worked down into the soil through the activity of earthworms and insects that live and burrow through the soil. The increased organic matter in the soil results in healthier plant growth.
Using locally produced wood chips is a sustainable activity, keeping a useful product out of the landfill, which is both environmentally and economically a good thing.
In order to get the optimum benefits from wood chips, they can be applied at a depth of 4 to 6 inches. They do break down rather quickly, and will settle after a few weeks. Like any organic mulch, they need to be replenished periodically to keep providing weed suppression and water conservation benefits.
Do not pile them up against tree trunks. That can lead to potential problems with insects and fungal diseases due to the constant moisture on the tree trunks. This commonly referred to as mulch volcanoes, a practice that is not recommended. Instead, spread them like a donut so they are not in direct contact with the trunks of trees.