Wood chips are a popular organic material used by gardeners to help soil retain moisture, manage soil temperature, and naturally suppress weed growth. Applying a layer of wood mulch helps mimic the natural decomposition processes that occur on a forest floor, which benefits soil structure over time. While most wood chips offer significant benefits to the garden, certain types of wood or improperly handled wood can introduce harmful substances or disrupt the delicate balance of soil nutrients. Understanding which wood chips to avoid, and why they pose a risk, is important for maintaining a healthy and productive garden space.
Chemically Treated and Contaminated Woods
Wood that has been chemically treated for preservation or sourced from industrial materials should never be used in a garden, particularly where food is grown. The most problematic material is older pressure-treated lumber, often treated with Chromated Copper Arsenate (CCA) before its residential use was restricted in 2004. CCA contains heavy metals—specifically arsenic, chromium, and copper—that are forced deep into the wood fibers to deter rot and insects.
When wood chips made from CCA material are placed in a garden, these toxic elements can leach into the surrounding soil and potentially be taken up by plants. Even small amounts of CCA-treated wood mixed into mulch can cause the entire batch to exceed safe limits for arsenic in residential soil. Gardeners using recycled wood from construction and demolition debris must be cautious, as it may unknowingly contain fragments of CCA-treated wood.
Other industrial wood sources also carry substantial contamination risks. Railroad ties, for example, are typically treated with creosote, a toxic, oily substance derived from coal tar. Utility poles and some older fence posts may have been preserved with pentachlorophenol or high concentrations of creosote. These substances can leach into the garden environment, creating hazardous conditions for soil health, beneficial organisms, and human contact.
Naturally Toxic and Allelopathic Species
Some tree species naturally produce chemical compounds that inhibit the growth of surrounding plants, a biological phenomenon known as allelopathy. The most common example is the Black Walnut tree, which produces a chemical called juglone. Juglone is a respiratory inhibitor that interferes with a sensitive plant’s ability to create energy, leading to wilting, stunted growth, or death.
Juglone is present in all parts of the tree, including the wood chips, though concentrations are highest in the roots and nut hulls. Juglone breaks down relatively quickly when exposed to soil, water, and microbial activity. However, using fresh Black Walnut wood chips is not recommended around highly sensitive plants like tomatoes, peppers, and various berry bushes due to the risk of localized toxicity.
While there is scientific debate about the true extent of juglone’s allelopathic effect when applied as a surface mulch, caution is necessary. Gardeners should avoid using wood chips from Black Walnut and other potentially allelopathic species, such as some types of Eucalyptus, directly on sensitive vegetable or flower beds.
The Issue of Nitrogen Depletion from Fresh Wood Chips
The most common issue with wood chips is not toxicity but a temporary nutrient imbalance known as nitrogen immobilization. This occurs when fresh, uncomposted wood chips are used as a mulch layer. Wood has a high carbon-to-nitrogen (C:N) ratio, often exceeding 400:1, which is far higher than the 30:1 ratio microbes prefer for efficient decomposition.
Soil microorganisms, primarily fungi and bacteria, are responsible for breaking down the complex carbon structures in the wood. To process this high volume of carbon, these microbes require nitrogen. Since the wood chips do not supply enough nitrogen, the microbes scavenge or “immobilize” available nitrogen from the surrounding soil.
This temporary depletion leaves less nitrogen available for plant roots, causing symptoms like yellowing leaves and stunted growth in shallow-rooted plants. This immobilization occurs primarily at the interface between the mulch and the soil surface. If the wood chips are mixed into the soil, the nitrogen depletion occurs deeper in the root zone, creating a more widespread problem for the plants.
How to Identify and Handle Potential Problem Mulch
The first step in safely using wood chips is to determine the source of the material. Reputable suppliers should confirm the wood’s origin and verify that it is made from virgin materials, not recycled industrial waste. Be wary of wood chips that exhibit an unusual color, such as a strong blue or green tint, which indicates older copper-based chemical treatments.
A strong, pungent odor, often described as smelling like vinegar, silage, or rotten eggs, suggests the wood chips have undergone anaerobic decomposition, sometimes called “sour mulch.” This process produces organic acids and compounds that are toxic to plant roots. Properly aged or fresh wood chips should smell earthy or of fresh wood.
To mitigate the risk of nitrogen immobilization, fresh wood chips should be aged or composted for a minimum of six to twelve months before application. This aging allows the initial microbial breakdown to occur and the C:N ratio to stabilize before the chips are placed near sensitive plants. If receiving wood chips from a tree service, avoid using material from visibly diseased trees, as it can harbor pathogens or fungal spores that could spread to garden plants.