Wood chips serve as an excellent organic mulch, improving garden health and reducing maintenance. Applied correctly to the soil surface, this layer conserves soil moisture by reducing evaporation. A dense layer of wood chips effectively suppresses weed growth by blocking sunlight and moderates soil temperatures, keeping roots cooler in summer and warmer in winter. As the material slowly decomposes, it adds beneficial organic matter.
Choosing Safe and Effective Wood Chips
The source of your wood chips is a primary consideration, as not all woody materials are safe for garden use. Strictly avoid any chemically treated wood, such as pressure-treated lumber, painted wood, or engineered products like plywood or particle board. These materials can leach harmful chemicals and heavy metals into the soil.
A particularly important species to exclude is black walnut, as all parts of the tree contain juglone. Juglone is an allelopathic chemical that can inhibit the growth or kill sensitive plants, such as tomatoes, peppers, and ornamentals. Arborist chips, a mixture of wood and leaves from trimming operations, are often the best choice because the leaves provide higher nitrogen content, which aids decomposition. Chip size also matters; a mix of fine and coarse pieces, often called “ramial chipped wood,” breaks down more efficiently and provides better coverage than uniform, large chunks.
Proper Application Depth and Placement
Correct application depth is paramount to maximizing the benefits of wood chip mulch. For established perennial beds, shrubs, and trees, a depth of 2 to 4 inches is recommended to effectively suppress weeds and retain moisture. If the soil is heavy clay or has poor drainage, apply a slightly thinner layer, around 2 to 3 inches, to ensure adequate air and water exchange.
One frequent application mistake is piling the mulch directly against the base of a tree trunk or plant stem, often called “volcano mulching.” This practice must be avoided because constant moisture trapped against the bark can lead to rot, disease, and create a habitat for rodents. Instead, pull the wood chips back, leaving a clear ring of soil a few inches wide around the base of the trunk or stem to expose the root flare.
In annual vegetable gardens, wood chips are best used in permanent pathways between growing beds, where a 4 to 6-inch layer provides excellent weed suppression and a clean walking surface. Applying chips directly to the soil where shallow-rooted vegetables grow can be risky, especially with fresh chips. If mulching vegetable beds, use finely aged or fully composted wood chips, or alternative mulches like straw, to protect the shallow root systems.
Debunking Common Wood Chip Myths
Many gardeners worry that wood chips will steal nitrogen from the soil, but this fear misunderstands the decomposition process. “Nitrogen theft” is only relevant if the wood chips are incorporated or tilled directly into the soil. Microbes that break down high-carbon materials require nitrogen, temporarily pulling it from the surrounding soil to fuel their activity and leading to a localized deficiency.
When wood chips are used as a surface mulch, this temporary immobilization of nitrogen occurs only at the narrow interface between the mulch and the soil. This does not significantly affect the deeper, established root systems of trees and shrubs. Over time, as the mulch fully decomposes, the nitrogen is released back into the soil, often resulting in a net increase in nutrients.
Another concern is that wood chips will cause the soil to become too acidic. Research confirms that most wood chips, even those from softwoods, are close to a neutral pH and do not meaningfully change the underlying soil chemistry. Any slight, transient acidity from the initial decomposition is confined to the mulch layer itself and is neutralized by the soil’s buffering capacity. If the soil is highly alkaline, the addition of organic matter may help pull the pH closer to the neutral range, which is beneficial for most garden plants.