The terms “wood chips” and “mulch” are often used interchangeably in landscaping and gardening, leading to confusion. This misunderstanding stems from the fact that one term describes a broad function, while the other describes a specific material. Clarifying this relationship helps in making informed decisions for managing soil and plants. The material properties of wood chips, such as their size and chemical makeup, result in distinct advantages and disadvantages compared to other ground coverings.
Understanding Mulch as a Category
Mulch is a functional concept defined as any protective layer spread over the soil surface. This layer is applied to help retain soil moisture, suppress weed growth by blocking sunlight, and regulate soil temperature. Mulch can be composed of a variety of materials, categorized as either organic or inorganic.
Organic mulches include once-living materials like shredded leaves, straw, grass clippings, compost, and wood chips. These materials break down over time, contributing organic matter and nutrients to the soil. Inorganic mulches, by contrast, do not decompose and include items such as gravel, pebbles, or rubber.
Wood chips are a specific type of organic mulch, typically created as a byproduct of tree trimming or processing logs. They are the raw, shredded pieces of wood, bark, and sometimes leaves that result from feeding tree material through a chipper. Other common forms of wood-based mulch, like shredded bark, are derived from the same source but are often processed to achieve a finer, more uniform texture.
Physical and Chemical Differences
The primary difference between raw wood chips and other mulches, such as shredded bark or compost, lies in their physical structure and carbon-to-nitrogen ratio. Wood chips tend to be coarser, chunkier, and more irregular in shape and size. This larger particle size means they interlock less, making them less prone to matting or compaction than finer materials, which allows for better water and air penetration into the soil.
The decomposition rate is directly related to particle size; wood chips break down much slower than fine compost or shredded mulches, providing a longer-lasting ground cover. This slow decomposition is due to the high carbon content of the wood, which creates a high carbon-to-nitrogen (C:N) ratio. Microorganisms require nitrogen to break down the carbon in the wood.
When applied as a surface layer, microorganisms only temporarily draw nitrogen from the thin zone where the wood chips meet the soil. This localized nitrogen tie-up primarily affects only the most shallow-rooted plants, such as annual seedlings. However, if wood chips are mistakenly mixed or tilled directly into the soil, the nitrogen depletion becomes much more widespread in the root zone, significantly impacting plant health.
Practical Applications and Placement
The physical and chemical properties of wood chips make them ideal for certain landscape applications. Because of their slow decomposition rate and coarse texture, wood chips are highly effective for long-term use in established landscape beds, around mature trees, and near shrubs. Their longevity means they do not need to be replenished as frequently as finer mulches. They are also an excellent choice for creating durable walking paths.
Finer organic mulches, such as shredded bark, aged compost, or straw, are better suited for vegetable gardens and annual flower beds. These materials break down faster, releasing nutrients more quickly to support the rapid growth cycle of annual plants. Shredded mulches present less risk of localized nitrogen depletion to the shallow root systems of young seedlings compared to raw wood chips.
The application depth should be maintained between two and four inches to ensure effective weed suppression and moisture retention. It is important to keep any mulch material several inches away from the base of trees, shrubs, and plant crowns. Piling mulch directly against the bark, often called “volcano mulching,” can trap moisture, which encourages rot and invites pests.