Wood chips are widely considered a superior choice for landscaping due to their multifaceted benefits as a soil covering material. Organic options like wood chips offer more than just coverage, actively contributing to a healthier environment for plants. Horticultural experts advocate for their use because the material’s structure and composition improve soil quality and plant survival.
Functional Benefits in the Landscape
Wood chips offer several scientifically supported roles that enhance the growing environment when applied correctly around trees and shrubs. One immediate benefit is the significant increase in soil moisture retention. The porous, coarse texture forms a thick barrier that dramatically reduces water evaporation, meaning plants require less frequent watering.
This insulating layer also provides effective soil temperature regulation, which shields roots from environmental extremes. During hot summer months, the mulch keeps the soil cooler, potentially by as much as 10°F compared to bare soil, mitigating heat stress on roots. Conversely, the layer acts as an insulator during winter, preventing rapid temperature fluctuations and protecting roots from sudden freezing and thawing cycles.
Wood chip mulch provides effective weed suppression primarily by blocking light from reaching the soil. This physical barrier prevents most weed seeds from germinating. Furthermore, the slow decomposition contributes organic matter to the soil over time, improving soil structure, increasing water infiltration, and fostering beneficial microbial activity. Arborist wood chips decrease soil bulk density, a measure of compaction, leading to better root penetration and overall tree growth.
Debunking the Nitrogen Depletion Myth
A common concern regarding wood chips is the belief that they will “steal” nitrogen from the soil, causing nutrient deficiencies in plants. This fear originates from an understanding of the decomposition process involving the carbon-to-nitrogen (C:N) ratio. Wood chips are considered a high-carbon material, and the microorganisms that break them down require nitrogen to fuel their metabolism.
When wood chips are mixed directly into the soil, the soil microbes temporarily draw nitrogen from the surrounding environment—a process called immobilization—to consume the carbon-rich wood material. This temporary nitrogen tie-up is confined to the specific area where the carbon and nitrogen sources are physically mixed. The nitrogen becomes unavailable to plant roots in the immediate vicinity until the decomposition process is largely complete.
However, when wood chips are used correctly as a surface mulch, nitrogen depletion does not occur in the root zone. Immobilization happens entirely above the soil line, in the interface between the mulch and the soil. Established plant roots, which draw nutrients from deeper soil layers, remain unaffected by this surface activity. Over the long term, as the wood chips break down, the immobilized nitrogen is released back into the soil as a slow-release nutrient source.
Best Practices for Application
To maximize the benefits of wood chip mulch, proper application techniques are necessary, especially concerning depth and placement near plants. For most landscape beds, trees, and shrubs, the recommended depth is a layer between two and four inches thick. A layer thinner than two inches may not provide adequate weed suppression or moisture retention, while a layer significantly thicker than four inches can restrict oxygen and create problems.
A common application mistake is creating a “mulch volcano,” where the material is piled high against the trunk or stem of a plant. This practice is detrimental because the excess mulch traps moisture against the bark, which the plant’s trunk is not adapted to handle. Constant moisture creates an environment conducive to fungal diseases, pest infestation, and encourages the trunk to rot.
Mulch volcanoes also lead to the development of stem girdling roots, which grow into the moist mulch layer rather than down into the soil. As these roots thicken, they can constrict the tree’s main trunk, strangling the vascular system and restricting the flow of water and nutrients. The mulch should instead be spread outward like a donut, leaving a clear ring of soil exposed around the base of the trunk or stem.
For source material, arborist chips—a mix of wood, bark, and leaves—are ideal because they offer a range of particle sizes that promote better aeration and decomposition. Use clean, untreated wood chips to avoid introducing unwanted chemicals or persistent herbicides. Timely application, typically once a year, will refresh the layer and maintain the optimal depth for continued plant health.