Cedar mulch is a popular landscaping material derived from the shredded bark and wood of cedar trees, such as Western Red Cedar or Eastern Red Cedar. This organic ground cover is recognized for its distinctive, pleasant aroma and attractive reddish-brown color. While many gardeners favor its aesthetic and functional qualities, its suitability for all garden applications is debated among horticulturalists. Using it requires understanding its unique chemical makeup and its specific interactions with soil biology.
The Unique Advantages of Cedar Mulch
The primary appealing feature of cedar mulch is its longevity. Cedar wood contains natural compounds, such as thujaplicin, which inhibit the fungal and bacterial decay that rapidly breaks down other organic mulches. This slow decomposition rate means a single application can last for three to four years, significantly reducing the frequency of replacement compared to softer wood or straw mulches.
The aromatic oils that give cedar its signature scent are a primary functional advantage, acting as a natural pest deterrent. These volatile organic compounds, like thujone, effectively repel common garden nuisances such as termites, ants, and moths. The pest-repelling potency is highest when the mulch is fresh, typically lasting six months to a year before the oils dissipate due to sun and rain exposure.
Beyond its functional benefits, cedar mulch offers superior aesthetic appeal for formal beds and pathways. Its rich, warm color and fibrous texture create a neat, uniform look that helps unify the landscape. Like all mulches, it also helps moderate soil temperature and conserve moisture by reducing evaporation.
Potential Negative Impacts on Soil Health
The most significant concern regarding woody mulches, including cedar, revolves around the high Carbon-to-Nitrogen (C:N) ratio inherent in the material. Wood is rich in carbon but poor in nitrogen, creating an imbalance for the soil microorganisms responsible for decomposition. When these microbes break down the carbon-heavy cedar mulch, they require a substantial amount of nitrogen to build their body mass.
To satisfy this demand, microbes actively draw usable nitrogen from the surrounding soil, a process known as nitrogen immobilization. This temporary nitrogen depletion can starve nearby plants, especially nitrogen-hungry annuals and vegetables, resulting in yellowing leaves and stunted growth. The average C:N ratio for materials causing this effect is roughly 30:1, but wood products can exceed 100:1. This issue is most pronounced when the mulch is mixed into the topsoil, such as during tilling.
Another potential drawback is the rumored allelopathic effect, where chemicals released by one plant inhibit the growth of others. Cedar contains various oils and phenolics that are sometimes implicated in suppressing seed germination or the growth of sensitive seedlings. However, scientific evidence suggests that the allelopathic effects of aged or shredded cedar mulch on established landscape plants are minimal and largely overstated.
A common physical issue occurs when finely shredded cedar mulch is applied too deeply, causing it to interlock and form a dense, hydrophobic layer. This matting effect can prevent rainwater or irrigation from penetrating the soil, leading to dry soil beneath the mulch despite surface moisture. The dense barrier also restricts gas exchange between the soil and the atmosphere, negatively impacting root respiration and soil health.
Proper Application and Placement
To mitigate negative impacts, strategic placement of cedar mulch is recommended. It is best utilized in permanent landscape areas, such as surrounding established trees, shrubs, and perennial beds, where the soil is not regularly disturbed. Cedar mulch should be avoided in annual vegetable gardens or beds containing young, nitrogen-sensitive seedlings, as tilling can incorporate the carbon-rich material directly into the soil, exacerbating nitrogen immobilization.
Temporary nitrogen depletion can be counteracted by applying a high-nitrogen fertilizer or a layer of well-composted organic material before spreading the cedar mulch. This proactive measure provides soil microbes with an accessible nitrogen source, satisfying their need for decomposition without taking the nutrient from plant roots.
Application depth is a primary factor in successful mulching with cedar. A layer of one to two inches is sufficient to suppress weeds and conserve moisture without risking the formation of a dense, matted barrier. When applying around trees and shrubs, maintain a space of several inches around the trunk, creating a “mulch doughnut” rather than a “mulch volcano.” Piling mulch directly against the base of a plant traps moisture, which encourages fungal diseases and can lead to the rotting of the trunk bark.