Cedar mulch is an organic ground cover made from the shredded wood or bark of cedar trees, typically Western Red Cedar or Eastern Redcedar, and is a byproduct of the timber industry. Its rich reddish-brown color and distinct aroma make it a popular choice for landscaping. Concerns about its natural composition and interaction with soil have led to questions about whether this material is harmful to plants. The answer requires separating common gardening myths from the science of wood decomposition and proper application techniques.
Debunking the Myths of Toxicity and Nitrogen Depletion
The primary concern about cedar mulch is the idea that its natural aromatic oils, such as thujone, are toxic to surrounding plants. While these compounds are potent in the living wood, there is no scientific evidence that cedar mulch harms established landscape plants. The concentration of these oils in processed, aged mulch is too low to create a toxic environment for deep-rooted plants.
The wood’s allelopathic properties are significantly reduced by the time the wood is chipped and sold as mulch. The small amount of leachate that does occur is rapidly diluted and neutralized in the soil environment. Concerns about toxicity are more relevant to young seedlings or sprouts, which have less developed root systems and are more vulnerable to chemical changes in the topsoil layer.
Another common worry is that cedar mulch will “rob” the soil of nitrogen, starving the plants above. This concern stems from the high carbon-to-nitrogen ratio of wood, which requires soil microbes to pull nitrogen from the environment to break down the carbon. This process, called nitrogen immobilization, only becomes an issue when the high-carbon material is mixed or tilled directly into the soil.
When cedar is applied as a surface mulch, the nitrogen immobilization is confined to the narrow interface between the mulch and the top layer of soil. Established plants with roots extending below this shallow layer are unaffected by the temporary nitrogen depletion occurring at the surface. As the mulch slowly decomposes over many years, it eventually releases its captured nutrients, including nitrogen, back into the soil.
Physical Effects on Soil Moisture and Temperature
The physical properties of cedar mulch offer substantial benefits to the garden environment. A layer of cedar mulch acts as a barrier, reducing water evaporation from the soil surface. This moisture retention helps keep the soil consistently damp, allowing plants to develop deeper, healthier root systems and reducing the need for frequent watering.
This covering also provides insulation, moderating soil temperature fluctuations that can stress plant roots. By shading the soil, the mulch keeps it cooler during summer days. The same insulating layer helps to retain warmth, protecting roots from freezing during the winter months.
Cedar is a slow-decomposing wood, meaning a single application provides coverage for a longer period compared to softer mulches like grass clippings or straw. This durability is an advantage for longevity and reducing annual maintenance. However, this slow decay means cedar contributes less organic matter and fewer nutrients to the soil in the short term than faster-decomposing options.
A potential physical drawback is that a thick or matted layer of cedar mulch, especially if it dries out completely, can become hydrophobic, meaning it repels water. When this occurs, rainfall or irrigation can run off the surface instead of penetrating to the soil below. Breaking up any matted sections with a rake and ensuring deep, slow watering methods can counteract this temporary issue.
Unique Pest and Weed Control Properties
The aromatic compounds are effective for pest control, deterring certain unwanted insects. Cedar’s natural oils repel pests like termites, ants, and some moths, making it a functional choice for garden beds near structures or for discouraging surface pests. The effectiveness against beneficial insects and pollinators, however, is a consideration, as the strong scent may also deter them from the immediate area.
The most immediate benefit of cedar mulch is its effectiveness as a weed suppressant. By creating a dense physical barrier over the soil, the mulch blocks sunlight from reaching weed seeds, preventing them from germinating. The slow decomposition rate of cedar ensures this barrier remains intact and effective for multiple seasons.
Best Practices for Applying Cedar Mulch
The benefits of cedar mulch are maximized through proper application. The ideal layer of mulch should be maintained at a depth of two to three inches. Applying a layer thinner than two inches may not provide adequate moisture retention or weed suppression, while a layer thicker than four inches can restrict air and water flow to the roots.
Avoid piling mulch directly against plant stems or tree trunks, often referred to as a “mulch volcano.” This practice is detrimental because the excess moisture trapped against the bark can promote decay, inviting pests and pathogens to attack the weakened tissue. The mulch should be pulled back several inches from the base of the plant, leaving a small, clear ring of soil around the trunk or stem.
After spreading the mulch, thoroughly water the area to settle the material and ensure moisture penetrates the soil. Subsequent watering should be done slowly and deeply to allow the water to soak through the mulch layer and into the root zone. The existing layer should only be replenished when it has decomposed below the two-inch functional depth, though a thin top layer can be added for aesthetics.