Cedar mulch, derived from the bark and wood of various trees like Western Red Cedar and Eastern Red Cedar, is a popular organic option for landscaping beds. Available in shredded or chipped forms, it offers an appealing, rich reddish-brown color. This material functions as a protective layer over the soil, helping to regulate temperature, suppress weed growth, and conserve moisture.
The Advantages of Cedar Mulch
One of the most praised features of cedar mulch is its impressive longevity compared to other organic mulches. Cedar wood contains natural compounds, like thujaplicin, which resist decay from fungi and bacteria. This causes it to break down much slower than pine or hardwood mulches, meaning it requires less frequent reapplication, often lasting three to five years.
The aesthetic qualities of cedar mulch are also a significant draw, with its natural color remaining vibrant for longer than many alternatives. The shredded texture tends to interlock, forming a dense mat that resists shifting from heavy rain or wind. Cedar is also well known for its pleasant, aromatic scent, which results from the natural oils within the wood.
These aromatic oils, including a compound called thujone, provide a natural deterrent effect against certain common garden pests. Cedar mulch is often cited for discouraging insects like termites, ants, mosquitoes, and some moths. This property makes it a preferred choice for areas near wooden structures or outdoor seating spaces. This is a repellent action, not a broad-spectrum insecticide.
Potential Concerns: Nutrient Impact and Leaching
A common concern with any wood-based mulch is the effect it may have on soil nutrients, specifically nitrogen availability. Woody materials possess a high carbon-to-nitrogen (C:N) ratio, meaning they contain significantly more carbon than nitrogen. When soil microorganisms begin decomposition, they require nitrogen to break down the carbon fibers.
If the mulch is fresh, these microbes will temporarily draw available nitrogen from the shallow top layer of the soil to fuel their growth. This process, known as nitrogen drawdown, can cause temporary yellowing or stunted growth in shallow-rooted plants like annuals or vegetables. However, this effect is localized to the soil-mulch interface and rarely impacts established trees or shrubs with deeper root systems.
Claims that cedar mulch is toxic to plants are often overstated and relate to the concept of allelopathy. While some studies suggest fresh cedar extracts can inhibit the germination of sensitive seeds, there is minimal documented evidence of cedar wood chips harming established landscape plants. The natural oils that give cedar its repellent properties are not phytotoxic to mature plants. Any observed growth inhibition is more likely due to nutrient competition or the physical barrier the mulch creates for seedlings.
Optimal Use and Placement
Given its properties, cedar mulch is recommended for use in foundation planting beds and around established trees and shrubs. Its slow decomposition rate and attractive appearance make it ideal for areas where long-lasting aesthetics and low maintenance are desired. The natural insect-repellent quality also benefits these permanent landscape areas.
Caution should be exercised when using cedar mulch in vegetable gardens or beds containing young annuals where rapid nutrient uptake is important. In these areas, a well-composted material that does not compete for nitrogen may be a better choice. If cedar is used, incorporating a nitrogen-rich material like compost or aged manure beneath the mulch layer can mitigate potential nitrogen drawdown.
When applying the mulch, the recommended depth is between two and three inches. Avoid the practice known as “mulch volcanoes,” which involves piling the material up against the base of a tree trunk. Mulch piled too high retains excessive moisture against the bark, which can lead to rot, insect infestation, and the development of girdling roots. Instead, spread the mulch in a donut shape, keeping a clear space of several inches around the trunk flare.