Cedar mulch is a popular landscaping material, created as a byproduct of processing cedarwood, such as Eastern Red Cedar (Juniperus virginiana) or Western Red Cedar (Thuja plicata). Beyond its aesthetic appeal and ability to suppress weeds, cedar mulch is widely reputed to deter insects. The aromatic quality of the wood has long been associated with keeping certain pests away. This article examines the scientific basis for cedar’s repellent action and identifies which insects are truly affected by its presence.
The Chemical Basis of Cedar’s Repellent Action
The insect-repelling power of cedar mulch stems from volatile organic compounds (VOCs) concentrated within the wood’s natural oils. These essential oils contain specific chemicals, such as cedrol and thujone, that are biologically active against certain pests. Cedrol is found predominantly in Eastern Red Cedar, while thujone is common in Thuja species.
These chemicals slowly evaporate, releasing the characteristic strong cedar aroma. The odor functions as a deterrent by interfering with the olfactory sensory systems of insects, making it difficult for pests to locate food sources, mates, or nesting sites.
Target Pests: Insects That Cedar Mulch Repels
Scientific evidence supports that cedar mulch deters insects relying heavily on scent for survival. One common target is the termite. While cedar is not toxic, termites generally avoid it, making the mulch a viable barrier in landscaping near structures.
The aromatic compounds are also effective against pests that infest stored materials, such as various species of moths (clothes moths and pantry moths), which are strongly repelled by the odor. Cedar mulch can also help manage biting pests in a yard. Research shows that compounds like cedrol exhibit strong effectiveness against ticks and can help repel fleas.
Furthermore, the strong scent interferes with the navigation of crawling insects. Specific species of ants may avoid foraging through cedar mulch because it disrupts the pheromone trails they use for communication. Mosquitoes and cockroaches are also among the pests repelled by the volatile oils present in fresh cedar mulch.
Limitations and Non-Targeted Garden Pests
While effective against certain pests, cedar mulch is not a universal repellent and has little effect on many common garden inhabitants. Many garden pests are not deterred by the wood’s aroma and may find the moist, decomposing material an attractive shelter. Pests like slugs and snails (mollusks, not insects) are completely unaffected by cedar’s chemical properties.
The physical nature of the mulch layer can also inadvertently provide refuge for non-targeted pests, such as earwigs, pill bugs, and vine weevils, which thrive in the dark, damp environment created beneath any thick layer. Furthermore, cedar mulch does not significantly deter beneficial garden organisms, including ladybugs, ground beetles, and earthworms.
Practical Longevity and Application Tips
The insect-repelling efficacy of cedar mulch is directly tied to the concentration of its volatile oils, which diminishes over time. Exposure to environmental factors like rain, sunlight, and wind causes the cedrol and thujone to dissipate from the wood fibers. The strongest repellent effect is observed when the mulch is newly applied, but protective properties typically begin to wane within six to twelve months.
To maintain the pest-deterring effects, the mulch requires periodic maintenance and replenishment. Gardeners should rake or turn over the mulch layer every few months to expose the lower, less-weathered chips, releasing a fresh burst of aromatic oils. For continuous protection, especially in areas prone to pests like termites or ticks, the top layer of cedar mulch should be refreshed annually or biannually.