Cedar Mulch & Fleas
Cedar mulch is desirable to home gardeners for its color, scent and general durability. The mulch is also said to repel fleas and other common insects. For that reason, it is often used in pet bedding. On the other hand, cedar is commonly rumored to have a toxic effect on some plants.
Types of Cedar Mulch
Many types of cedar mulch are not made from true cedars (species Cedrus). Some are from the species Juniperus or Thuja. The eastern red cedar, for example, is actually a juniper tree. Cedar mulch is also not always the bright red color that is so attractive to many home gardeners. It can be much lighter (almost white) or darker brown, depending on the species.
Cedar Oil and Fleas
Cedrus, Thuja and Juniperus species all contain the essential oil (called thujone) that repels insects. Mulch that has been "de-oiled" will not repel fleas and other insects, so be sure to check the bag if you want mulch that has retained the oils. It is the distinctive scent of this oil that keeps the fleas away. The mulch will also repel slugs and snails and even rodents.
Cedar Mulch Toxicity
Cedar mulch is also rumored to be allelopathic. This means that chemicals in the cedar will kill some plants. There is no scientific evidence to support this rumor, according to Washington State University. It's more likely that overuse of the mulch by gardeners stifled the growth of small seedlings or that very fine cedar sawdust was used, which can seep into the ground easily and be absorbed by the roots of seedlings. Use thin layers of large-wood cedar mulch sparingly around very young plants, and do not use sawdust at all.
Due to its ability to repel fleas, cedar mulch is often used in dog and cat bedding. Never wash such bedding, however. If the cedar gets wet inside the fabric, it will never dry out and will rot. Instead, spot-treat stains on dog and cat bedding filled with cedar mulch. The mulch can also be used in an outdoor dog kennel or dog run or around a doghouse.