Concerns

• Some resources suggest you should avoid use of cedar wood mulch due to toxins that can leach from the mulch and kill plants. Other concerns suggest that volatile chemicals produced by cedar mulch will stop seeds from germinating and even kill them. Using mulch of any kind around seedlings is not recommended, since seedling root structure has not yet formed.
  
  

Benefits

• While allelopathy is evident in some tree species, cedar trees do not produce toxins. Instead, cedars rely on chemical weapons they've developed over thousands of years. These chemicals help keep pests and disease away when used around ferns. The substances do not harm plant tissues.
  
  

Essential Oil

• Cedar trees produce an essential oil called thujone. Thujone is known for repelling moths, carpet beetles, and termites, among other pests. The natural oil does not dissolve in water, but becomes airborne, making it highly unlikely that thujone leaches into the soil around ferns.