Is camphor laurel mulch toxic?
Camphor laurel (Cinnamomum camphora) is a stately tree native to East Asia. It was introduced into Australia possibly as early as 1822 as an ornamental, and quickly gained popularity as a shade tree. Unfortunately for the Australian environment, camphor laurel is a highly invasive species that is crowding out native plants and the animals that depend on them. This is helped by its fruits, which birds favour, distributing the seeds long distances.
Camphor laurel is a declared noxious weed on the far North Coast of NSW, and in other parts of the state no permission is necessary to remove it.
Because of the tree’s fast growth and extensive coverage, camphor laurel removal is a continual process in many parts of Australia. This generates much plant material that can be used as mulch (in addition to excellent timber for wood carving). But given that camphor laurel suppresses most plants beneath it, this raises the question of whether it is toxic to plants when shredded and used as mulch.
No chemical warfare
Many tree species are allelopathic – that is, they produce chemicals from the leaves or bark that, when washed out in rain, suppress the growth of plants beneath them. Many eucalypts, for example, are known to use allelopathy to reduce competition.
But camphor laurel is unlikely to be allelopathic. Its dense, shallow roots are efficient at sucking moisture out of the soil surface, and its dense leaf canopy cuts off most light from reaching the ground. These effects on their own are enough to suppress most plants.
Workers with Tweed Shire Council have seen no difference in the growth of plantings between those mulched with camphor laurel chips and those mulched with chips of other species. In addition, the Florida Conservation Lodge Foundation chips camphor laurel into mulch for landscaping. So evidence suggests that shredded camphor laurel is safe to use as mulch.
Composting for safer mulch
The safety of any plant material as mulch can be increased by appropriate composting.
A properly prepared compost heap or windrow creates conditions that encourage the rampant growth of bacteria and fungi. High humidity and warmth foster hundreds, if not thousands, of species of bacteria and moulds, which break down the vast array of chemicals present in plant matter, including toxins.
As the microorganisms break down the organic matter, they release heat as a byproduct. This heat speeds up the composting process, reducing the time needed for the chemical reactions and allowing the microorganisms to build up huge numbers rapidly. This acceleration also speeds up the consumption of oxygen, so a reliable oxygen supply is essential if the composting is not to be derailed. Regular turning of windrows or the injection of air is vital.
The heat is also critical in killing weed seeds. Properly composted camphor laurel mulch will contain fewer seeds that can germinate.
SESL has three decades’ experience in composting, and Simon Leake has helped create and test the Australian composting standards. SESL staff will be happy to advise you on composting chipped plant material – including camphor laurel – or you can speak to your preferred laboratory.