Choose mulches carefully

Mulches play an important role in northern Nevada landscapes. They can reduce the water requirements of plants, cool soil temperatures, reduce the occurrence of weeds, control soil erosion and dust, and visually enhance the landscape. Unfortunately, some popular mulches are also combustible, a bad characteristic for residential landscapes located in high-fire-hazard areas.

Organic mulches are mulches made from plant materials. Examples include pine needles, wheat straw, pine bark nuggets of a various sizes, shredded western red cedar and redwood sometimes referred to as “gorilla hair,” and wood chips from recycled pallets or wildfire fuel-reduction projects. These materials vary considerably in terms of size, shape and texture, which also influence their flammability.

Pine needle and straw mulches pose the greatest fire hazard. They are easily ignited, burn fast and produce considerable heat. Wood mulches that are compact, arranged so that there is very little airspace between the mulch particles, tend to smolder. An example is small pine bark nuggets. Don’t be fooled by these products, as the absence of flames does not necessarily mean they are safe. Smoldering mulches can generate high temperatures and ignite other materials near them. Investigations of the flammability of shredded wood and bark materials have produced mixed results. This could be attributed to the high variability in shredded mulch products.

Inorganic mulches are derived from nonplant materials and include rock, stone and gravel. Most inorganic mulches are noncombustible. One important exception is ground rubber. Ground rubber, which is often used in playgrounds, burns intensely and can be difficult to extinguish.

Here are some tips on using mulch in landscapes located in high-fire-hazard areas:

  • Do not use organic mulches or ground rubber within 3 feet to 5 feet of the house. This is particularly important for wood-sided houses. During a wildfire, burning embers may accumulate in this area, providing ample sources of ignition for wood and bark mulches.
  • Keep organic mulches and ground rubber at least several feet away from combustible materials, such as wood posts, firewood stacks, decks, stairs, etc.
  • Irrigating organic mulches, such as gorilla hair in a flowerbed, may improve ignition resistance.
  • Do not allow thick layers of pine needles to accumulate within 30 feet of the house.

This summer, University of Nevada Cooperative Extension and University of California Cooperative Extension, in collaboration with the Carson City Fire Department and Nevada Tahoe Conservation District, plan to conduct a local evaluation of the flammability of some of northern Nevada’s popular organic mulch materials. The results of this project will be available on the website, www.livingwithfire.info, this fall.

For more information about good mulch and plant choices for high-fire-hazard areas, go to www.livingwithfire.info, or contact your local University of Nevada Cooperative Extension office.

Ed Smith is a natural resource specialist for University of Nevada Cooperative Extension.