Breaking up fuel continuity and fuel ladders

Defensible space is that area between a house and an oncoming wildfire where the vegetation has been modified to reduce the wildfire threat and provide an opportunity for firefighters to safely defend the house.

Are you concerned about the fire risk in your area (see I-Zone article page 1)? The arrangement of fuels is critical to a defensible space and is a key to making sound defensible landscaping decisions.

“Fuels” are trees, shrubs, leaf litter, houses, and other flammable materials. Fire requires fuel to burn so where the fuels are reduced, fire intensity will be reduced accordingly. The more continuous the fuel layer, the more rapidly a fire can spread and the greater the potential for high intensity fires.

Shrubs in the defensible space should be planted in (or thinned into) clumps, or islands, with open space between each. Some landscapes are designed with areas of walkways, driveways, patios, and other “hardscaping” to provide aesthetically pleasing fuel breaks.

In addition to horizontal continuity, vertical fuel ladders are another concern. Fuel ladders are created by layers of flammable material that allow a fire to move from the ground to the tree canopy. For example, pine needles on the ground can ignite and burn shrubs which in turn ignite tree limbs or leaves. Fuel ladders are a major cause of high intensity crown fires.

Fuel ladders can often be removed by pruning branches of trees or removing shrubs. A rule of thumb is that, within the defensible space area, vertical separation between fuel layers should be at least three times the height of the lower fuel layer. Following that formula, a 4' shrub growing next to an incense cedar should be separated by 12 vertical feet (4' x 3'). This could be accomplished by removing the lower tree branches, reducing the height of the shrub, or both.

Trees planted in defensible spaces should be far enough apart that a fire cannot travel across tree canopies. This separation is especially important around structures like houses.