Plant Flammability

Fire is generally sustained through sufficient oxygen, heat and fuel following an ignition. Different fuels have varying degrees of ignitability, combustibility, sustainability and consumability, all critical components of their overall flammability.

The wildland and urban environments contain a range of potential fuels. An important question in our research focuses on the flammability of different plant species commonly found in our local vegetated areas as well as those utilized by homeowners in and around fire hazard severity zones.

Assessment of flammability involved placing a plant sample on a wire mesh just above hot plates to test the time taken to ignite the vegetation in a controlled space. Over time, the heat causes the plant to release flammable vapors, which can catch fire if they come into contact with a pilot flame. Temperature and heat are measured at different points in the chamber. Videos are taken to document:

• Time taken for the plant to ignite
• How long the flame lasts
• Height of flame

Plant characteristics like live fuel moisture, stem thickness, number of branches, and more are measured and related to flammability outcomes.

A goal of the research is also to quantify and qualify plant desirability in Santa Barbara County, making use of the flammability triangle graph to break down fire risk into three factors:

1. Ease for plant to catch fire
2. Duration of burn
3. Likelihood of sustained fire

Thicker and longer arrows in the graph indicate higher scores in these areas for each species.

Flammability Components

All three sage species and herbaceous bush mallow ranked highest for ignitability while lemonade berry shrubs ranked lowest. Shrub species tend to be large and woody while tree species are often small and dense. Compact, leathery, waxy leaves are drought-resistant characteristics common across shrubs and trees. Chamise stood out with the highest combustibility score, followed by coyote brush and the toyon, coastal live oak, and California bay trees. Herbaceous species ranked lower for combustibility. The three sage species also scored the lowest for sustainability for combustion while eucalyptus demonstrated the highest sustainability score.

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Overall Flammability

Despite having a light lemony scent, which might indicate the presence of volatile oils, the lemonade berry ranked very low for flammability. Throughout sampling, including towards the end of the summer dry season, this species was observed to leak watery sap from its thick, succulent-like stems when cut. The lemonade berry’s high distribution of live fuel moisture and well-hydrated stems allowed it to resist ignition and provide little material for combustion.

References

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