High Moisture Content
Plants with high moisture content are usually more difficult to ignite and they burn slower. Green, healthy, and actively growing herbaceous (grasses and forbs) have a much greater percent moisture content than wood plants (trees and shrubs.) When cured the herbaceous plants possess a much lower percent moisture content that wood plants. Consequently, herbaceous plants, if kept green throughout fire season by irrigation, are usually more desirable than wood plants in the defensible space.
Unfortunately there are NO fire-proof plants. Any plant can burn during extreme fire conditions. There are however, some plants which are harder to ignite, burn slower, produce less heat when burning, and produce shorter flame lengths than other plants.
Fundamental to making the residential landscape an effective defensible space is the proper selection of plant materials. Besides addressing the other needs (providing shade, adding color, controlling erosion, fruit production, etc.) and being adapted to local growing conditions, plants used in high wildfire hazard areas should possess desirable fuel characteristics.
Plant Materials for Defensible Space
Low Growing Habit
Plants which are low-growing usually produce shorter flame lengths and have less fuel volume than tall plants. Select plants that grow to a height of less than 18 inches at maturity, or can be maintained at this height by pruning.
Low Fuel Volume
There is variability between plant species regarding the amount of fuel produced. Select plants which produce relatively small amounts of vegetation, especially plant parts that are less than one-half inch in diameter.
Desirable Chemical Content
Avoid selecting plants with resinous, oily, or waxy plant parts. These characteristics are often associated with plants which possess an undesirable chemical content that increases their flammability.
Maintain Your Landscape
The manner in which plants are maintained is as important as the species of plants selected. For example, plants which are considered to be low fire hazard can become high hazard plants without proper care (irrigation, removal of dead branches and leaves, etc.) Likewise, the risk of high hazard plants can be reduced through maintenance practices.
Plants which are high in resins and oils are extremely flammable, and should be avoided in defensible space landscaping. Examples of pyrophytes include pampas grass and junipers. Also avoid plants which are listed as noxious weeds.
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