Fire Resistant Trees & Shrubs

Species selection and maintenance practices are key to developing a firewise landscape. Although all plants are flammable under the right conditions, the following practices may decrease their flammability and reduce their accessibility to fire:

Select species and varieties that are fire resistant:

• Plants that are well-adapted to the local climate, microclimate, aspect, slope and local environmental conditions.
• Plants with low fuel volumes: low growing, limited spread, and “clean”
• Plants that are deep-rooted and proficient at water uptake
• Plants with relatively fire resistant foliage:
  • deciduous trees and shrubs.
  • trees and shrubs with large, fleshy leaves.
  • trees and shrubs lacking volatile chemicals, oils, waxes, etc.
    

Increase fuel moisture:

• Place trees and shrubs near natural water sources, e.g., moist soils, seeps, springs, ponds, streams, seasonal or ephemeral drainages, etc.
• Cultivate, amend and mulch the soil surface surrounding plants to enhance water penetration, retention/storage, reduce evaporation and to prevent compaction. Use permeable materials for drives, walks and patios.
• Place trees and shrubs in or adjacent to areas that are currently irrigated.
• Deep irrigate trees and shrubs every 20 to 30 days during the fire season.
• Maintain plant health and vigor:
• Avoid crowding and over-competition for available soil, water and nutrients.
• Remove unhealthy, dying or dead plants, and prune out dead branches and foliage.
• Treat pests and diseases promptly and monitor for recurrence.
• Fertilize and irrigate plants as needed to maintain vigor.

Disrupt the horizontal and vertical continuity of shrub and tree fuels:

Separate shrubs and shrub islands by a distance of no less than two times their height. Limit island groupings to 18 feet diameter. Thin shrub cover to less than 1/3 of the area.

Place only fire resistant, low growing (less than 18") shrubs under trees.

Raise tree crowns to at least 10 feet above grade or to a maximum of 1/3 of their height.

Privacy screens. Although aesthetically desirable, they can be hazardous in four ways:

1. They interrupt the flow of wind, forming a partial vacuum, causing turbulence on the leeward side that draws “firebrands” down onto homes and decks.
2. They provide a highly dense fuel cluster, often with much deadwood from crowding and shading of internal and lower branches.
3. They typically are composed of species that maintain their lower branches which form a ground-to-crown, vertical “ladder fuel” architecture.
4. They are often composed of highly flammable (pyrophytic) plants.

Staggered planting provides screening while maintaining plant spacing

Trees

• When thinning out trees, remove dying and dead trees first. Then thin out or “clean up” trees with excessive deadwood. Next, thin highly flammable species (needle leaf and blade leaf trees with volatile leaves.)
• Tree crowns should be separated by at least ten feet. Add five additional feet for every ten percent increase in slope (10 feet of separation on slopes 0 to 10%, 15 feet of separation on slopes from 11 to 20%, 20 feet on slopes from 21 to 30%, and so forth).
• Raise all tree crowns at least ten feet above soil grade.
• Thin tree crowns (up to 25%) to reduce total fuels.
• Remove dead branches and large areas of dead foliage, all vines and loose, papery bark.
• Remove flammable undergrowth and woody debris.

Shrubs

• Separate individual shrub crowns by two times their height, or group shrubs into islands less than 18 feet in diameter, and separate the islands by a distance of no less than two times their canopy height.
• When thinning brushy areas, remove dead, dying or stressed shrubs first, then the highly flammable shrubs, e.g., highly twiggy shrubs, shrubs with small woody leaves, shrubs with volatile oils (smell them!). The shrub cover should not exceed 30 percent of your defensible space landscape.
• Remove dead, declining or diseased branches. The maximum dead to live ratio is 20 percent.
• Limb-up shrubs (raise the skirt), but no greater than one-third their height.
• Remove all vines, papery bark or other debris in the crown.
• Remove or mow undergrowth to 3", and remove all loose, woody debris.

--Ray Moritz is a fire ecologist and urban forester in Marin County.