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Fire Safety at the Wildland/Urban Interface

Use Fire-Resistant Plants

No plant is fire proof, but some plants are less flammable than others. If you must use really flammable plantings in your landscape, plant them farther from the house, don’t group or isolate them from fire by using hardscapes.

There are lists of plants assorted by their flammability. Some of this knowledge is amassed by experience and common sense, but a good deal of Cooperative-Extension-based research has gone into determining the flammability of certain plants. At the end of this chapter there is a large list of plants; however, no list can hope to keep ahead of the number of plants available or newly introduced into the nursery trade. When you are going to select a plant for your landscape, remember what you know about fuels and fire behavior from the beginning part of this chapter,and try to evaluate a plant selection based on what you know about flammability. The things that influence a plant’s flammability are:

      • Moisture content

      • Leaf and twig shape

      • Presence of oils or resins

      • Branching pattern

      • Deciduous or evergreen

• Retention of dead material on the plant.

    • Moisture Content

    The moisture content of the leaves and plant have a considerable influence on the ability of a fire to ignite the fuel in the plant. The more moisture in the plant, the more energy must be expended to drive off the moisture before the leaves can ignite. This moisture is dependent on the nature of the plant to a certain extent. Succulents have more moisture in their leaves than grasses or pines.

    Equally important, however, is the moisture that has been available to the plant in the recent past. Fire seasons are fire seasons because the fuel that is available is dryer. Fire season is usually one of the drier seasons of the year when most plant moisture is below ground. Any season can be drier than normal, however. After an extended drought, even leafed-out deciduous trees in summer can be as dry as they are in winter. This is one reason to be aware of your surroundings when you live at the Wildland/Urban Interface. Choosing plants that are less prone to drought stress is one approach, while another is to minimize the use of plants that are prone to dryness, or even to eliminate them from the Defensible Space of your landscape altogether. Masses of dried warm season grasses are stunning in the landscape and very important areas for wildlife; however, they shouldn’t be the main planting on that steep slope up to your deck, or allowed to run as a meadow within your Defensible Space perimeter.

    • Leaf and Twig Shape

The finer the cut on the leaf, the more flammable it is likely to be. That is one of the reasons pine needles tend to be more flammable than round leaves like on dogwoods. Finer, divided leaves have a much larger surface to volume ratio, so just like smaller pieces of food, they will heat up rapidly, allow the moisture to be cooked off, and ignite.

    Minimize the contact of your fine leaved plants with your structures and the outside of your protected space perimeter.

    • Presence of Oils or Resins

    It should not be a surprise to you that pine trees, cedars, and the like are filled with flammable resins. They are particularly susceptible to ignition. It would be a good idea to keep them apart from plantings that could be considered ladder fuels, and also from steep slopes.

    Many other plants such as rhododendrons, laurels, etc., also have high levels of resins and other flammable compounds.

    • Branching Pattern

    The branching pattern of the plants can effect a whole plant’s flammability. Just like finely cut leaves, the finer the branches, the easier it is to drive off the moisture and reach the ignition point.

    • Deciduous vs. Evergreen

    Deciduous trees shed their leaves each year. This build up of dry leaves on the ground can lead to surface fires when they are dry. In the summer the leaves are still on the trees and filled with moisture. Much higher temperatures or prolonged contact with a flame are required to cause deciduous trees to ignite during summer months.

    Evergreen trees often contain more plant resins to enable them to withstand freezing temperatures. The resins are more flammable, and can ignite at lower temperatures.

    • Retention of Dead Material on the Plant

    If a plant retains its leaves throughout the year, or if it only slowly drops off dead parts (like palmettos or palm trees, American Beech, and some oaks like Scarlet Oak), this dead material is quite flammable. It is quite easy for a ground fire to ignite the dead material on a plant and then cause the fire to become a crown fire. If you have these types of plants in your landscape it is best to frequently remove and maintain the dead materials.

    Standing warm season grasses are quite flammable when they are dry in the winter. The tall, standing dead leaves are striking and normally the reason why people plant these grasses. They are also important as habitat for a variety of animals. It is best to use these types of plants at the edges of your Defensible Space, or to make certain that they are isolated by hardscape from where a fire could approach.

    • Plant Lists

The following plant lists in Tables 7.2 and 7.3 were developed for Virginia. There are lists of FireWise Planting selections for almost every state. Ask your Cooperative Extension Agent about the lists, or check with your state Department of Forestry.

 

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Last modified: 09/10/13