By Kelly Miller
It may come as a surprise some gardeners that many popular trees and ornamental plants are highly flammable. In fact, quite a few plant species native to regions prone to wildfire actually depend on occasional fires to support the germination of their seeds or reduce competition from other plants. These plants evolved with fire and they produce oils, resins, and waxy substances that exploit fire to their benefit.
In the event of a wildfire, major property damage is more likely for homes that are surrounded by plants that can easily catch fire. Flammable plants are also fire hazards if they’re too close to yard activities like fire pits or grills or used to create privacy screens that surround a wooden deck or a patio next to the house.
It’s unfortunate that some of the beautiful native plants that are best adapted to landscapes in high fire-risk regions, are also the same plants that can make fires worse. But when it comes to fire risk, you can never be too safe. In regions where wildfires commonly occur, flammable plants must be used with caution when designing home landscapes.
In the sections below, we’ll explore some of the flammable trees and plants you should be cautious about using when landscaping in fire-prone areas. We’ll also identify some fire-resistant plants to help you create a fire-smart landscape.
What Makes a Plant Flammable?
Some plants contain volatile organic compounds, such as terpenes, which are highly flammable. (Volatile compounds are substances that turn to vapor at relatively low temperatures.) Often, the very purpose of these volatile oils or resins is to interact with fire. By encouraging a hotter fire, these flammable plants may improve the reproductive success of their species. Fire may be needed to release seeds from a cone, break down hard seed coats to allow water absorption, or destroy understory brush to clear space for seed germination and the growth of seedlings.
Popular trees that contain terpenes include pines, cedars, spruce, and eucalyptus trees. Terpenes, like many other volatile compounds, have a variety of commercial uses, and they can be found in an array of products such as turpentine, surface and food preservatives, perfumes, and cleaning products.
Terpenes also have anti-inflammatory, anti-microbial, and anti-fungal properties that make them useful for a wide range of pharmaceutical applications from topical ointments to cancer treatments.
Other plants, junipers for example, may contain volatile organic compounds similar to terpene, that also make them more flammable. These are the very same compounds that are found in plant essential oils.
More About Plant Essential Oils
Many plants contain essential oils that impart flavors and fragrances we enjoy, but those same oils instill these plants with a higher degree of flammability. This would include most herbs, citrus trees, cinnamon, and bergamot just to name a few.
Essential oils may be found in various parts of a plant; the leaves, seeds, fruit peels, flowers or roots, but the potency found naturally in plants is lower than that of concentrated essential oils that you might purchase for crafting fragrant soaps or candles, using in skin care products, vaporizing in a diffuser, or for cooking. (Note that if you do have bottles of essential oils in your home, you’ll want to be sure to store them appropriately to avoid creating a fire hazard.)
While you should have little worry that your herb garden is a fire hazard, the higher potential for combustibility of these plants is worth mentioning. Under severe drought conditions if herb plants dry out and the oil concentrations increase due to the lack of water, they may become somewhat more flammable as dried plant material.
Highly Flammable Trees and Shrubs to Avoid in Fire-Prone Areas
These types of trees and shrubs tend to be highly flammable:
- Trees with gummy sap or waxy leaves.
- Trees that retain dead leaves on branches.
- Trees that drop large amounts of leaves or needles.
- Fine-needled evergreen trees and shrubs.
- Trees with papery, peeling bark.
- Conifers and other softwood trees (compared to hardwood trees).
The tree and shrub species listed below are very popular for landscaping, but highly flammable and should be planted with caution in fire-prone areas:
- Eucalyptus
- Leyland cypress and Italian cypress
- Juniper
- Arborvitae
- Cedars
- Birches
- Firs, spruces, and pines
Use these trees and shrubs sparingly in fire-prone areas. Avoid clustering these species and don’t plant them within 30 feet (9 m) of structures. Additionally, check your local municipal code to see if certain species or landscaping arrangements are prohibited for fire safety purposes.
Highly Flammable Plants and Grasses
Along with trees, certain grasses and other plants can significantly contribute to a property’s fire risk.
Plants that ignite easily and add large amounts of fuel to fires include:
- Tall, dry grass.
- Invasive grasses and weeds that spread easily and tolerate heat and drought.
- Plants with excessive amounts of dead material due to lack of pruning.
- Plants that have become extremely dry due to drought or lack of watering.
The plants and grasses listed below are especially flammable and should be avoided in a fire-safe landscape:
- Bamboo
- Rosemary
- Black sage
- Buffalo grass, Guinea grass, molasses grass, pampas grass, fountain grass, and feather grasses
In areas that are prone to both drought and wildfire, tall, dry grass is a major concern. In this condition, a field of grass can quickly become engulfed in flames.
To minimize fire risk, only plant grass, groundcover, and other plants that you’re able to keep sufficiently watered. Don’t let invasive grasses take over large areas, and consider replacing grass with fire-resistant native groundcovers or nonflammable hardscaping. Succulent plants, such as sedum or hardy ice plant are also good choices for their naturally high moisture content and showy flowers.
Which Trees and Plants Are Fire-Resistant?
No plant is fireproof. But, some plants provide more fire resistance than others. Compared to highly flammable plants, fire-resistant plants are harder to ignite and provide less fuel. Traits like dense trunks, thick bark, and moist leaves contribute to fire resistance. Plants and trees with good fire resistance include:
- Deciduous hardwood trees like ash, dogwood, and crabapple.
- North American native groundcovers like stonecrop, creeping phlox, and sedum.
- Cacti and succulents.
For recommended species lists, see this guide to fire-resistant trees from Oregon State University, and this one on fire-resistant groundcovers from Colorado State University. If you live in an area prone to wildfires you can also contact your local fire department to see if they have recommendations on plants to avoid. If you are moving into a new home, you may want to consider removing any potentially flammable trees if your house is in a high fire-risk region.
Remember that species selection is just one part of fire-smart landscaping. Along with choosing the right trees and plants, perform these maintenance activities to reduce your property’s fire risk:
- Around your home, establish defensible space—a buffer zone free of flammable materials.
- Limit the use of dense plantings, such as shrub privacy screens.
- Keep grass, trees, and plants sufficiently watered.
- Keep trees and shrubs properly pruned.
- Remove dead plant material from the landscape.
- Follow local fire safety code requirements.
Designing a Fire-Smart Landscape
In areas with high wildfire risks, fire-smart landscaping is a must. Learn more in our guide to Landscaping for Wildfire Protection.
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