- Author: Luca Carmignani
How can weed control help with wildfire preparedness?
Wildfires are part of California's ecosystems, and they do not have to lead to the destruction of structures and livelihoods. Each of us can contribute to improving wildfire resilience, from individual homeowners and businesses to entire communities. Managing the vegetation and landscape around our homes can play a crucial role in preventing the spread of fires and sources of ignition.
Given the large amount of rain in the winter of 2022-2023, you might have experienced a surge in annual grasses and fast-growing plants that cover most of the ground around your home and community. In my area, I observed invasive species like wild oats and mustard growing rapidly in the spring, then drying out as summer approached (Figure 1). Dry vegetation poses a major threat to our homes and communities, both in terms of ignition (possibility of starting a fire) and fire path (creating ways for a fire to spread).
Understand fire risks.
Fires require fuel to spread, and any combustible materials, including vegetation, wooden fences, or sheds can serve as fuel. Once ignited, these materials can create a direct fire path toward a residence. Fires also generate embers (small fuel brands transported by the fire plume or wind) that can ignite leaves or debris on roofs and gutters or penetrate directly into a building through vents. Embers can also accumulate near the house, especially within the first 5 feet. You can reduce your home's exposure to flames and embers by implementing defensible space and home hardening strategies. Creating a defensible space involves managing the landscape around buildings (such as houses, sheds, detached garages) to prevent fires from reaching them. Home hardening focuses on improving building components, such as vents, roofs, and gutters, that could reduce exposure to flame and ember ignition. Though it can seem overwhelming to figure out where to begin reducing your home's fire risk, in this article I will walk you through a few simple, cost-effective recommendations that have been shown to make a difference.
Fires can start from weeds.
How do we build a fire in a firepit? We start by adding the smaller kindling, and then the larger pieces of wood. Why? Because twigs and small branches are easy to ignite, they burn quickly, and they can be used to ignite larger logs. Similarly, dry grasses and herbaceous plants are easier to ignite than other types of vegetation such as big shrubs or trees.
Weeds can be ignited directly by flames, or by embers and sparks landing nearby. A mower hitting a rock or sparks from a power tool can easily ignite dry grasses around your property. Using the firepit analogy, ground fuels such as weeds serve as kindling to spread the fire to larger fuels nearby such as fences, decks, and shrubs. Therefore, removing weeds from vulnerable locations, such as near fences (Figure 2), is a very effective way to prevent ignitions around homes and communities, and reduce potential fire paths. It's easier to remove weeds while they are still green. This reduces the risk of ignition caused by mowers and prevents invasive species from reaching maturity and producing viable seeds.
Weeds and fire paths.
In addition to being easy to ignite, dry grass and herbaceous plants can also create fuel continuity. Fuel continuity, or fire path, refers to the way a fire could spread toward a building. There can be both horizontal and vertical paths (Figure 3). Weeds can provide horizontal continuity between shrubs or other combustible materials, increasing the intensity of a fire and bringing it closer to the house. Thus, it is important to create horizontal separations between groups of plants when maintaining vegetation. When burning grass ignites a fence, the fire “climbs up” from the ground, and if the fence is attached to the house, the fire can continue to climb. A fire can reach a building by using this vertical path, often called a “fuel ladder.” The risk of fire spreading to your house can be significantly reduced by removing these potential fire paths, starting with ground fuels like annual grass. However, other sources of ignition, such as embers, may create additional paths. It is therefore crucial to harden house components like vents (for example, by replacing their screens with a metal mesh of 1/8” or smaller) and keep your roof and gutters clean.
What can you do?
Maintaining the landscape and vegetation around your home and community is crucial to preventing losses during a wildfire. Prioritize your actions to reduce the risk of ignition and fire spread around your home starting from the building and working outwards. Below are some recommended actions for creating and maintaining a fire-resilient landscape:
- Remove annual weeds and litter from vulnerable locations such as fences, sheds, siding, and under decks.
- When mowing or removing grass, be careful of sparks from power tools or other machinery, especially near open areas. Make sure you have access to water in case of a fire emergency.
- Break horizontal and vertical fire paths by removing weeds and other vegetation that are easy to ignite (grass, dead twigs, and dry leaves).
- Prune lower branches of shrubs closer to the ground and clean their understory; trim lower limbs of trees that are close to other plants or buildings.
- Mulch can be effective for weed control, but it is also flammable. Do not place mulch in vulnerable locations within the first 5 feet around a structure.
- A fence creates a direct path for fires. If your fence is attached to your house, replace the last 5 feet with a noncombustible section or gate.
- Install metallic 1/8" mesh screens on vents to prevent ember entry.
- Regularly clean roofs and gutters, especially near roof intersections.
- Maintain your landscape throughout the year.
Preventing the ignition of your home during a wildfire is possible, but it requires a combination of home hardening and defensible space strategies. For more information related to wildfire preparedness, check the additional resources below:
- UC ANR Fire website: https://ucanr.edu/sites/fire/
- Reducing the vulnerability of buildings to wildfire: Vegetation and Landscaping Guidance: https://anrcatalog.ucanr.edu/Details.aspx?itemNo=8695#FullDescription
- Wildfire home retrofit guide: https://www.readyforwildfire.org/wp-content/uploads/Wildfire_Home_Retrfit_Guide-1.26.21.pdf
- Combustibility of landscaping mulches: https://naes.agnt.unr.edu/PMS/Pubs/1510_2011_95.pdf
- Landscaping and home hardening: https://defensiblespace.org/
- Author: Help Desk Team
A poet once said, “A weed is but an unloved flower.” Sometimes though, a weed, an insect, or a four-legged critter can become a dangerous pest. In a state like California where so much of our economy is agriculturally based, these pests can wreak havoc. In our home gardens they threaten our landscape and ornamental plants and make the creation of natural areas a significant challenge when they displace native plants and wildlife.
Exotic and Invasive
California's native ecosystems were uniquely adapted to our Mediterranean climate, with its dry summers and wet winters. However, as the population changed and grew with immigration alongside increased international travel and commerce, new species of plants, many bringing insects and pathogens with them, were imported from Asia and Europe (often inadvertently) and introduced into the landscape. These exotic plants sometimes failed and sometimes flourished. Sometimes we move them unwittingly from state to state as we travel. The result is that some exotics have become invasive, spreading through the native ecosystem.
You might recognize some of these pests. The pathogen that causes sudden oak death was accidentally introduced on nursery stock and is estimated to have killed more than 1 million oak and tanoak trees over the last decade. In addition to disease, invasive plants can change the composition of soil as scotch broom does by adding nitrogen to the soil, or outcompete shallow rooted native species during dry summer months as the star thistle does with its deep root system.
Of special concern currently is the Asian Citrus Psyllid (Diaphorina citri), a tiny insect that attacks all varieties of citrus, and is a vector for the bacterium that causes Huanglongbing (HLB) disease. An infestation can spread quickly and there is no cure for HLB. Although the psyllid is rarely seen in Northern California, it has become a serious problem in Southern California where it arrived from Mexico in 2008 and is slowly spreading north. The USDA notes that HLB “has devastated millions of acres of citrus production around the world, including in the United States.” http://ipm.ucanr.edu/PMG/PESTNOTES/pn74155.html
Steps to Take to Stop the Spread
We can all have an impact on the spread of invasive species into our ecosystem.
1. Become familiar with invasive pests, how to identify them, and where to find information. The UC IPM website is a good source of information about managing exotic and invasive pests. https://ipm.ucanr.edu/Invasive-and-Exotic-Pests/
2. When you see suspicious organisms, get help identifying them. Contact your local UC Master Gardener Program Help Desk or Agricultural Commission to report invasive species and to get help with managing them. https://www.cdfa.ca.gov/plant/
3. Inspect new plants carefully before planting them. When possible, plant native species — they are better adapted to our climate, support butterflies and other pollinators, and are less likely to have pest problems! https://plantright.org/
4. Don't bring plants into California from outside the state, and don't purchase invasive plant species. This includes planting gifts from friends across the country into your garden, and ordering online from nurseries that are outside of California. https://www.cal-ipc.org/
5. Buy your firewood where you burn it. Many pest insects and pathogens move with firewood. Don't move it far from its source. http://www.firewood.ca.gov/
To learn more about the invasive species prevalent in California, their impact and how to address them, the UC IPM website is a wealth of important information. We can all make a difference in protecting our beautiful state.
Help Desk of the UC Master Gardeners of Contra Costa County (RDH)
- Author: Mackenzie Patton
The Invasive Pest Spotlight focuses on relevant or emerging invasive species in California. In this issue we are covering brooms, a group of invasive shrubs.
Invasive Broom facts
Brooms are upright shrubs in the legume family that typically produce small, yellow, pea-shaped flowers. Shrubs range from 3 to 10 feet tall. They produce flowers from mid spring to summer and produce seed pods in late summer. All brooms are prolific seed producers, with a single shrub producing as many as 2,000 to 3,500 pods containing up to 20,000 seeds.
While brooms are attractive plants, they grow in dense stands that outcompete many native plants. These dense stands are highly flammable and increase the risk of wildfires. The most common species found in California are Scotch broom, French broom, Spanish broom, and Portuguese broom. Scotch broom is often found on interior mountains and on lower slopes in Northern California and is very prevalent in the Sierra Nevada foothills. Without management, these plants can survive for about 12 to 17 years, producing thousands of seeds.
Actions you Can Take
First, avoid planting any broom species. While most retailers do not sell the most common invasive brooms, many do sell hybrids that could become highly invasive in the natural landscape. There are similar-looking alternatives to brooms, such as forsythia and golden currant. Contact your local UC Master Gardeners or visit PlantRight.org for a list of other alternatives.
If you have brooms on your property and want to remove them, there are many different nonchemical and chemical methods that are effective in controlling these plants. These options are extensively detailed in the UC IPM publication Pest Notes: Brooms.
- Author: Ben Faber
I recently went looking for our copies of Walter Ebeling's book on pests of subtropical tree crops. It was created over 40 years ago, but is still a great reference for past pests and those that still pop up with less frequency than avocado thrips and persea mite do today. But both copies have disappeared from the office. and we have a new entomology advisor - Hamutahl Cohen - who is coming up to speed on the pest issues of our coastal area.
But all is not lost. It's online. This is the section of Subtropical Fruit Pests by Walter Ebeling that covers avocado pests in not only California, but what was and is known to exist in other avocado growing regions around the US and the world. It was reproduced at the Hofshi Foundation's Avocadosource website. At this point it only contains the chapters pertaining to avocado. Other chapters in the full text cover citrus, grape, walnut, almond pecan, olive, fig, date and other "Minor Subtropical Fruits". The beauty of the book is not only historical, but that it is still current (although the DDT recommendations are out of date) for many pests. It also chronicles pests that have appeared in the past, disappeared and then reappeared. An example is Avocado Bud Mite - here, gone, here and seemingly gone again, probably to reappear sometime in the future. This is no replacement for the UC-IPM website, http://ipm.ucanr.edu/PMG/crops-agriculture.html , but it is a good look at how the pest has been managed in the past and is done so currently. http://www.avocadosource.com/papers/research_articles/ebelingwalter1959b.pdf
I thought I had the only copies of this book in Ventura County, but you could too. There are some listed on ABE Books for cheap, which is where I got our replacement. At one point the University of California made hard copies of all manner of valuable material available for cheap or free. It still does, but much is online now. Check out the selections: https://anrcatalog.ucanr.edu/
This pestiferous book was compiled by Walter Ebeling at UC Riverside/Los Angeles. He was of some note, considered the Father of Urban Entomology. As you can see from the descriptions of avocado pests, he was a good all round entomologist, as well. Urban entomology really forces you to know a lot because of the diversity of arthropods in urban settings. He passed in 2010 and was recognized world-wide for his work.
They may seem too tiny to do much damage to a mature, healthy tree, but invasive shothole borers (ISHB) are responsible for the deaths of tens of thousands of trees in Southern California. These beetles bore into trees and infect them with Fusarium dieback, a fungal disease that kills the trees. Many native California trees like California sycamore, valley oak, and arroyo willow can be killed when invasive shothole borers attack them.
While these pests are currently only found in Southern California, they could spread to many other parts of the state. Limiting the infestation will reduce their impact. Controlling the beetles is difficult but includes regular monitoring of trees to quickly identify sources of beetles, disposing of infested cut wood, and appropriate pesticide treatments.
What can you do to help?
- Don't move firewood around the state. These beetles and other potentially damaging beetles are easily moved on cut wood. Buy it where you burn it.
- Learn more about host trees, symptoms of infestation, and what to do.
UC IPM's new publication, Pest Notes: Invasive Shothole Borers is written by various state experts on this pest and contains everything you need to know about the beetle. Visit the UC IPM website for specific management recommendations, identification of the beetles, and lists of trees affected.