When Invasive Plants Take Over
Invasive plants do not simply become part of the scenery. When they are allowed to dominate a landscape, they fundamentally change how that landscape functions.
The Santa Clara River shows what that can look like.
RIP Team Lead Jordan Corral has worked with researcher Tom Dudley and regional partners on Arundo management in the Upper Santa Clara River watershed, helping identify priority infestations and working with private property owners to secure right of entry so crews could reach areas that otherwise could not be treated.
By the time large scale restoration efforts began across the watershed, Arundo had already transformed immense portions of the river. Dense stands displaced native willow and cottonwood habitat and created monocultures. After the 2003 Simi/Verdale Fire burned through riparian habitat along the Santa Clara River, researchers found that Arundo rapidly resprouted and accounted for 99 percent of the vegetation cover at monitored sites within the first year after the fire. National Assessment of Shoreline
And the consequences extend far beyond which plants are growing there.
It takes water
Arundo is extraordinarily water intensive. Santa Clara River restoration planning has found that it can use approximately three to four times as much water as native riparian vegetation through evapotranspiration. Large infestations reduce water available in the river, soil and groundwater. Removing Arundo is expected to increase water availability and improve streamflow for native plants and aquatic wildlife. Santa Clara River Conservancy
RIP Team Lead Jordan Corral identified a priority Arundo site along the Upper Santa Clara River, secured right of entry from a private landowner so the site could be accessed, and then helped install and monitor sensors measuring water use in Arundo and native plants.
That work is being carried out with researcher Tom Dudley and regional partners as part of a broader effort to better understand Arundo’s impact on the watershed.
Earlier planning estimated that replacing one acre of Arundo with native vegetation could save roughly 2.6 million gallons of water per year.
For Southern California, where water is already a limited resource, allowing acres of invasive vegetation to consume substantially more water than the native ecosystem is not a trivial consequence.
It replaces wildlife habitat
A wall of green is not necessarily a healthy ecosystem.
Arundo replaces diverse willow, cottonwood and other native riparian vegetation with dense stands that provide far less habitat value. Santa Clara River restoration research has documented reductions in arthropod diversity and abundance and declines in bird diversity associated with Arundo dominated habitat. The river supports sensitive wildlife including least Bell's vireo, southwestern willow flycatcher, western yellow billed cuckoo, southwestern pond turtle and other species that depend on functioning riparian habitat. Santa Clara River Conservancy
When the native plants disappear, much of the food, shelter and breeding habitat that evolved around them disappears too.
It changes water quality and the river itself
Native riparian vegetation helps shade waterways, stabilize soil, filter sediment and nutrients and support healthy aquatic conditions. When invasive plants replace that vegetation, those functions are diminished.
Santa Clara River conservation planning identifies invasive plants as a threat to both water quantity and water quality. Loss of native canopy increases water temperatures and reduces dissolved oxygen, while Arundo can alter water flow, contribute to debris accumulation and increase erosion and instability during floods. CERC
It turns fire breaks into fire fuel
Healthy Southern California riparian vegetation can act as a relatively moist break in the landscape. Arundo changes that relationship. It produces large quantities of dry combustible material and recovers extremely quickly after burning.
Santa Clara River restoration planners describe Arundo as a fire promoting species capable of transforming riparian areas into more fire prone systems. Fire damages native trees while Arundo rapidly resprouts from underground rhizomes, allowing it to gain even more ground after each disturbance. Santa Clara River Conservancy
That is the cycle we are trying to interrupt in the Palisades.
Nearly two years after the Palisades Fire, we are no longer wondering whether invasive plants will exploit the disturbed landscape. They already have. We are seeing dense castor bean, Arundo, pampas grass, black mustard, fennel and other invasive vegetation establishing across recovering areas.
The question now is how much ground we allow them to take.
Giving up would not preserve the landscape we see today. It would allow invasive populations to mature, reproduce and spread into new areas, making them progressively harder and more expensive to control.
The Santa Clara River shows what that trajectory can become. What starts as a costly but manageable restoration problem can grow into a watershed scale effort requiring researchers, public agencies, nonprofits, private landowners, specialized crews, years of follow up and millions of dollars in investment.
Walking away does not make the cost disappear. It turns an expensive project into a much larger, more complex and potentially unmanageable problem.
We do not want to spend the next several decades trying to reclaim the Palisades from invasive plants that we had an opportunity to control now.
Giving up would not preserve the landscape we see today. It would allow invasive populations to mature, reproduce, spread and become increasingly difficult to control. Native plants would face greater competition. More invasive seed and plant material would move through canyons and waterways. The cost and complexity of restoring those areas would continue to rise.
A dense stand of invasive Arundo along Topanga Canyon Road in 2023, before the Palisades Fire, showing how established infestations can dominate disturbed corridors long before a wildfire occurs.
California Invasive Plant Council found that by 2011, about $71 million had already been spent controlling Arundo in central and Southern California, and estimated another $124 million would be needed to control the remaining mapped infestation. The report specifically concluded that early control is more cost effective.
The current Upper Los Angeles River Watershed Arundo Eradication Programis budgeted at $12.36 million to address roughly 80 acres across 72 stream miles.