The California Invasive Plant Council (Cal-IPC) defines invasive plants as plants that are not native to an environment, and once introduced, they establish, quickly reproduce and spread, and cause harm to the environment, economy, or human health.
One objective of the OCPW Mitigation program is to restore our County’s natural habitats to self-sustaining native ecosystems; however, if invasive plants are allowed to persist, they can outcompete native species for resources, such as sunlight, water and nutrients. This competition can lead to habitat conversion, reducing the acres of habitat available to support special status wildlife species and important pollinators. Invasive plant species can also contribute to increased fire risk. For example, what may appear as a beautiful yellow blanket of “wildflowers” on a hillside in the spring, is actually a monoculture of mustard that will soon dry up and provide quick burning fuel for a fire in the presence of a spark or lightening strike.
While not exhaustive, the list below contains the “dirty dozen” invasive species that we aggressively target in our restoration sites. Invasive plant species profile and management data are from the most current scientific research data available.[1]
The smart phone app Seek by iNaturalist can be a useful tool to help with in-field species identification. Observations of invasive species should be entered into Calflora so populations can be tracked. Cal-IPC works closely with Calflora to support invasive plant mapping in California, data which is added to CalWeedMapper. CalWeedMapper shows landscape-level distribution of invasive plants and suggests management priorities by region.
Artichoke thistle is a large perennial forb (family Asteraceae) from the Mediterranean region found below 500 m elevations throughout California, except in the Great Basin and Desert Regions. Artichoke thistle prefers disturbed open sites, including grassland, chaparral, coastal scrub, and riparian areas. This thistle is closely related to cultivated artichokes (Cynara scolymus), and the two species hybridize frequently. Artichoke thistle is also sometimes grown as an ornamental plant and is available commercially.
Environmental Impacts: Dense colonies displace desirable vegetation through competition for space and soil moisture, and can exclude wildlife and livestock.
Management: Most of the plant’s large taproot must be removed to avoid resprouting.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Castor bean is a semi-woody, large, perennial shrub (family Euphorbiaceae). Native to the southeastern Mediterranean, eastern Africa, and India, it has escaped cultivation and become a noxious weed in many mild climates, including in California. It has a shrubby habit but can reach the height of a small tree depending on growing conditions. Leaves are large, palmately shaped, initially crimson/maturing to dark green, and glossy, with several pointed lobes and serrated edges.
Environmental Impacts: Plants grow very quickly and can outcompete natives, especially in more disturbed environments. The plant is also a very strong trigger for asthma, and allergies can be severe. Its seeds are a source of oil as well as the highly potent toxin Ricin.
Management: This weed is best addressed when immature prior to flowering and seed production. It is important to remove as much viable seed and root as possible to prevent reestablishment. Under certain conditions, its seed bank may last several years.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Fountain Grass is a coarse tufted perennial grass (family Poaceae). It is cultivated as an ornamental. The red cultivar is sterile and not considered invasive. This plant is widely available in the nursery trade and still appears on lists of plants recommended for drought tolerance. It is a popular landscape plant in Southern California. Its seed is wind-dispersed and can be spread by vehicles, livestock, and humans, especially along transportation corridors from urban landscapes to sensitive habitats. This plant is often mistaken for being desirable in urban ornamental grass plantings. It burns readily and rapidly regenerates after brush fires, often displacing native species.
Environmental Impacts: Fountain grass is well-adapted to fire and can increase in density following a burnout, competing with native species and establishing dense monocultures, especially on windward slopes in difficult terrain. It also increases fuel load and readily contributes to the frequency, intensity, and spread of fire.
Management: If inflorescences are present, they should be cut, bagged, and correctly disposed of. Mowing is not an effective method for fountaingrass control. Manual removal is difficult as the plant readily regenerates from leftover root tissue.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Giant reed is a tall perennial grass (family Poaceae) that typically forms dense stands on disturbed sites, sand dunes, riparian areas, and wetlands. It has invaded central and coastal Southern California river valleys. Giant reed is threatening California’s riparian ecosystems by outcompeting native species, such as willows, reducing the nesting and foraging value of habitat, changing streamflow, and utilizing more water than native vegetation. It is native to northern Africa and western and central Asia.
Environmental Impacts: Giant reed is primarily a problem in riparian corridors. It develops dense fire-prone stands which displace native riparian vegetation, diminish wildlife habitat diversity, and increase flooding and siltation in natural watercourses. Giant reed is also adapted to periodic fire regime. Large stands of giant reed can increase water loss from underground aquifers in semi-arid regions due to a high evapotranspiration rate.
Management: Giant reed spreads by rhizomes and resprouts readily if the rhizome root mass is left intact following mechanical removal. Sections of this plant can raft downstream and root. Targeted control is best achieved starting upstream and working downstream.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Various invasive species that originated from other Mediterranean climates are present in Orange County. Some were intentionally introduced during European colonization. Most produce bright yellow flowers during Spring and are often erroneously mistaken for native California wildflowers. Established populations often harbor ticks.
Environmental Impacts: Establishes dense monocultures that outcompete native plants for water and nutrients, increasing wildfire risk due to the dry, flammable biomass in the late Summer. These impacts reduce biodiversity, destabilize ecosystems, and degrade wildlife habitat, creating monocultures of invasive plants that are difficult for native species to recover from.
Management: Mustards are best addressed when young prior to seed production and control can be achieved with correctly timed mowing. However, some species can rapidly produce seed. Usually control requires multiple approaches. Mustards have a high affinity for sunlight so the shade created by dense forest tree canopies can be an effective deterrent.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Black Mustard (Brassica nigra):
Cal-IPC Species Profile
Calflora Taxon Report
Perennial monocot trees with dense fibrous trunks and expansive shallow fibrous root systems. Most produce large quantities of small waxy fruit containing a single seed. The seed is readily dispersed by wildlife.
Environmental Impacts: Palms compete for natural resources, especially water, with large specimens capable of intaking over 100 gallons of water daily. In addition to competing for available light, the large trunks offset the volumetric capacity of flood control conveyances. Detached burning fronds can travel up to a mile during windstorms and act like firebrands resulting in erratic wildfire spread. Be aware that there is a native California fan palm, Washingtonia filifera, which typically occupies desert washes and oases in the Southwest. The taller Washingtonia robusta, iconically associated with Southern California, is an introduced species native to Baja California and parts of northwestern Mexico.
Management: Most large invasive palms can be controlled by felling, since actively growing meristems are located terminally in the canopy. Detachment from the root system results in death. However, the remaining canopy persists in the habitat and contributes to fire fuel load as well as compromises flood capacity. Detached trunks and fronds readily flow downstream during flood events and may become hazardous by clogging channels and impacting vessels. Many species are easily controlled economically with herbicide trunk injections, where small amounts of product are applied to a drilled cavity in the trunk, which is then sealed, minimizing potential for contamination.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Mexican Fan Palm:
Cal-IPC Species Profile
Calflora Taxon Report
Canary Island Date Palm:
Cal-IPC Species Profile
Calflora Taxon Report
Perennial forb related to mustards (Brassicaceae) originating southern Europe and Asia and thought to have been introduced to North America in contaminated seed. This edible plant can spread through underground rhizomes and form dense monocultures in riparian, lacustrine and estuarine ecosystems.
Environmental Impacts: Monocultures prevent the establishment of desirable native species that provide critical habitat for many endangered species. It thrives in moist areas and can tolerate high salinity. It burns readily once dried.
Management: Hand pulling is only effective when targeting newly established individual plants. The species spreads rapidly by subsurface rhizomes that regenerate if left in place, often producing additional “daughter” plants. The root system is extensive and can penetrate several feet deep in certain soils. Mowing is not an effective control. The plant has a high affinity for sunlight but is not easily shaded by dense canopies due to its extensive root system that can benefit from canopy breaks.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Although a member of the Sunflower family (Asteraceae) and often incorrectly called Spanish Sunflower. This plant is known in its native Europe as Spanish False Fleabane. It can be an annual, biennial or perennial depending on growing conditions and can produce large quantiles of seed due to its prolific flowering.
Environmental Impacts: This species readily outcompetes low growing grasses and forbs along roadsides and wetland habitats. The seeds are readily spread and dispersed. The plant has a wide range of tolerances and can grow in both wet and dry conditions and multiple soil types.
Management: Hand pulling of newly established individual plants can be effective, but this species can resprout from its root mass. Mowing while still green can reduce the amount of seed produced. Mowing dried plants can facilitate seed spread. Spot treatment with herbicides can be effective as systemic products can control the root system as well thus reducing regeneration. As with most weeds, control is best initiated when the plant is young before it produces seed. This species aggressively competes with low-growing natives, including wetland rushes and sedges. Site sanitation to prevent spread to new areas should be practiced along with early detection and rapid removal/response (EDRR).
Annual forb, reproduces extensively by seed. Originating in Southern Africa. Its first observation in the U.S. was recorded at Lake Perris in 1984. Multiple generations can germinate in a single year. Each plant produces many round yellow flowers without petals. Each flower can give rise to as many as 300 viable seeds. Additionally, recent research has indicated that established monocultures can change the soil mycoflora, thus impacting the symbiotic relationship of native plants with soil microbes. This highly invasive plant has been seen being sold at swap meets as “wild chamomile”.
Environmental Impacts: High rate of spread due to extensive seed production, multiple generations, and fire resilience. The resulting monoculture negatively impacts native species as well as native pollinators that have co-evolved with them. The impact on pollinators is not yet understood, but is probably negative due to a reduction in diversity.
Management: Early detection and rapid removal (EDRR) should be practiced wherever new sightings are observed. This plant can be easily hand pulled, but resin glands on the leaf produce a foul substance that can be an irritant. Recent research has indicated that pre-emergence herbicide application in the Fall may provide relief to established native plantings as a strategy to limit the spread. Equipment, sanitation and surveillance prior to and after moving soil during construction and maintenance activity is also recommended.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Annual forb, reproduces by seed, flowers in the fall producing small yellow flowers with reddish centers. Best observed in the Fall as it remains green when many other plants have senesced. It first showed up in the Bay Area in 1984 originating from Mediterranean regions and spread to southern California along right of way corridors. This plant has a distinct odor similar to camphor. Seeds readily attach to clothing and fur. Stems and leaves are sticky, and contact can produce skin irritation.
Environmental Impacts: This plant aggressively competes with native species in disturbed areas particularly roadsides.
Management: Early Detection Rapid Removal (using gloves) should be practiced wherever feasible. Timely mowing can reduce seed production. Targeted herbicide applications can be very effective especially on young plants. Correct equipment and clothing sanitation along with site surveillance should be practiced especially where soil movement is anticipated.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Perennial evergreen tree capable of seed and vegetative reproduction, flowers over multiple seasons and a prolific producer of wind and water dispersed seed. Originate in Eurasia.
Environmental Impacts: Highly aggressive competitor of native riparian vegetation and is well adapted to saline and alkaline sites. Additionally, their reproductive potential is great as the plant is a prolific seed producer and is well adapted to recover vegetatively by regenerating from segments of root and shoot tissue. This tree is a halophyte and is capable of exuding salt, thus changing the surrounding soil chemistry to its advantage. It is of little value as a food source for native species, although some pollinators are attracted to the flowers. Tamarisks are more efficient utilizers of available soil moisture, and heavy infestations are thought to be capable of lowering the water table in arid regions.
Management: Trees can grow very large. Control is best achieved prior to maturity. Small seedlings can often be hand-pulled easily from sandy soil. Cut wood should be disposed of off-site or chipped and placed on high ground or solarized to prevent regeneration. Stump treatments with systemic herbicides can require several applications but can be effective in controlling viable root tissue with minimal soil disturbance.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
Winter annual forb originating in Europe but introduced to California via South America. It reproduces by seed and is very prolific with individual plants capable of producing over 70,000 seeds. Flowering takes place in the summer peaking around June depending on water availability with each plant producing multiple yellow flowers surrounded by large spines that deter grazers.
Environmental Impacts: Yellow star thistle rapidly depletes available soil moisture resulting in less species diversity. It is also toxic to horses and ingestion can be fatal.
Management: Multiple approaches are needed to address this species. These include sanitation and screening of hay, seed and mulch. Grazing best practices where ruminants are not allowed to graze uninfected areas following having grazed yellow starthistle infested area. Also, surveillance of hay placement areas in pasture. Early Detection Rapid Removal/Response (EDRR). Timely grazing, selective herbicide treatments as well as introduced biological control natural enemies are all being employed to address this species that has invaded over ten million acres of California.
See WeedCUT plant species profile for specific management methodologies and effectiveness.
