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Common Invasive Plants in Orange County

Updated: Dec 5, 2023

Southern California is rich with diversity; one can see an assortment of plants throughout Orange County. Orange County features a variety of diverse biomes, such as grasslands, coastal scrub, and wetlands. Unfortunately, the biodiversity of these areas are decreasing due to invasive plant species taking over, meaning we are seeing less and less variety of native plant species due to replacement by non-native plants which out compete the local plant species.

Why do we care about invasive species?

Source: California Department of Fish and Wildlife

Invasive species are non-native plants that are able to reproduce and spread quickly. It is an important distinction to make that not all non-native plants are invasive, but all invasive plants are non-native. Invasive plants have the ability to outcompete with the native species and eventually take over an area, once introduced. This can lead to a variety of problems including extinction of native species, significant decreases in biodiversity, altered natural fire frequencies / intensities, and changing vital nutrient and moisture availability, and this can cause permanent disruptions to the affected ecosystems. Disturbances to the natural environment like agriculture or herd grazing assist invasive plants’ ability to establish themselves in a habitat and spread quickly. Invasive species become near impossible to eradicate after enough time without intervention. California in particular is at a higher risk for invasive species because it has a unique climate that allows for great growing conditions since it is warm for most of the year. This allows many plants to be able to thrive when introduced to California habitats. Next, we are going to discuss invasive species that are commonly seen around Orange County.

The grasslands of Orange County have almost all disappeared, and what is left has been dominated by non-native annual grasses and Mediterranean herbs, many of which were introduced accidentally. After colonization the grasslands were maintained through rigorous livestock grazing. Grasslands were also expanded during this time by burning shrublands to create more room for ranching. In the present day, Southern California's grasslands are hard to come by due to urbanization, but there are some non-native grasslands including Simi Hills of Los Angeles, the Perris plain in Riverside County, and Camp Pendleton in San Diego County. There are still native grasses that can be seen around the Santa Monica Mountains and the Irvine Ranch Conservancy, but the best example of native grasslands in Southern California would be the Santa Rosa Plateau in southwestern Riverside County. A common invasive species that resides within the California grasslands is Centaurea solstitialis, also known as yellow starthistle. This plant is a member of the sunflower family that was introduced accidentally through contaminated seed. Yellow starthistle is considered by the USDA to be a priority invasive species. It is widespread throughout California and has grown from 1 million to 12 million plants in just 20 years. They begin to emerge with rains and germinate throughout the rainy season and are mitigated commonly by cattle grazing. Unfortunately, since ranching is not as profitable as it once was, ranches are being replaced with urban development. This means that the California grasslands may disappear. Irvine Ranch Conservancy and other grassland sites are currently undergoing restoration projects to preserve and restore California grasslands.

Yellow Starthistle; Credit: Eric Coombs, Oregon Department of Agriculture; Source:

Another amazingly diverse biome is the coastal sage scrub (CSS). CSS is similar to chaparral, but it is known for its low growing aromatic shrubs. If you are hiking trails along the coast you are likely to see this invasive species the Brassica nigra, also known as black mustard. It produces chemicals that prevent other plants from growing around them; these are called allelopathic chemicals. Their seeds are too small for seed eating animals, so they do not provide nourishment to the surrounding animal population. Black mustard is said to have been introduced to California after it was planted to connect the missions with a beautiful yellow path of flowers.

Photo Credit: Ron Vanderhoff

Lastly, we are going to talk about wetlands. They have a high saturation of water, which is where they get their name. This means that in a wetland, water will cover the soil, or is present near the soil all year long or through different periods of the year. Salsola tragus, also known as Russian thistle, is a major problem in wetlands around Orange County. Russian thistle prefers sandy, saline soil so they will often thrive around wetlands. One can see this plant around the wetlands/marshes of Huntington Beach. Russian thistle is a type of tumbleweed that will blow with the wind and disperse its seeds to reproduce. Their seeds contain toxic levels of nitrates that are able to damage crops/plants and have even caused sickness or death in grazing livestock from contamination from the seed.

Russian Thistle; Credit: Joseph M. DiTomaso, University of California - Davis;

As you can see, invasive species affect many of the habitats around OC. If you are now thinking, “How can I help?” There are many programs around Orange County that are involved in habitat restoration and are looking for volunteers! Another way is to plant more native plants in your garden or lawn. This will also cut down on your watering costs since California native plants have adaptations for drought resistance. Help support your local plant communities! If you would like to learn more about native species, check out the links below.

Resources (Links to more information)

Resources for Other States

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