Non-native plants, called exotics or aliens, may look good but are a threat to native California species. Statewide, about 5,600 native species and 4,400 exotics exist. In some cases, exotics are destroying entire habitats and threatening plant species.
These alien plants were introduced intentionally or accidentally from Europe and Asia. Seeds are spread by wind, water and animals. Their presence is widespread, from urban yards to trails and hillsides. Hardy and tenacious, many have no known predators. They upset the balance of native habitat with their aggressive growth. A habitat overrun with non-natives usually cannot sustain wildlife. One of the worst range-and-open-land pests in the country is Russian thistle, or the tumbleweed, which proliferates by dropping seeds as it rolls.
“Ground cover species like star thistle make areas impassable,” said Los Angeles County Natural Areas Coordinator Frank Hovore. “It doesn’t provide food or homes for any wildlife. Larger (invasive) trees like eucalyptus crowds out native species-where they grow, nothing else does. If this continues, we’ll have a biological vacuum.”
Efforts to eradicate unwanted plant species vary, depending on area and type of plant. Arundo proliferation is so widespread that the U.S. Forest Service and local governmental organizations are attempting to wipe it out by cutting and spraying with herbicides. Space Invaders: A Primer A sampling of the more common non-native plants crowding out native flora in the San Fernando Valley: Arundo (Arundo donax) Description: Commonly referred to as bamboo or cane. Can reach a height of 25 feet, with leaves two feet long. Needs rich, moist soil such as wetland habitat. Origin: Europe Use: Planted as windbreak or for erosion control. Threat: A guzzler, its roots tap into ground water, denying moisture to any plant nearby. Flourishing in Little Tujunga Canyon and Tujunga Wash. Castor bean (Ricinus communis) Description: Shrublike, with highly toxic seeds that can poison humans if eaten. Origin: Europe Use: Quick-growing and tall, provides shade or leafy background. Castor oil, long used as a laxative, is a nontoxic derivative of its seeds. Threat: Aggressive threat to riparian systems. Grows densely around ponds and dry washes, and is often seen in vacant lots. Pampas grass (Cortaderia selloana) Description: Also called sawgrass, has sharp, serrated edges. May reach 20 inches in height. In late summer, stalks bear one-to-three foot white or pink flower plumes. Origin: Eastern South America. Use: Ornamental. Threat: Spreads rapidly, invading alluvial or wetland habitats and washes; houses pests such as roof rats. Spanish broom (Spartium junceum) Description: Tall, nearly leafless shrub with fragrant yellow flowers. Origin: Mediterranean Use: Planted along roadsides as an ornamental. Once believed to be a fire-retardant, but biologists now say the plant can ignite “like a torch.” Threat: Called “an attractive nuisance,” it spreads aggressively, crowding out native plants within chaparral. Star thistle (Centaurea melitensis) Description: Small, yellow flower heads with long sharp spines. Origin: Southern Europe Use: None. Introduced accidently. Named for its spiny heads, which were similar to a “morning star,” a metal ball on a long handle set with spikes used as a medieval weapon. Threat: Forms dense thatch on ground and solid columns along trail sides and in chaparral, crowding out everything else. Sources: California Native Plant Society; Los Angeles County Dept. of Parks and Recreation, Reader’s Digest North American Wildlife, Sunset Western Garden Book