Invasive plants can significantly impact many of the complex relationships in California’s beautiful and biodiverse landscape.

California is home to some of the most beautiful and biodiverse areas in the world: oak woodlands to redwood forests; serpentine and valley grasslands to alpine meadows; coastal wetlands to riparian corridors. These communities support an astonishing variety of insects, plants, and other animals in a diverse, interdependent web of life.

Carpobrotus edulis Cape iceplantCarpobrotus edulis – Cape iceplant (Vernon Smith)Unfortunately, invasive plants can significantly impact many of these complex relationships. Invasive plants such as water hyacinth (Eichhornia crassipes) blanket waterways; iceplant (Carpobrotus edulis) smothers dunes. Weeds like blue gum (Eucalyptus globulus) and Acacia spp. from Australia, African veldt grass (Erharta erecta), French broom (Genista monspessulana), pampas grass (Cortaderia spp.), and poison hemlock (Conium maculatum) invade grasslands and forests. Grasslands once comprised of native annual grasses, perennial bunchgrasses, and annual and perennial wildflowers are now dominated by annual grasses and forbs from the Mediterranean region. Roadsides are now being invaded by rampant stinkwort (Ditrichia spp.).

Invaders displace native plants and animals. They diminish forage for livestock, native fish, and wildlife. Some invasive plants consume enormous amounts of water; some block natural waterways causing flooding. Many increase wildland fuel loads, making adjacent residential neighborhoods and wild areas more fire-prone.  Erharta erecta IMG 8683Erharta erecta – African veldt grass (Vernon Smith)Our warming climate may cause invasive plants to expand into new areas posing an increasing threat to biodiversity.

How do non-native plant species get here?  International travel, shipping containers, and imports of agricultural products can spread propagules (seeds or plant parts that grow into new plants), but the greatest culprit is the horticultural trade. Gardeners find many of these invasive species attractive, providing a market for commercial growers and nurserymen to supply. Property owners and landscapers often lack information about a plant’s potential to spread outside landscaped spaces.  In the past, even landscape restoration projects used invasive plants such as cordgrass (Spartina spp.) and ice plant (Carpobrotus edulis) to control erosion quickly.

Not all non-native species are invasive, but those that are have a competitive edge for several reasons. They often lack natural predators to hold them in check.  For example, no insect in sufficient numbers and no browsers in Marin eat French broom, a shrub that continues to invade our wildlands. Many invasive species are strong colonizers, easily dominating damaged or denuded landscapes.Genista monspessulana IMG 0688cGenista monspessulana – French broom (Vernon Smith)

CNPS works with its partners Cal-IPC and Calflora to monitor and map rare and threatened plants and plant communities, identify immediate and potential threats of new invasive plants, promote restoration of native plant ecosystems, and encourage horticulturalists to use California native plants (locally sourced when available). Marin Chapter members have successfully lobbied to require 70% CA native vegetation for new developments in the recently approved update to the Housing Element (see pg. 62) of the Marin Countywide Plan.

Spartium junceum Spanish broomSpartium junceum – Spanish broom (Vernon Smith)

Ditrichia graveolens stinkwortDitrichia graveolens – stinkwort (Vernon Smith)

Mats of ice plant cover huge parts of the Point Reyes dunes. Photo: Laura Lovett