Yellow Starthistle Tops the Plant "Most-Wanted" List

At first glance, the Yellow Starthistle looks like a beautiful wildflower.... one that you would be delighted to view during a nature hike.  However, this plant has quickly risen to the top of the "plant most want listed" as an invasive species in the United States. As you know, an invasive species is one that is introduced into an area or habitat to which it is not native.  In this new area, the checks and controls that keep a species from overpopulating are not present.  As a result, the species increases so rapidly that the local (and native!) species are often driven to extinction.

The Yellow Starthistle ( Centaurea solstitialis) is native to Eurasia. It grows as part of balanced ecosystems there because it is kept in check by a variety of local herbivores, as well as other plant species that have co-evolved with it in its native habitat. It is thought that the Yellow Starthistle was introduced into California in the 1850's during the California Gold Rush when imported alfalfa seed arrived that was contaminated with the seed from the Yellow Starthistle.  Once introduced into the United States, it quickly took hold, and has been problematic ever since.

Here is the quick-list of facts about this highly invasive species:

• A single staked plant may produce a root system that extends to an area of three feet outward from the stalk.  It rapidly depletes moisture in the soil and out-competes the local and native species in its ability to obtain water.

• It has become very common along roadsides, pasture lands, and wildlands.  
• It is poisonous to horses and causes a disease called "chewing disease."  This can be fatal to the horse if treatment is not received.  Horses are the only animals known to be affected.
• The plant may grow to a height of 5 feet.
• Seed output may be as high as 30,000 seeds per square meter.  
• Invasion of the species is increased by cars along roadsides and by livestock.  Livestock that is suspected of ingesting the Yellow Starthistle should not be shipped to uncontaminated areas.

Yellow Starthistle seedling

What are methods of controlling this invasive species?  In 2003, 4 insect species were introduced from Europe as a means of control.  These agents of biological control included 2 weevils and 2 flies. Each of these insects lays its eggs on the flower/seed head of the plant and, as a result, reduce the number of seeds that the plant produces. All four insects are host-specific to the starthistle and do not attack native species.  Grazing animals such as sheep, goats and cattle are also an effective means of control.  A variety of chemical agents are also used.

In an interesting, but sad turn of events, the Yellow Starthistle has invaded and is quickly taking over the Sierra Nevada habitats in Yosemite National Park.  You can read all about Yosemite's attempts to control this plant at this link.