We’ve all witnessed its prickly presence — the dreaded yellow starthistle (Centaurea solstitialis) — which is estimated to cover 10 million to 15 million acres in California, making it the most widespread invasive weed in the state.
This nonnative weed is a threat to indigenous plants, depletes soil moisture levels and is poisonous to horses, so controlling it is important on many levels.
Unfortunately, thwarting a full-blown starthistle infestation requires stubborn diligence and a multistep process implemented over a protracted period of time. Fortunately, home gardeners are a hardy bunch ready for the task.
Prevention is the first line of defense in controlling yellow starthistle. Because it thrives along roadsides and in vacant fields, roadwork and construction can cause an invasion, so monitor your property regularly near such sites.
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Seeds are often carried on vehicles, spread during livestock transport, and are found in hay and grass seed and in hay used for feed or mulch. You can reduce the likelihood of contaminating your property by using only certified grass and hay seed and by thoroughly inspecting any hay brought onto your property.
Also, beware of livestock that have grazed in areas known to have starthistle — such animals should not be allowed on land currently free of the weed.
If yellow starthistle does rear its ugly head, diligence is a must.
Controlling this weed requires removal of existing plants, prevention of seed production and competition from desirable plants.
Starthistle seeds germinate during the entire rainy season so it’s impor tant to remove young plants after the last rain but before seeds are produced. If rain occurs after you’ve removed existing plants, seeds will continue to germinate and the process must be repeated.
Preemergent and postemergent herbicides are also necessary to control growth. For information about herbicides and for photos of yellow starthistle’s growth cycle, please visit http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7402.html .
Finally, effective management of starthistle includes revegetation. Planting affected areas with desirable plants discourages starthistle regrowth by creating competition for soil, sun and water. Be sure to select plants that are suited to the site and that won’t become invasive themselves.
GOT A GARDENING QUESTION?
Contact the University of California Cooperative Extension Master Gardeners: at 781-5939 from 1 to 5 p.m. on Monday and Thursday; at 473-7190 from 10 a.m. to noon on Wednesday in Arroyo Grande; and at 434-4105 from 9 a.m. to noon on Wednesday in Templeton. Visit the UCCE Master Gardeners website at http://ucanr.org/sites/mgslo or email firstname.lastname@example.org .