The magnificent beauty of Trinity County’s mountains and the charm of our towns are compromised by the presence of noxious weeds, notable yellow starthistle, Centaurea solstitialis. Noxious weeds depreciate land values; diminish wildlife habitat and ecological health.

Yellow starthistle, an unwanted occupant of Trinity County that has the potential to diminish property values and wildlife habitat, needs to be controlled. Disturbances created by cultivation, road building and maintenance, overgrazing and poorly timed mowing aid in the spread of this invasive weed. Starthistle forms a dense thicket that rapidly depletes available soil moisture, which in turn inhibits native grasses from becoming established or expanding their coverage. The competitive ability of this invasive weed is tremendous; one mature plant can produce up to 100,000 viable seed. Most of the seed will germinate upon dispersal; however, seed may remain viable in the soil for up to 3 years. It is critical to use control measures before the seed becomes viable. The picture depicts the developmental stages of the yellow starthistle flower.

At Stage 6 some of the seed are viable, most however, are partially or unfilled seed. At Stage 8 the seed has become viable and will germinate shortly after dispersal unless high temperatures and low moisture occur causing the seed to become dormant. This adaptive mechanism accounts for the extended duration for germination to occur. It is imperative that control measures be implemented at or before Stage 5.

Manual removal of yellow starthistle is most easily achieved upon smaller acreages when the soil is moist and the entire root system can be pulled from the soil. Depending on the rapidity of soil moisture depletion, one may wish to wait until the plant has fully elongated to minimize the soreness of one’s back.

There is new product labeled for yellow starthistle control, Brush-Weeds & Grass Herbicide for organic production. The constituents of this non-synthetic herbicide are vinegar and citric acid. Greenergy, Inc., www.greenergyinc.com, produces it. It works by drying plants out. Therefore it should be applied before the plants become too woody and prior to seed viability.

Unfortunately it is very difficult to control yellow starthistle by simply mowing, because of the plants ability to grow close to the ground below the level of most mower blades. To Mowing after the plants have fully bolted and the flowers have expanded minimizes the formation of low branching plants. Researchers recommend mowing when about 2 to 10 percent of the starthistle is at Stage 6. It is critical that one does not aid in the dispersal of the seed by not employing a bag attachment when seed has become viable. Mowing is most effective when soil moisture is low and no precipitation follows.

Grazing has been demonstrated to be a very effective control. Sheep, goats and cattle eat yellow starthistle before spines form on the plants. Goats will continue to browse on yellow starthistle even after the spines have formed.

Burning 2 consecutive years can deplete the seedbank substantially. It should be performed in spring when the flowers have expanded. Burning can also increase the recovery and density of native plants. Burning should only be conducted under the guidance of someone with extensive experience along with copious quantities of available water.

Lastly, the best control is to minimize soil disturbance. If soil is disturbed, competitive non-invasive annual seed should be broadcast or perennial grass plugs planted so that the soil is occupied by more desirable vegetation.