Grown In Marin 

Not all thistles are the same. These four species are found in Marin, each with its own way of making trouble. Control and eradication measures differ for some species. Mowing, pulling, grazing and biological controls are all practiced with varying success rates. Spraying can be effective, yet it is expensive and may have negative environmental impacts. Here is a short description of four thistles.

Italian thistle (Carduus pycnocephalus)

Italian thistle, from the Mediterranean,  was accidentally introduced to California in the 1930s. The flower heads are small, pink, with five to twenty heads per cluster.

Italian thistle

       On grazed lands this annual thistle reduces productivity by interfering with grazing and displacing desired grasses. It reproduces by seed, with flowering in September to December. A single plant can produce over 20,000 seeds, which last up to eight to ten years in the soil.

       Hand pulling can control small populations. The plants should be cut four inches below the soil surface early in the season, before seed is set.  Mowing is not a reasonable method. The plants can regrow and produce viable seed.

       There is a biocontrol program for Italian thistle. A Rhinocyllus weevil has been introduced, but is not yet providing the desired level of control.

Purple star-thistle (Centaurea calcitrapa)

Purple star-thistle, from the Mediterranean, was found in California in 1886.  It has large thorns below purple flower heads, and invades highly disturbed areas like roadsides and over-grazed pastures. It also invades well-managed rangelands. The spines discourage feeding by livestock. 

Purple star-thistle

       This biennial can grow 1 to 4 feet and produces a purple flower head surrounded by sharp spines. It reproduces by seed even after the plant has been cut.

       Grubbing and digging can control small populations. The plants should be cut 2 inches below the surface early in the season and removed from the site.

     There is no biocontrol program for purple star-thistle. 

Yellow star-thistle (YST) (Centaurea solstitialis)

YST originated in southern Europe and western Eurasia and has been in California since 1869. It is one of the most prominent invasive weeds in California, infesting over 14 million acres. It is recognized by the large thorns growing below yellow flower heads. It is toxic to horses, causing a neurological disorder. YST is spread by contaminated forage, heavy equipment, mowers, hikers, bikers and equestrians. 

Yellow star-thistle

       YST reproduces by seed, producing 30-80 seeds on 1-1,000 seed heads per plant. 

       Tillage can control YST, but it exposes the soil to rapid reinfestation if not mitigated. Mowing is effective, but multiple mowings are required for maximum control. Mowing reduces spines and allows cattle to graze the plant.  

       There are 6 USDA approved insects for YST. Three weevils, and 3 flies, that affect the seedheads, with a 50 to 75 percent reduction in the seed populations.

Distaff thistle (Carthamus lanatus)

Distaff originated in the Mediterranean and is an aggressive rangeland pest, recognized by its spiny yellow flower heads. Their large, sharp spines can injure the eyes and mouths of livestock that are forced to graze within dense populations. Distaff causes lameness in animals whose hooves have been penetrated by its spines.

Distaff thistle

       This winter annual grows in pastures, rangelands, and grasslands displacing desirable forages. It reproduces by seed, which disperse passively, but can remain in the seed head to mature. Seeds are dormant and viable for up to eight years. A plant is capable of producing 18,000 seeds that spread by wind, animals, and machinery.

       Hand pulling controls small populations. As with any annual thistle, plants should be cut or mowed after bolting, but before flower buds form. If they are cut after flowers develop, they should be removed from the site as seed can still ripen.

       There is no biocontrol program for distaff thistle because it is so closely related to commercial safflower.

Chemical control of thistles

There are a number of herbicides that will control these thistles. The most effective is Transline. The best time for application is December through April. Spraying is an effective way to control thistles, but may have negative effects on the environment. It is also costly to the operator. Most ranchers prefer not to spray, but economic realities foster some continued use of herbicides on pastures.

- Amanda Stephens, Marin County Agricultural Commissioner's Office (415) 499-6700

Photos by Charles Kennard