What is three feet tall, armed with sharp needles, loiters in the grass and kills a dozen or more horses in California every year?

It is California’s meanest weed--and may also be, in at least one way, the most painful side effect of the state’s runaway population growth.

Yellow star thistle is a foreign interloper without natural enemies here to slow its invasion, but with plenty of accomplices. It thrives in the disturbed earth beside roads, around new housing tracts, in yards and trampled pastures--all conditions that have become more common as California absorbs more people.

“Every vacant lot in Sacramento is waist high with the stuff,” said Kathy Brunetti, a state Department of Food and Agriculture biologist who is part of the army of government scientists joined in battle with the weed. “Once you’ve seen yellow star you don’t forget it.”


Few residents of urban Southern California have yet to suffer the thistle’s painful sting. Elsewhere in the state, the weed is making picnickers, ranchers and home gardeners curse. Despite exotic insects brought from Greece to attack it, yellow star is on the march and may come to your neighborhood next.

Almond grower Cliff Koster, who has fought a losing battle trying to keep yellow star thistle out of his orchards along Interstate 5 here, said: “It’s an awful tough weed. . . . You go a little bit down toward Stockton and all of (Interstate) 5 is solid with yellow star.”

Tough as rawhide cord, the weed stalks tangle up lawn mowers and farm equipment. Horses that eat enough of the thistle heads can suffer brain lesions and starve to death, unable to chew and swallow. For most people, the most persuasive reason to hate yellow star thistle may be the nasty thorns that make traversing a patch of yellow star memorable.

“It hurts!” Paso Robles equestrian Callie Fisher said from experience.


Yellow star has been around California at least since Civil War days. Based on seeds found in old adobe bricks, it may have come in with contaminated hay or seed in the Mexican colonial period of the early 1800s.

But it has only been in recent decades--as California’s population soared--that the weed has spread across more than 8 million acres and become a matter of official concern. The Mediterranean climate here nurtures yellow star, and like most weeds, it takes advantage of the opportunities nature or humankind provide.

In California, its best gift has been the physical spread of California’s population into rural areas. In the 1980s the state’s population soared by 7 million, to more than 30 million people, and tens of thousands of acres of grass and wild lands were colonized by suburbs.

Yellow star did not miss its chance, growing to become a serious nuisance in state parks--especially those heavily used parks in the dry hills that ring San Francisco Bay--and becoming the worst botanical pest on cattle pasturelands.


“I’ve been here 23 years--I’ve seen it spread from a few isolated patches to where we’ve got it over most of the county,” said Jack Schrock, agricultural commissioner of Marin County, north of San Francisco.

The drought has also helped yellow star thistle spread. Given ideal habitat in the miles of farm fields lying fallow for lack of irrigation water, the weed has taken off across the Central Valley.

Cattle and sheep herds that have moved around the state in search of grazing pasture have also spread the weed. The dearth of winter rains has helped the yellow star thistle conquer California’s native grasses, said Jim Trumbly, a state parks ecologist in Sacramento with a personal dislike for the weed: “Oh, I have it in my back yard. I hate it.”

Herbicides can kill yellow star, but there is far too much of the weed for the spray to be used economically. Scientists think the best answer may lie with insects captured in the Mediterranean region, where the plant is native, and released here.


The best performer so far is a Bangasternus orientalis weevil, collected in Greece, that has been introduced in 38 counties, said Charles Turner, biologist at the U.S. Agricultural Research Service lab in Albany, near Oakland.

Scientists are gambling that the B. orientalis weevil will work with a second weevil and gallfly also from Greece to fight yellow star thistle. The bugs lay eggs whose larvae feed on the seeds that spread the weed, which is an annual plant that needs to re-establish itself every spring, Turner said.

Elsewhere in the country--the weed is found over a wide area of the West, but is most troublesome in California--scientists are experimenting with a fungus from Turkey that they hope will also attack the hardy thistle.

“We never know what’s going to be effective until we try it,” Turner said.


Sending biologists off to hijack foreign bugs hungry to attack California’s exotic pests is an honored tradition here. So much of California’s signature flora is non-native--including eucalyptus trees and the Russian thistle, or tumbleweed--that scientists are always looking for insects with the right stuff.

For the last 30 years, a stone monument in Eureka has acknowledged the Klamathweed beetle. Humboldt County ranchers say the beetle saved their cattle and sheep from a poison plant known as St. Johnswort or Klamathweed. The beetle was introduced from Australia in 1946 and in 10 years all but eliminated the weed.

Orange and lemon growers credit the Australian vedalia beetle with saving the California citrus industry more than 100 years ago. The beetles controlled a devastating outbreak of cottonycushion scale.

Most recent, a stingerless wasp from Israel was enlisted to fight the ash whitefly, the tiny pest that is stripping leaves from hedges and ruining gardens in a wide swath of California.


In the case of yellow star thistle, the control effort is not without dissenters. Yellow star’s flowers make a tasty honey and commercial beekeepers cherish the weed. But there’s no chance of the favored honey vanishing from stores, Turner said.

“There will always be yellow star around,” said Turner. “It’s a very invasive weed.”

And if you never run into any yellow star thistle, you can always look for its nastier but less common (so far) cousin, the purple star thistle. “It’s larger, with stouter spines,” Turner warned.

California’s Meanest Weed Here is a look at the yellow star thistle and the effort to combat it. THE PLANT * WHAT: Yellow star thistle, an annual weed native to the Mediterranean region, with no natural enemies here. * WHERE: Has spread across more than 8 million acres in California. * SIZE: Can grow to three feet high. * EFFECTS: Sharp spines deliver painful sticks to humans. Horses that eat star thistle can die of starvation because of brain lesions that affect the chewing function. The weed also crowds out native grasses from pasture used by cattle and sheep. * THE WEAPON * WHAT: Bangasternus orientalis weevil, captured in Greece. * WHERE: Weevil colonies have been bred at labs in Northern California and released in 38 counties in a joint federal-state effort to halt the spread of yellow star thistle. * HOW: The weevil’s larvae feed on the plant’s seeds, and are a major reason yellow star thistle is not a problem in other parts of the world. * BACKGROUND: Herbicides will kill the weed, but too many California pastures and vacant lots are infested to economically spray.