The Dreaded Yellow Star Thistle

Our five suburban acres have been attacked by the Dreaded Yellow Star Thistle--a really nasty late summer plant. The source? Seed blown down from the ridgeline on the ranch above our property, whose owner seems not too concerned with keeping nasty, pricky, aggressive plants controlled on his own property (let alone anyone else's). At the moment it's flowering. But we need to take steps to kill it all soon or else we'll have a huge amount of seed broadcast over all our five acres to sprout next spring.

It's weed-whacking time!

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Arastradero Preserve | Yellow star thistle plague

...the now-ubiquitous nonnative weed has blossomed into a major menace throughout much of the West -- ruining rangeland, choking out native plants, killing horses and stabbing both man and beast with its needlelike spines. "The star thistle has become the most common plant in the state, and it is out of control," said Joe Di Tomaso, a weed ecologist in the Department of Weed Science at the University of California at Davis. "It already occupied the valleys and foothill regions. Now it is expanding into the coast ranges and in the Sierra up to 5,000 feet."

Found along roadsides in grasslands, landfills, vacant lots, parks and recreation areas, the yellow star thistle often grows in dense stands and can reach higher than four feet. A star-shaped cluster of spines surrounds the flowers, which number from a few to hundreds per plant. Even when the thistle dies in the fall, leaving only a brown skeleton, the sharp spines can pierce skin and clothing. The fierce weed quickly dominates any area it invades. Like some alien from "The X-Files" it kills off its floral competitors in part by sending a tap root as deep as eight feet into the ground and sucking the available moisture out of the soil. Some scientists believe that the plant also may emit a substance, called an allelochemical, that stunts the growth of nearby plants.

The plant is especially harmful to horses, which sometimes eat the plant in the early spring before the spines come out. In large quantities, it is poisonous, causing brain lesions and a nervous disorder called chewing disease, which can be fatal....

El Nino hastened the spread of star thistle, but the plant shows few signs of withering away with the departure of heavy rains. That is in part because the star thistle has numerous advantages over competing plants, said Michael Pitcairn, a research scientist at the state Department of Food and Agriculture.

>For one, it is a prodigious seed producer, pumping out as many as 29,000 seeds per square meter. Moreover, an astonishingly high 95 percent of its seed germinates, and can do so in just a few days.

While other plants are sending out foliage in the rainy season, the star thistle is growing its long tap root, which will later rob other plants of moisture. In May, when other annuals start to wither, the thistle bolts, shooting up its stalk, then branches out and flowers in June and July.

Combatting the weed is proving difficult. Burning works but must be done annually and causes pollution. Mowing also can be effective, but the weed needs cutting several times a year in successive years. Even so, star thistle rebounds quickly.

Others prefer intensive grazing, first with cattle -- early in the year before the spines appear -- then with goats, which seem impervious to the sharp points....

Those battling the prickly invader are caught on the thorns of another problem: The weed is expensive to eliminate and inhabits areas generally considered of low economic value, such as landfills, roadsides, hillsides and hiking areas.

But, experts say, inattention to the star thistle has allowed much of California to become a vast noxious nursery for the aggressive plant.

"Right now, it's gotten to the point that I don't think eradication of the star thistle is a possibility at this time," Assemblyman Oller said.

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AN EVIL WEED

The yellow star thistle, which first arrived in California during the Gold Rush days, has run amok in recent years. The noxious weed with sharp pointed spines now heavily infests 22% of the state, an area equal to more than 20 million acres. In part, its success is due to its tap root, which the plant sends down as far as 8 feet below ground, sucking moisture from the soil.

Yellow star thistle (Centaurea solstitialis) is an annual plant varying from 6 inches to 3 feet. The sharp spines are 0.5 to 1 inch in length.

Posted by DeLong at July 20, 2002 09:17 PM | Trackback