Invading Thistle Plagues Horses, Hikers


They're coming.

They're everywhere.


Getting a strong foothold, the yellow starthistle blossoms brightly and is considered one of the hardest weeds in the world to eradicate.

They're pretty from a distance -- but up close they're very, very pointy.

They'll drive horses crazy -- and drive hikers away.

And they're running Forest Service Range Manager Bill Barcus ragged, in his long war of attrition against the insidious Yellow and Malta Starthistle. Now he's hoping to enlist Rim Country residents as foot-soldiers in his war on what he calls "the most toxic invader weed in existence."

And with the flowering and seed setting season upon us, he's hoping residents will learn what the invader looks like and help uproot them.

Residents can get free, heavy-duty plastic bags at the Forest Service office in Payson or the town halls in either Star Valley or Payson. Then they can yank up the toxic weed, bag it and drop it off for burning at the Blatner Pit just east of Star Valley every Saturday between 8 a.m. and 4 p.m.

Barcus hopes residents will act quickly, since we're entering the period in which each plant normally produces about 200 seeds -- but in a good year, a single plant can put out thousands of seeds. In an infested area, the thistles can leave behind 2,700 seeds per square foot -- with 96 percent of them viable.

As a result, the import from Spain can cover whole hillsides with dense stands of spiky yellow flowers that will deter the most hardy of hikers -- and kill horses who graze on the tempting forage. Already, the Malta and Yellow Starthistle have taken root in scattered locations throughout Rim Country -- including many roadsides, the Payson Event Center and much of Star Valley. The infestation is widespread in Star Valley, but so far in Payson, limited to 60 to 80 acres.

The hardy, deep-rooted plants have also infested millions of acres in California, since they arrived onboard ships from Chile during the California Gold Rush in the mid 1800s. Although goats gobble them happily, toxins in the weed cause horses to go glassy eyed and stop eating or drinking, causing irreversible brain damage -- a condition called "chewing disease."


A bee lands on the flower of the yellow starthistle, pollinating the prolific weed.

Although the plants do no damage for most of the year, when they flower and develop their long protective spines they can make many areas all but impassable to human beings and most wildlife.

"It's the Bubonic Plague of weeds," says Barcus.

But the lack of natural insect enemies, the high cost of pulling them up by their long taproots and the side effects of herbicides have made the infestation tough to fight, said Barcus. He noted that a two-inch-high plant can put out a foot-long root.

The Payson Ranger District this year had budgeted about $5,000 to pay for work crews to pull up the weeds in some of the infestation hot spots. Barcus noted that crews recently filled up 100 large bags with the weeds from a seven-acre site at the Doll Baby Trailhead.

The key to limiting the infestation is to yank out the plant before it produces seeds -- and before they sprout the yellow flowers with their nasty protective spikes. Once the seeds have developed starting in about August, going after the plants generally just knocks loose and spreads the seeds. The tough seeds can persist for years in the soil -- and can sprout after fires. The tenacious plant thrives in disturbed areas and can choke out all competitors.

Once established, they're almost impossible to get drive out -- so the focus shifts to containing their spread.

The seeds often arrive in shipments of hay and grain, since the weeds often grow in agricultural fields. Some areas have been treated with a combination of herbicides and mowing -- but that can be costly and often inflicts unwanted side effects.


Forest Service is urging homeowners to dig up any yellow starthistles they find on their property like the one pictured here.

Some areas have had success in containing the thistle by importing the insects that feed on them in Spain and Greece. Five different insects lay eggs on the plant and produce larvae that eat the flowers and seeds, including three weevils, a gall fly and a peacock fly. All have evolved digestive systems that allow them to handle the defensive chemicals of the plant. As a result, they all munch on Star and Malta Thistle and don't attack other plants. Several field trials have yielded promising results.

But in the meantime, Barcus must rely on the help of local residents willing to fill bags with the thistle plants.

"We're not going to get rid of it anytime soon -- but we can at least slow it down," he said.

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