Thorn Burns

Just when you thought it was safe for that summer hike or picnic in the Santa Monica Mountains, state parks officials are warning of an outdoor predator whose sheer ruthlessness has made it a leading trailside enemy for plants, animals and people alike.

It’s milk thistle, Southern California’s scraggliest, nastiest weed, a foreign interloper without natural enemies that has staked its claim all along the well-trod meadows and trails in popular Malibu Creek State Park--sticking wildlife and hikers and all but choking off colorful breeds of native grasses and wildflowers.

On Tuesday, parks officials fought back against the unruly thistle through a “prescribed burn” of about 100 acres of parkland just south of Calabasas. Such fires, which officials stress are cheaper and more effective than chemical warfare and machine leveling of the weeds, are not without their risks.

Shortly before noon, the carefully planned fire jumped across man-made lines to briefly burn uncontrolled, forcing parks officials to temporarily close a portion of Las Virgenes Road and summon three Los Angeles County helicopters for water drops.

Despite the mishap, officials say fire is still the best way to cope with the storm-trooping weed, which can grow into gargantuan-size plants more than eight feet tall, standing like huge dried-flower arrangements.

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After drying to the consistency of old wood, the milk thistle can also quickly turn killer, displacing plant life and causing numerous complaints from park users wounded by its spiny thorns.

“It looms in the fields like stalks of corn, but with a big spiked head,” said Dan Preece, Angeles district superintendent for the California State Parks Department. “But I’m doing corn an injustice to use it as a comparison. Corn is friendly. But this thing isn’t. This thing is a pest. This thing is a habitat killer.”

Spreading million upon millions of airborne seeds that explode from its bulbous seed heads, the milk thistle spreads out in every direction after taking root in dirt disturbed by hikers or animals.

Park users call a tangle with the milk thistle a good walk spoiled.

“If you’re in it, it’s a pain in the neck,” said David Brown, conservation chairman of the Sierra Club’s Santa Monica Mountains task force and an avid hiker.

“It’s not natural. It doesn’t belong here. If you’re hiking to see the California landscape before the coming of the white man, the last thing you want to run into is a patch of these weeds.”

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Officials say there are more than a dozen varieties of pesky thistle weeds now populating California, and several--including the artichoke thistle and the Italian thistle--are taking root in otherwise graceful mountain meadows of Los Angeles and Orange counties.

Another predator giving park officials throughout Southern California concern is the yellow star thistle. Like its prickly cousins, yellow star thrives in the disturbed earth beside roads and mountain trails, around new housing tracts, in yards and trampled pastures. In recent years, its quick spread has caused alarm among scientists who attribute its proliferation in large part to California’s rampant development.

Tangling up lawn mowers and farm equipment, yellow star also kills a dozen or so horses a year, who suffer brain lesions and starve to death after eating too much of the weed.

“That’s the thing about the yellow star and the other thistles,” said Preece. “They’re pretty when they’re blooming. But they’re no friend of ours.”

Tuesday’s burn was aimed at the milk thistle, a native of southern Europe that scientists believe probably made its way to California with the Spanish centuries ago.

The thistle emerges as a low, leafy plant in January or so, after the year’s first rains, scientists say. By May, it has often risen to heights of eight feet or more, towering over and displacing native plants--including several native thistles--which are not nearly as aggressive.

“One of the things we’ve found, the invasion of these weeds depletes the area’s natural diversity,” said W. James Barry, a senior state park ecologist in Sacramento. “Species like the purple needle grass become rare or disappear altogether.”

Like its relatives, the milk thistle grows, well, like a weed, experts say.

“They just kind of take over,” Barry said. “The grasses, and everything else that would use that area just take a secondary spot. Like weeds in your lawn, they invade and if you don’t do anything about them, pretty soon you’ve got a lawn full of weeds and not grass.”

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In recent months, parks officials say they have received numerous complaints from hikers, picnickers and other park users who have been stung by the thistle while walking along stream banks and under towering trees where the plants invade.

“These plants can set up a wall of defense that makes it difficult for hikers and animals to get through,” Preece said. “They’re also well-armored and people get stung. The little spines come off and they prick. It hurts.”

Starting with white-blotched leaves that give it its name, the milk thistle blooms with pretty blue and purple flowers, not to mention the barbs on its leaves, spine and seed heads. Few if any native animals--other than the occasional luckless horse--will eat the plant, turning scientists to other means to exterminate it.

“Burning can make a drastic difference,” he said. “But the fires have got to be staged at the right time. And for that area of the Santa Monica Mountains, the time is summer.”

Fires started over three summers in Sonoma County reduced the number of seedlings per acre from 60,000 to just 50, he said.

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Tuesday, more than 100 firefighters from the National Park Service, California Department of Parks and Recreation, Los Angeles County Fire Department and other agencies conducted a 120-acre prescribed fire for the second consecutive year.

Last year’s burn was called off after 80 acres because weather changes made it dangerous to continue. On Tuesday, the 14-engine team scorched 90 acres before officials halted the controlled burn to concentrate on the spot fire that began climbing west up a hillside a half-mile south of Lost Hills Road shortly after 11:30 a.m.

Cmdr. Frank Padilla Jr. of the state Parks Department, estimated that the eight-acre blaze would be contained by 5 p.m. Tuesday.

Park officials say the fire was a timely step to take back an area of the park before the summer rush peaks. “In one park, people had deserted a campground to this thistle,” Barry said.

“But after several burn-offs, we took it back. It was a prime example of mind over plant.”