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  • Rachel Brownsey / California Agriculture Journal

    Stinkwort seedlings grow into a tumbleweed-like plant that has a turpentine-like odor, hence its name.

  • Rachel Brownsey / California Agriculture Journal

    Stinkwort seedlings grow into a tumbleweed-like plant that has a turpentine-like odor, hence its name.

Stinkwort's fast growth could threaten California's wine growers

Published: Monday, Aug. 12, 2013 - 12:00 am | Page 1A
Last Modified: Monday, Aug. 12, 2013 - 12:49 pm

A new invasive weed has been expanding exponentially along roadways in the Sacramento region and the state, prompting plant scientists to warn that its unchecked growth could end up costing taxpayers millions of dollars.

The new weed, named stinkwort for its turpentine-like odor, threatens to choke out native plant species. The weed also has begun appearing in vineyards, raising a concern that it could affect the flavor of some grapes in the state's premier wine-growing region.

The low-lying weed, known by the botanical name Dittrichia graveolens, is a relative newcomer to the state, making its first appearance near Milpitas in 1994. It resembles a tumbleweed, and rarely grows taller than 3 feet. Its flowers are small and sport tiny yellow petals.

For a decade, the weed made only small and slow incursions in California. Its first appearance in Sacramento County came in 2003, said Joseph DiTomaso, plant science specialist at the University of California, Davis.

"It was just an oddity back then, and no one ever mentioned it," DiTomaso said.

But after 2005, the weed spread quickly. Today stinkwort is found in 36 of 58 counties, according to UC Davis data.

In Sacramento County, stinkwort can be found proliferating on roadways. Weed scientists have inventoried its presence on Interstate 5 between Sacramento International Airport and downtown, as well as along stretches of Highway 16 and Highway 50 west of Folsom Boulevard. The weed has also been spotted spreading south of the Freeport Boulevard intersection of I-5, and on the American River Parkway.

To the east, the weed has been seen on the side of roadways such as South Shingle Road in Placer County and in Diamond Springs in El Dorado County.

Outside California the weed is found only in a tri-state area that includes New York, Connecticut and New Jersey.

"Not too many people know what to do or how to control it," DiTomaso said. The weed's growth is being curtailed somewhat through mowing and the use of herbicides, but other methods – such as fire control, insects or grazing the weed – are still in their infancy.

For DiTomaso, a worrisome aspect of the weed is its potential to grow rapidly in open, overgrazed land and along the banks of streams, rivers and wetlands areas – where ample water can fuel the plant.

"If it gets a major foothold and produces millions and millions of seeds, then the seedlings will grow and they can form a carpet," DiTomaso said. "Then it would block light and prevent the growth of more desirable species – like native plants. It will out-compete them, and that is a concern."

Another troubling aspect is that the weed has been seen in vineyards, said John Roncoroni, weed science adviser for Napa County.

"I've seen it on the roadsides in Napa, and it's just encroaching into the vineyards at Napa Valley College," he said.

While the weed is yet to become a problem at any vineyard, its odor may pose a danger, Roncoroni said.

"My concern is that if it grows up tall, in an area where grapes are trellised fairly low, there may be some mixing of that awful smell into the grapes," he said. "This may or may not impact the flavor."

Having an aromatic plant or tree close to vines is no small matter for some vintners. Some have taken to removing aromatics from vineyard properties – as is the case with eucalyptus trees because their odor was affecting the flavor of the grapes, Roncoroni said.

The fact that stinkwort grows in autumn gives it an advantage in vineyards because the pesticide Roundup, which is one of the most effective herbicides in controlling the weed, is typically applied before fall.

Weed scientists advocate controlling invasive species in their ascendancy rather than after they have become well established. Controlling invasive weeds is no small matter in California. The state spends roughly $82 million yearly to control and monitor the progress of invasive weed species, according to the nonprofit California Invasive Plant Council.

DiTomaso believes that stinkwort is poised to become as much of a nuisance as yellow star thistle, California's most troublesome and pervasive weed.

Yellow star thistle is considered a noxious weed because of the way it edges out native plants. The plant also is poisonous to horses, and the barbs of the flower have the potential to prick human skin.

The weed was introduced in Oakland in 1849 and became prevalent in alfalfa farms. "Once tractors were invented … it really started to spread," DiTomaso said.

It occupies 14 million acres in California and is found in all but two of the state's 58 counties.

Managing the spread of yellow star thistle has proven costly. California spent $12.5 million to prevent the spread of star thistle on just a half-million acres, according to a 2006 California Invasive Plant Council report.

"People are now asking the same question: Is stinkwort going to be the next yellow star thistle?" DiTomaso said.

Crucial to stinkwort's eradication will be controlling its growth on roadways. That job falls to CalTrans, which manages more than 15,000 miles of roadway and more than 230,000 acres of right of way. A lot of that is done by mowing roadsides as well as targeted spraying of herbicides.

Caltrans has an inventory of 45 weeds it seeks to control, said Parviz Lashai, chief of roadside maintenance at CalTrans. However, the priority at CalTrans is clearing roadways of fire hazards and poor visibility, Lashai said.

Clearing roadsides with mowing can kill the stinkwort, but can also benefit it. The weed cannot grow unless it has full light. Roadside mowing, especially the bulk that is done in the summer for fire control, clears most of the stinkwort's competition for light.

"Stinkwort is listed as one of our noxious weeds, so it is a weed of concern," Lashai said. "We do not have any specific strategy to address it other than how we address all other noxious weeds."

Caltrans uses Roundup, or glyphosate, and Capstone to control stinkwort. Currently, the agency has identified roughly 240 acres of roadside for spraying in the Sacramento region.

DiTomaso said he believes CalTrans will have to treat stinkwort as an invasive of high and unique concern if it has any hope of controlling its spread.

"If this is very early in the process … then it is worth doing something now," DiTomaso said. "Do you want to wait until you have 10 million acres occupied? Because at that point, you're kind of stuck."

Call The Bee's Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.

© Copyright The Sacramento Bee. All rights reserved.

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