Weeds seen as threat to desert

Monday, April 11, 2005 | 9:39 a.m.

Conservationists had expected an invasion of weeds along with a spectacular wildflower bloom this spring in Southern Nevada, but the profusion of blooms the past two months has exceeded their expectations.

And it's not all about pretty flowers. Scientists from coast to coast are sounding the alarm about an unwelcome new resident: the tenacious mustard weed.

Conservation scientist Marilyn Jordan of the Nature Conservancy on Long Island, an expert on the spread of mustard weed in the Southwest, is calling it "the invasion of the century" as three kinds of yellow-flowering plants are spreading.

Mustard weeds compete with wildflowers and native plants alike.

In addition, the invasive weeds could threaten existing habitat conservation plans designed to protect endangered and threatened species, scientists said.

The predominant plant at Lake Mead, Sahara mustard (Brassica tournefortii) has been a target for eradication for a number of years within the Lake Mead National Recreation Area, said Elizabeth Powell, a National Park Service botanist.

"We have never seen such bad infestations as we are seeing this year," Powell said in an e-mail message.

Mustard plants are growing in areas at the lake where botanists have never documented a single weed before this year, Powell said.

Mustard seeds growing in sandy areas are so thick and so extensive, "we have had to throw up our hands," Powell said about efforts to remove the weeds.

Even with extra hands loaned by volunteers to pull out the plants, the mustard keeps spreading.

In addition to sandy soils, the mustard seeds sprout in salty soils along with another pest, salt cedars, Powell said. The seeds also thrive in desert pavement and gypsum, soils where nothing else grows.

First spotted at the lake in the 1970s, Sahara mustard has spread across at least 2,500 acres of the park's 1.5 million acres, Powell said.

Rodents rapidly move the sticky seeds from roadsides deep into the desert areas. Seeds clinging to vehicle tires also help perpetuate the species throughout the area.

Seeds can start to grow within 24 to 48 hours after being wet by rain in cooler weather, Powell said, which is how this year's bumper crop of mustard plants began to grow.

New plants flower and begin to set seeds within a month.

Powell discovered 125 acres blanketed with new yellow mustard blossoms inside the park this year.

The Nevada Department of Agriculture is expected to place Sahara mustard on its list of noxious weeds, but not until the end of this year, Dawn Rafferty, an invasive plant specialist with the department, said. That would allow the state Department of Transportation to spray weed killer along highways such as U.S. 95 and U.S. 93. The money would have to come from state and local budgets.

Mustard weeds could become the newest threat to western deserts, said Craig Dremann, a plant specialist in Redwood City, Calif.

A century ago European grasses invaded the Southwest. They included wild oats, ripgut grass and foxtails that now blanket about half of California between sea level and 3,000 feet. Nuisance grasses have also spread into Nevada.

Dremann compared the mustard plants to a tsunami wave, which covers everything inland after it hits.

This year scientists nationwide have taken note of the infestation of mustard weeds. The Society for Ecological Restoration has called for a summit on mustard weeds, urging scientists to develop ways to contain the plant's spread.

"Eradication seems highly unlikely," the society's Web page said.