LAS CRUCES - Although New Mexicans battle common weeds in their yards each year, the worst invaders are nonnative plants that are taking over streambanks, ranges and public land.

A New Mexico State University weed expert ranks salt cedar, African rue, Russian knapweed, yellow and purple starthistle at the top of the list of the state's worst weeds. Others edging up the roll are camelthorn, yellow toadflax and onionweed.

Several weeds on this list are ominously called "ecosystem transformers." In other words, they can change the environment they're in so even if these weeds are removed native plants cannot re-establish, said Mark Renz, a weed specialist with NMSU's Cooperative Extension Service.

The common characteristic of these problem weeds is that none are native to New Mexico and the United States, he said. They're thriving because New Mexico lacks insects and diseases that keep them in check in their native habitats. While the definition of a weed is subjective, they're normally considered a plant that goes against the intended purpose of the land.

"Identification of problem weeds can be a big problem," said John White, Doña Ana County horticulture agent with NMSU Extension. "That new wildflower growing on your property could be a fast-spreading, invasive weed."

To measure the spread of these invaders, NMSU Extension and the New Mexico Department of Agriculture are working with the U.S. Geologic Survey to update a statewide weed map that can be used for regional management.

A major obstacle in controlling weeds in New Mexico is that so much of the land is public, said Frank Holguin, a Valencia County Extension agent who is deeply involved in weed tracking, mapping and eradication. Public lands and roadside rights of way are not actively managed in the same way a farmer would his fields, he said. Agencies simply don't have the funds to manage weeds on such a massive scale. That's why prevention is the key.

"We've also had problems with absentee landowners," Holguin said. "They don't know that they've been invaded. They need to stay involved."

Over the years, New Mexico agricultural experts have compiled a list of the toughest weed outlaws. Called the noxious weed list, the registry ranks weeds in three classes: bad, worse and worst.

The 'bad' weed class, which holds the possibility of eradication, includes camelthorn, yellow toadflax and onionweed, along with purple and yellow starthistle.

"Camelthorn is a big problem in Arizona and we really want to keep this one out," Renz said. A few small pockets have crept into southern New Mexico, near the Texas-Arizona border, he said. Toadflax is a problem throughout northern New Mexico.

Officials have found yellow starthistle, which is poisonous to livestock, in an isolated area of southwestern New Mexico, near rural Cliff. Purple and yellow starthistle has also been found in Lea County. Starthistle has taken over more than 15 million acres of land in California and is present throughout the Southwest. "It really has the potential to spread long distances, so we must be especially proactive in stopping this particular weed species," Renz said.

Weeds in the 'worse' category have spread, but are still regionally isolated. Containment is the goal here. Atop the list is Russian knapweed, which is poisonous to animals. Already a problem in Western states, it has established a strong foothold in northern New Mexico.

"It was a big problem in the past, and now it's bigger," said Gary Hathorn, San Juan County Extension agent. "We've seen a tremendous increase in the last two years because of the drought, which has knocked down its competition." Research projects in Bosque Farms and Albuquerque aim to slow Russian knapweed's progress.

Another worry is African rue, a poisonous, drought-tolerant plant that was first discovered near Deming, said Laurie Abbott, an NMSU range scientist. The rangeland weed that first appeared in North America in the 1920s has established an 11-county foothold in southern New Mexico.

"This plant takes hold in disturbed soil - along roadsides, abandoned fields and near livestock watering tanks," Abbott said. "It often dominates where it occurs."

The 'worst' list contains weeds that are already widespread and difficult to control. Leading the list is water-hogging salt cedar, also known as tamarisk. The invader was brought over from Eurasia and the Middle East in the 1800s as a feathery pink-flowered ornamental for erosion control. It thrives along the state's rivers and streams, forcing out cottonwoods, willows and other natives.

Besides sucking up too much water, the tree increases soil salinity, making it almost impossible for other plants to survive. "We're starting to get hold of the problem, but salt cedar is so widespread that eradication is unrealistic," Renz said.

If you run across a suspicious weed, contact your local county Extension agent, or Renz at (505) 646-2888 or markrenz@nmsu.edu to identify the plant and learn if any control measures are required.

Norman Martin is a writer for New Mexico State University.

e-mail: nsmartin@nmsu.edu