Goal for these desert troops? Bag the buffelgrass.

Just after sunrise on the second Saturday of each month, Claudia Bloom and 20 friends scale the slopes of Piestewa Peak in central Phoenix – but not just for an invigorating hike or the splendid vistas. This small army has come to wage war, wielding pickaxes and crowbars. Their enemy? Clump after clump after clump of buffelgrass.

The weed is so invasive that it threatens the ecology of the Sonoran Desert, choking out native plants, including the iconic saguaro cactus.

Volunteer groups like Ms. Bloom's – part of the recently formed Phoenix Weedwackers – have dedicated thousands of hours to hacking and prying buffelgrass out of the rock-hard Arizona earth. State agencies are now taking up arms, too, spraying roadsides to kill it.

But buffelgrass is one tenacious species, propagating so often that seven plants seem to spring up for every one yanked out and packed off, ignominiously, in a 30-gallon trash bag.

"It's probably impossible to completely eradicate now," says Ed Northam, a weed biologist and invasive-plants manager for the University of Arizona Cooperative Extension. The best these volunteer groups can do, he says, is develop an "approach to figure out which areas we want to protect and start developing buffer zones."

That's not to say no ground has been won.

Some 120 miles south of Phoenix near Tucson, the Sonoran Desert Weedwackers for seven years have spent a morning a month clearing buffelgrass from the slopes of the 25,000-acre Tucson Mountain Park. Other informal groups have taken on smaller projects, as did the volunteer squad that recently cleared the roadsides of Galvin Parkway in Phoenix's Papago Park, home of the Desert Botanical Garden and the Phoenix Zoo.

A native of South Africa's savannahs, buffelgrass was introduced in the United States in the 1940s, after the hard lessons of the Dust Bowl. It seemed to be the answer to government officials' desperate search for a plant that would hold the soil and provide forage for cattle.

After testing and further development in Texas, its seeds were sold to area ranchers. Since then, it has marched across northern Mexico and southern Arizona and is now invading the central part of the state.

Buffelgrass thrives with very little water and germinates easily and often, producing seed heads three or four times a year, experts say. It began its wild trek through Arizona on the wind, taking root on roadsides, then spreading up surrounding hillsides. It starts in tiny low clumps and grows into larger ones that can reach three to four feet wide.

"It competes with and eventually overtakes wildflowers," says Raul Puente, curator of living collections at the Desert Botanical Garden here. "Its root system is thicker and more developed than [those of] wild plants, so it eventually chokes out all the native plants."

Perhaps more worrisome is that buffelgrass is in cahoots with wildfire. In the arid climate here, it dries and becomes very combustible, providing both fine fuel, which is a fire starter, and secondary fuel, which causes wildfires to spread.

The effect, these experts say, appears to be devastating. The magnificent saguaros, which grow nowhere else in the world, cannot survive wildfires. Neither can other cactus species such as ocotillos and barrel. Worse, buffelgrass burns hotter than garden-variety desert grasses, killing cactus seeds that are lying dormant in the soil.

Wildfires weren't unheard of here but occurred naturally only every 100 years or so. With the new tinder, they are flaring much more often. What the Sonoran Desert may ultimately have left, experts warn, are buffelgrass and other nonnative grasses.

Dr. Northam, the invasive-plants expert, recently visited a burned area about 20 miles north of Phoenix, where the 2005 Cave Creek Complex fire scorched nearly 248,000 acres.

"Hundreds of saguaros were lost," he reports. It's too early to tell if their seeds will eventually replenish the stock. But already buffelgrass is beginning to grow there at elevations below 2,500 feet, which seems to be as high as it can grow before succumbing to the cold.

Mr. Puente of the Desert Botanical Garden thinks back about 10 years, when he first hiked to the 2,600-foot summit of Piestewa Peak. "I saw some plants at the base, and I remember thinking, 'Wow, this is buffelgrass,' " says Puente, who earlier had been involved in efforts to fight buffelgrass in his native Mexico. Now it covers the mountain.

The Phoenix Weedwackers working on the mountain have succeeded in clearing buffelgrass from its base. Now the volunteer corps is working its way up the 50- to 60-degree slopes. On their last workday this month, the whackers packed out 150 30-pound plastic bags of the grass. Since December, when they first began work at Piestewa Peak, they've removed about 32,000 pounds, Bloom estimates.

Since 2000, the Sonoran Desert Weedwackers in Tucson have removed about 50 tons of the invasive grass from Tucson Mountain Park. The group has crafted a program to at least control the spread of buffelgrass there, says co-leader Marilyn Hanson,

"We take a spot thick with buffelgrass, pull it out, bag it, get it out," says the former high school science teacher. "We then go back next year, and the next year after that, and so on until we get all the seedlings. In five years, native flowering plants come back, and there is no buffelgrass in that area."

It's a task that requires the patience of Job, but Ms. Hanson notes the price of not doing it. "We're all alarmed at what buffelgrass could do to our Sonoran Desert," she says. "We've been working this for seven years very steadily, just knocking it out."