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 …