As the flames spread, the clumps of brown grass withered into slick, black mounds. Several mesquite trees, which seemed to withstand the blaze, cut ominous shapes in the thick smoke.
Firefighters emerged from the haze, flames licking the air behind them as they marched down the length of the field, some 50 or so feet between each man.
The firefighters spent much of the day last Wednesday, May 28, burning a 160-acre stand of buffelgrass in Avra Valley.
The noxious weed covers much of the Sonoran Desert, filling in bare patches and overcoming native plants.
When buffelgrass burns, “it’s a hotter fire … a flashier fire,” according to Chris McDonald, a Ph.D. student at the University of Arizona who studies how the plant reacts to fire.
The UA, Tucson Water, which owns the Avra Valley property where the prescribed burn took place, and Saguaro National Park teamed up last week to study the buffelgrass-fueled fire, with researchers like McDonald hoping to learn more about how firefighters might control the fast-moving blazes.
Scientists first brought buffelgrass to the Sonoran Desert from Arica in the 1930s and 1940s as a means of controlling soil erosion, according to McDonald. Of course, back then scientists had no way of knowing just how successfully the weed would consume this wet desert, much in the same way the kudzu vine (native to Southeast Asia) has enveloped much of the Southeastern United States since its use for erosion control became widespread in the 1930s.
If kudzu could be called “the vine that ate the South,” then surely buffelgrass could be called the weed that is strangling the desert.
Even burning buffelgrass won’t truly kill it, according to McDonald.
“We’ll wait ’til it greens back up and hit with Roundup,” said Mitch Basefsky, of Tucson Water, who came out last week to watch the prescribed burn.
The herbicide glyphosphate (the active ingredient in the commercial weed killer Roundup) has proven an effective destroyer of actively growing stands of buffelgrass. Of course, during the drier months, when the weed remains brown and lays dormant, only pulling it up by the roots will stamp out re-growth.
Tucson Water owns about 22,000 acres of former farmland throughout Avra Valley, according to Basefsky. The agency aims to restore all of it to native vegetation.
McDonald and other researchers spent the last year scouring the region for large stands of buffelgrass to burn. The Tucson Water-owned former cotton farm in Avra Valley seemed like the perfect spot.
But, before firefighters could light the controlled blaze, wind had to blow at the right speed, fast enough to fuel the fire, and in the right direction, from south to north.
Crews had cordoned off three sections of the 160-acre plot off Reservation Road and planned to burn them one at a time. It wouldn’t take long.
About 11:45 a.m., firefighters carrying canisters of diesel and gasoline dripped liquid fire onto the field. Almost instantly, the fire burned hot and high.
Onlookers, who had gathered on Reservation Road along a barbed wire fence, watched the flames skirt higher and higher in the distance. Several hawks began circling in front of the column of smoke, perhaps scanning the ground for rodents and other creatures fleeing the oncoming flames.
As the blaze approached the fenceline, the heat pushed onlookers back across the road. Just then a column of flames shot high into the sky, forming a “fire whirl,” according to firefighters. A mini-tornado formed, sending dust and ash swirling across the road.
“Slow and steady!” shouted a firefighter, one of several from the nearby Ironwood Hotshots crew.
He urged his colleagues to form a “chain” along the length of the blaze. In firefighting terms, a “chain” equates roughly to 66 feet between each man.
Moments after the fire whirl subsided, a large jackrabbit zipped across the field, looking to escape the flames. Smaller cottontail rabbits slalomed their way through the fire lines as well.
It took 30 minute for the flames to consume the entire stretch of buffelgrass.
The weed grows quickly, according to McDonald. It quickly recovers from fire, growing stands large enough to help fuel up to two wildfires per decade.
It’s an especially troubling reality for the caretakers of Saguaro National Park to the east of the Reservation Road fire site.
The stands of saguaros in the national park are hundreds of years old. As buffelgrass moves further into the forest of slow-growing cacti, the risk of fire increases. The protected cacti simply cannot recover from a major fire, McDonald said.
Soon, the Ph.D. student hopes to publish his findings about buffelgrass and the way it burns. Eventually, his research could help firefighters more effectively control the weed-fueled fires and protect the mighty saguaros and the homes that continue to crop up deeper and deeper into the region’s desert areas.
His work, however, seems likely to get harder.
In Texas, ranchers love buffelgrass. “They sell the stuff at stores,” McDonald said.
It grows as quickly as grazing cattle can chomp it down. The same holds true in Mexico.
Some researchers even have begun developing a strain of the grass, known as “frio,” that can withstand colder conditions. Seeds of the newer strain likely will spread throughout the region over time, according to many experts.
McDonald yanked a clump of the stuff out of the ground last Wednesday, revealing its dense network of hardy roots.
Stamping out the noxious weed, he said, requires diligent effort. Effort alone, however, may not be enough to halt its inexorable march across the desert.
WHAT IS IT? Native to most of Africa, buffelgrass is a noxious perennial weed that was introduced to the Sonoran Desert in the 1930s and 1940s as a means of erosion control. It remains popular as forage for grazing livestock in Mexico and Texas.
WHY IT’S A PROBLEM: It spreads quickly, filling in bare areas in the desert and taking water from native plants. It has a low ignition threshold, so a buffelgrass-fueled fire can spread quickly. Native plants, especially saguaro cacti, cannot recover from frequent fires.
CONTROLLING IT: Herbicides only work if the grass is actively growing. Manual removal — pulling the weed out by its roots — works best.