An alien invasion is creeping into the north East Valley in the form of non-native weeds, threatening to endanger the Sonoran Desert ecology as well as the local economy, plant experts say.
Malta star thistle has taken root in the Tonto National Forest just beyond north Scottsdale and Carefree and is spreading onto scenic land that Scottsdale intends to make part of its McDowell Sonoran Preserve, said Patti Fenner, weed control manager for the forest.
The plant is one of the most relentless of about 20 nonnative weeds in Arizona categorized as "noxious" because they kill native vegetation in their path.
Noxious weeds "can destroy whole ecosystems," Fenner said.
By constricting biodiversity, the weeds erode wildlife habitat, depriving animals of food and shelter. They also provide an extremely flammable fuel for wildfires.
Infestation also makes landscapes ugly and often inaccessible, a major blow to any economy supported by the lure of nature and outdoor pursuits.
"People won’t want to camp, hike, hunt, fish or sightsee in areas overrun by noxious weeds, and the weeds ruin scenic vistas," Fenner said.
Malta star thistle has the potential to do such serious damage. Its close cousin, yellow star thistle, has spread over at least 15 million acres of California, growing so thick and thorny that it has forced some state parklands to close.
Fenner is working with local plant scientists to organize and seek funding for a regional weed-control project involving government land management agencies. But experts stress that the battle can’t be won solely by plant specialists armed with herbicides.
"Public education is the big thing. . . . People need to become aware of the problem and learn to identify the plants so they don’t spread them," said John Brock, an Arizona State University ecologist.
A small patch of noxious plants can spread over a few acres a year on its own, but seeds can be dispersed over hundreds and even thousands of acres by using humans for transport, Brock said.
The seeds stick to clothing, pets and even vehicles. That’s primarily how they’re being spread across more than 3,000 acres a day in the West, said Kai Umeda, a weed scientist with the Maricopa County Cooperative Extension Service.
So far, only a few of the most prolific noxious plants have found their way to Arizona.
"Right now the problem is in its infancy here, so we have an opportunity to keep these weeds from getting out of control. . . . Other states have significant infestations that are costing them millions of dollars to control," Umeda said.
The star thistle in the north East Valley is among the most difficult to combat, said Ed Northam, a plant scientist and former weed control officer for the Arizona Department of Agriculture.
It grows long root systems and imbeds seeds deep in the soil. Even when controlled fires are used to clear the surface of the weeds, buried seeds can sprout anew in a few years.
Camelthorn, another noxious weed that has been found in the Valley, is so strong that it often pushes through pavement, cracking roads and sidewalks.
Some of the more aggressive plants threatening Arizona and the Valley’s environment:
Malta star thistle: A yellow-flowered plant that is finding the Sonoran Desert to its liking. It is one of the most voracious in wiping out native vegetation as it spreads.
Yellow star thistle: A yellow-flowered weed similar to Malta star thistle, and even more aggressive. It is spreading in higher elevations, mostly in the Payson area and Pleasant Valley near the town of Young.
Camelthorn: A tall shrub that spreads rapidly along canal banks and roads, and in irrigated fields. By sprouting and growing through asphalt, it has grown through concrete house foundations and has damaged roads. It has been found in the Valley.
Russian knapweed: A purple-flowered shrub. Roots can penetrate soil to a depth of 16 feet. Like other invasive plants, it produces chemicals that inhibit growth of other plants. It is toxic to horses. This plant covers only about five acres in the Tonto National Forest, but it has great potential to spread.
Diffuse knapweed: A shrub with white to purplish flowers. A highly competitive plant that quickly spreads and crowds out other plants. Can be toxic to horses.
Salt cedar: The small tree outgrows native wetland vegetation by exuding salt from its leaves, rendering the stream bank surface unsuitable for germination of native plants such as cottonwood and willow. It is a major threat in New Mexico and a growing one in Arizona.