Yellow Pest Spreads Through Rim Country


The color yellow is generally reserved for all things sunny, bright and happy. But in the case of a weed crowding the landscape in many areas of Rim Country, it may help identify the villain.

The Yellow Starthistle is an annual, parasitic weed that steals water from other plants, causing their demise and increasing the fuel for wildfires. It crowds out native species and can be fatal for horses, if consumed. Livestock cannot consume the spiny weed and once it disperses its seeds, it is nearly impossible to destroy.


Yellow Starthistle (mature)

The weed originated in Europe and was first brought to the United States during the gold rush era of the mid-1800s in California, said Patti Fenner, Tonto National Forest's noxious weed program manager.

California and the Western coastal states have the highest incidence of the weed, but it is quickly spreading in Arizona and can be found in all 48 contiguous states.

"It's a fairly small problem now, but it's beginning to become a large problem, if nothing is done about it," Fenner said. "We know it's just going to explode here if we don't do anything."

The weed lives and completes a full life cycle in a year. The plant sprouts in the winter and its flower shoots in the spring. When the plant dies in the summer, it releases up to 50 seeds per flower, which can remain dormant for up to 10 years.

"The seeds can live as long as 10 years," Fenner said.

The seeds spread through animal or human contact by sticking to fur, pant legs or shoes, by the wind or, as Fenner estimates, much of the population has spread in and around Payson, in drainage systems.

"We've seen it all along (Highway) 87 as it goes through town," she said.

The weed is often ignored when it first grows because it doesn't seem unusual. It is low to the ground and looks like a normal, green weed. But if ignored, the weed can grow up to three-feet tall, branches out extensively and has taproots that stretch far into the soil.

The penetrating taproots reach water that native plants cannot reach and monopolizes the available water.

"It makes it so that nothing else can grow," Fenner said.

The weed blooms yellow flowers that look like daisies, she said. And perhaps the most noticeable characteristic is the plants' inch-long spines underneath the flower blossom.

Erosion is a major, long-term problem caused by the Yellow Starthistle. Native plants die from the weed-imposed drought and the landscape is left to erode.

The noxious weed also causes Chewing Disease in horses, which will kill them.

Various methods to kill the weed include, pulling the entire plant before it has dispersed its seeds, using herbicides at certain stages in its life cycle or burning it.

"You have to manage the seeds," Fenner said.

More information is available on the Tonto National Forest Web site at Click the "natural resources" or "invasive weeds" sections.

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