Standing on the east rim of the canyon, the river thousands of feet below, you can see a lot of that ground, and it’s all at risk, particularly from a weed called yellow star thistle.
“It’s nasty stuff,” Talsma says. “It’s just a crying shame.”
... The plant first came to North America from Mediterranean countries in tainted loads of clover seed or alfalfa shipped to California in the Gold Rush days. From there, it has marched steadily east. It bears a pretty yellow flower, but that’s the only nice thing to say about it.
Here's even more pictures, courtesy of UCB.