From the Sacramento Bee:
It doesn't take a botanist or even a nature nut to identify yellow star thistle along a favorite hiking trail. Every outdoor enthusiast has felt the familiar stab of introduction.
The invasive weed can reach 6 feet high with stiff limbs that seem to sprout daggers from every pore. Each arm is topped with a crown of inch-long spikes – and one showy yellow flower – that seem perfectly placed to stab a shoulder or poke an eye.
Yellow star thistle is public enemy No. 1 in the California weed world, found in every county but one and covering as much as 14 million acres. It is blamed for altering native landscapes, turning meadows into deserts and even killing horses, which are uniquely vulnerable to a toxin in the plant's leaves.
"Yellow star thistle is kind of like the state weed," said Doug Johnson, executive director of the California Invasive Plant Council, a nonprofit research group. "There's tons of it."