Between 1985 and 1995, the spread of weeds - exotic plant species - increased on public rangelands in the West from 4 million acres to 17 million acres. Unlike native species, exotic weeds have no native insects, fungi or diseases to control their growth or spread.


Exotic weeds are spreading at about 4,600 acres per day on federal lands in the West.


Invasive weeds are generally non-native plants introduced to North America from Europe and Asia. Weeds began entering this country in earnest in the mid-1800s, and new arrivals continue to this day.


There are hundreds of exotic species in the West. Some of the most problematic include leafy spurge, spotted knapweed, scotch thistle, purple loosestrife and yellow starthistle.


In Montana, spotted knapweed increased from a few plants in 1920 to more than 5 million acres in 1995.


In Idaho, skeletonweed increased from a few plants in 1954 to 4 million acres in 1995.


In Northern California, yellow starthistle increased from 1 million acres in 1981 to 10 million acres in 1995.


Of the approximately 350 million acres of federal public lands in the West, more than 90 percent are not infested with exotic weeds - yet.


Source: U.S. Department of Interior, Bureau of Land Management