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
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
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.
Idaho, skeletonweed increased from a few plants in 1954 to
4 million acres in 1995.
Northern California, yellow starthistle increased from 1 million
acres in 1981 to 10 million acres in
Of the approximately 350 million acres of
federal public lands in the West, more than 90 percent are
not infested with exotic weeds - yet.