For many of the most troublesome cropland weeds there is little hope of wide eradication, but cultural farming practices combined with the use of selected herbicides offer reasonable control. Noxious weeds are spreading at an alarming rate across the Western United States and California is no exception. Weeds typically spread through seeds dispersed in a variety of ways. Wind, water, animals, machinery, and people carry seeds from one location to another. Because society has become increasingly mobile, weed seeds can and do travel great distances quickly.
Weeds usually become established and advance along highways, roads, trails, and river corridors.
Some noxious weeds, such as purple loosestrife, a major threat to California's wetlands and riparian areas, have been spread through ill-advised horticultural and home garden plantings. Others have been inadvertently introduced through planting of contaminated crop seeds or spread through the feeding of weed seed contaminated livestock forage. Yellow starthistle was estimated to infest 4 million acres in California in 1973. Today, it is established on, and severely degrading, over 12 million acres and continuing to expand. Weeds left unchecked can spread at a surprising 14 percent per year.