Native plants evolved over millions of years to fill unique ecological niches. What we know as weeds today (non-native, ecologically damaging plants) did not exist in the wilderness then. These plants developed in and are native to other countries. Like our native plants, they are kept in check in their native environment by insects or diseases and by competition with other species. In order to survive in their native ecosystems, many plants develop characteristics that make them especially hardy.
Early European settlers in North America inadvertently brought weed seeds with them, perhaps in the hay they brought for their animals or in the dirt they used as ballast for their ships, or even in their clothes or bedding. Some activities, such as clearing the land, opened up niches that created places for weeds to grow. Settlers also purposely brought plants from their "home country" to reseed areas, make dye for clothing, and use as ornamental plants.
These plants have spread at an alarming rate because, unlike native species, there are no native insects, fungi, or diseases to control their growth and spread in this country. What began as a handful of plants introduced in the 19th century, now number in the hundreds of millions. Noxious weeds destroy wildlife habitat and forage, threaten endangered species and native plants, increase erosion and groundwater loss, and prevent recreational activities.
Weeds are continuing to spread rapidly in many areas across the country. Weeds spread to an estimated 4,000 acres (over 6 square miles) each day on public lands managed by the Bureau of Land Management and the U.S. Forest Service. They have invaded approximately 17 million acres of public rangelands in the West -- more than quadrupling their range from 1985 to 1995. In northern California, yellow starthistle expanded its range from 1 million acres in 1981 to 10 million acres in 1997. But weeds know no boundaries. They also are spreading on private and park lands. In fact, no one really knows how fast or how far they are spreading.Generally, the term weed is used to describe any plant that is unwanted and grows or spreads aggressively. The term exotic weed describes an invasive unwanted non-native plant. Terms such as invasive weed or noxious weed are used somewhat interchangeably to refer to weeds that infest large areas or cause economic and ecological damage to an area. The term noxious weed has legal ramifications in some states that maintain official lists of noxious weeds. What is considered a weed in one area may not be a weed in another.