They grow unchecked by natural predators and enemies such as insects or diseases. In their native environment these forces prevent the weeds from taking over. A common characteristic of all noxious weeds is their aggressive, competitive behavior. Typically, they steal precious moisture, nutrients, and sunlight from surrounding plants. Some wetland weeds even rob waterfowl and mammals of their food sources, nesting areas, and access to water which they need for protection from predators.
Noxious weeds establish themselves in soil disturbed by construction, travel, recreation, etc.
Then wildlife, livestock, machinery, recreational vehicles, people, wind, and water transport seeds from existing noxious weed infestations to new sites. Once established, noxious weeds have a tremendous capacity to invade adjacent, undisturbed natural plant communities. This is particularly true in the drier moisture regimes at lower elevations. These areas most closely resemble the conditions in Eurasia where virtually all of the alien noxious weeds evolved. The biology of noxious weeds allows them to progress from a small, manageable problem to a huge, economically and environmentally impossible one when given the opportunity.