Most of them arrived in America free of their natural enemies in the Old World and found few native plants equipped to offer much competition to their rapid spread in cultivated soil. They spread westward with the early settlers. In Illinois, sandburs popped up along the Sangamon River in the 1830's. By 1852, black mustard, sheperd's purse, purslane, wild parsnip, mullein, hemp and several foreign grasses were growing near Peoria. Each decade has seen the arrival of others.
Weeds produce large numbers of seeds, usually quite small, often lying dormant for many years.
Different weeds scatter their seeds in different ways. A weed has also been defined as a plant for which we have discovered no use, and certainly some of them seem unpromising. Too many are either stringy, bitter, smelly, prickly, fuzzy, poisonous, or give us hay fever. On the credit side, many have gay flowers, some make fine salad greens, and a few yield useful drugs. Best of all, they give first aid to injured soil and prevent erosion.