That's about 4600 football fields every day . Without the normal checks and balances associated with their natural environment, plants that posed few problems in Eurasia became aggressive noxious weeds in the North America. In addition to having few natural predators, many noxious weeds have means by which they adapt and spread rapidly in new environments. For example, production of massive seed numbers with long seed viability, extensive root systems, seeds easily spread by wind or water, thorns, and excretion of compounds that inhibit growth of other plants, are some of the mechanisms that allow noxious weeds to outcompete native vegetation. Thistles develop thorns that protect them from grazing animals.
Most knapweeds produce chemicals that inhibit growth of surrounding vegetation.
Unfortunately, noxious weeds continue to spread, in large part, because people are often oblivious to the consequences of their actions. Weed seeds can be spread to new areas by attaching to vehicles, clothing, camping gear, and pack animals. People may unknowingly spread noxious weeds by picking and taking them home for flower arrangements which are later discarded in non-infested areas. Noxious weeds can be costly to all Arizonans. They can adversely affect our agriculture, economics, environment, wildlife, recreation, and health.