In the United States, the legislation that defines a noxious weed is the Federal Noxious Weed Act, 1974. It defines a noxious weed as,

any living stage (including seeds and reproductive parts) of a parasitic or other plant of a kind which is of foreign origin, is new to or not widely prevalent in the U.S., and can directly or indirectly injure crops, other useful plants, livestock, poultry or other interests of agriculture, including irrigation, navigation, fish and wildlife resources, or the public health (United States Congress 1974).

The Act further differentiates between a noxious weed and an "undesirable plant" as a,

species classified as undesirable, noxious, harmful, exotic, injurious, or poisonous under state or federal law, but not including species listed as endangered by the Endangered Species Act, or species indigenous to the area where control measures are to be taken (United States Congress 1974).

Under the Act, the Secretary of Agriculture has the authority to prohibit the importation and interstate transportation and sale of species that the Secretary has deemed noxious through actions such as inspection and quarantine. The Secretary is allowed to seize, treat, destroy and dispose of items that have been contaminated with a noxious weed.

This Act does not invalidate any state or local laws regulating noxious weeds. States are encouraged to have parallel legislation and to add species that may only be noxious within their areas. Most states have parallel legislation as a part of their agricultural regulation, and many defer to the federal list and regulatory language. For example, Michigan law regulates the sale of certain noxious weeds classified as prohibited (the plant can not be sold or grown in the state) or restricted (plants that already occur in the state, but are "nuisances or economically detrimental;" Michigan Department of Agriculture). While, North Carolina classifies regulated noxious weeds into classes: Class A, "any noxious weed on the Federal Noxious Weed List or any noxious weed that is not native to the State, not currently known to occur in the state, and poses a serious threat to the State; Class B, " any noxious weed that is not native to the State, is of limited distribution statewide, and poses a serious threat to the State; and Class C, " any other designated noxious weed" (North Carolina Department of Agriculture). The Act also allows federal agencies to cooperate with state agencies to control the spread of noxious weeds.

References

Federal Noxious Weed Program. United States Department of Agriculture Animal and Plant Health Inspection Service.
http://aphisweb.aphis.usda.gov/ppq/weeds/weedsrisk99.html

Filbey, M., Kennedy, C., Wilkinson, J. and J. Balch. 2002. Halting the Invasion: State Tools for Invasive Species Management. Environmental Law Institute, Washington D.C.: 112 pp.

Michigan Department of Agriculture. "Prohibited and Restricted Weeds."http://www.michigan.gov/mda/0,1607,7-125-1569_16993-11250--,00.html

North Carolina Departmentment of Agriculture. "Noxious Weed Regulations." http://www.agr.state.nc.us/plantind/plant/weed/nxwdrg.htm

United States Congress. 1974. Federal Noxious Weed Act. 2801-2814.