The definition of "weed" is always debatable. Traditional definitions include "plants out of place" or "plants that by their presence conflict with management objectives for the site." The definition included here also tries to incorporate the concepts of public land health and sustainability. A weed is defined as "a non native plant that disrupts or has the potential to disrupt or alter the natural ecosystem function, composition and diversity of the site it occupies. It 's presence deteriorates the health of the site , it makes efficient use of natural resources difficult and it may interfere with management objectives for that site. It is an invasive species that requires a concerted effort (manpower and resources) to remove from its current location, if it can be removed at all." This definition is adapted from Noss and Cooperider (1994) and Beck (personal communication).

"Noxious" weeds refer to those plant species which have been legally designated as unwanted or undesirable. This includes national, state and county or local designations. According to the Federal Noxious Weed Law, native plant species are not designated "noxious". Native plant species that may be of a management concern, such as poisonous plants or desert shrub and sub-shrub species are not considered priorities for noxious weed work or funding and are not included on the following list.

All weeds are not of equal importance for management. The focus of this list is on exotic species that are highly invasive in natural systems. Those species that are of a concern in agricultural situations, but do not pose a threat to rangelands, forestlands and wetlands have not been included. Many annual forbs have not been included. Exotic species typically used in range restoration which are known to be invasive have not been included. Invasive annual grasses which disrupt natural ecosystem function have been included.