Rationale for regulating weeds
Some plants are particular threats to agricultural producers and/or to native ecosystems. In Alberta, the Weed Act Regulations are based on the following:
Prohibited noxious (species which landowners must eradicate)
Non-native species with currently restricted or local distribution in Alberta that present risks of spreading and causing significant economic or ecological impact. Examples: nodding thistle, (Carduus nutans), yellow clematis (Clematis tangutica).
Non-native species not currently established in Alberta but that occur in neighbouring jurisdictions, cause significant economic or ecological impact in those jurisdictions, and are well adapted to Alberta conditions. Examples: yellow star-thistle (Centaurea solstitialis), salt-cedar (Tamarix ramosissima).
Noxious (species which landowners must control to prevent from spreading)
Non-native species already widely distributed in the province that have significant ecological or economic impact, and that can spread easily from existing infestations onto adjoining properties (e.g. those with windblown seed or creeping roots). Examples Canada thistle (Cirsium arvense), leafy spurge (Euphorbia esula).
Non-native species that are relatively easily controlled when a few individuals are found but that can easily get out of hand if left uncontrolled, and can have significant impacts when abundant Examples: scentless chamomile (Tripleurospermum perforatum).
Exotic species that have been long established in Alberta and have not shown significant ecological or economic impact. Examples: knawel (Scleranthus annuus), cypress spurge (Euphorbia cyparissias).
Exotic species that are found virtually everywhere in the province, so that it is too late to prevent further spread. Examples: stinkweed (Thlaspi arvense), wild buckwheat (Polygonum convolvulus).
Elaboration of the reasoning behind these classifications is expected to be posted on the web by the Pest Surveillance Branch, Alberta Agriculture.
In addition to agricultural and ecological concerns, some species may need to be regulated to ensure that neglect by some landowners does not have adverse impacts on others. The federal Controlled Drugs and Substances Act regulates Cannabis sativa and Papaver somniferum, for instance. Municipal By-laws to enforce community aesthetic standards (tall grass, dandelions) are common.
Some poisonous plants are often found in urban areas, both in neglected lots and in well-tended gardens. Black henbane (Hyoscyamus niger), foxglove (Digitalis purpurea), Tartarian honeysuckle (Lonicera tatarica, burningbush Euonymous atropurpureus, jimsonweed Datura stramonium, angel's trumpet Datura innoxia, yellow lady's-slipper Cypripedium calceolus, Virginia creeper Parthenocissus quinquifolia, and castor bean Ricinus communis are examples varying from somewhat to extremely toxic to animals, humans, or both. See  for more information.
Urban areas function as staging areas for invasive exotic species. They have extensive abandoned and industrial lands that are often poorly tended. They are points of introduction of new horticultural material. Weed control in surrounding areas is frequently made more difficult by lack of control in cities. While the most contentious weed issue in urban areas is probably unwanted species in lawns, the most serious urban weed issue is neglect of areas where weeds are dominant. An additional concern is that most cities include natural areas which are often the first places where exotic species become naturalized and begin to spread.
Mechanical or ecological approaches to control should be used wherever feasible. Herbicides should be a last resort. When used, the choice of herbicide should minimize collateral damage (including persistence and pollution as well as damage to non-target species). ANPC has adopted the following policy with regard to herbicide use: .