Purple Loostrife (Lythrum salicaria) is considered a noxious weed that invades wetland areas. It is considered invasive in all lower 48 US states. Photo credit: Ron Schott Flickr CC BY-NC-SA 2.0
Burning bush also called Winged euonymus (Euonymus alatus), is considered an invasive shrub in some parts of the US. Photo credit: Susan Buffler
Norway maple (Acer platinoides) an invasive, yet widely popular, street and lawn tree readily reseeds itself. Photo credit: geneva_wirth Flickr CC BY-NC 2.0
English Ivy (Hedera helix), a noxious weed, takes over a tree in Virginia. Photo credit: Miles Grant Flickr CC BY-NC-ND 2.0
An invasive plant is " an alien species (non-native) whose introduction does or is likely to cause economic or environmental harm or harm to human health.”
Highly invasive plants that are threats particularly to agricultural production and clog waterways are considered 'noxious'. These plants are highly regulated and are illegal to buy, sell, or grow.
According to the United States Environmental Protection Agency (EPA) invasive and noxious weeds are bad for the following reasons:
Invasive plants were brought to the United States for ornamental reasons, as forage for livestock, and for soil stabilization. Many of these plants arrived accidentally on ships or other means of transport and are spread by people, birds, and other animals accidentally carrying seeds on clothing and fur.
We tend to think of invasive plants as undesirable weeds, however, many popular landscape plants are considered invasive.
Some states have made certain landscape plants illegal to sell. For instance, the state of New Hampshire has banned the sale of burning bush, Norway maple, and Japanese barberry.
Some plants may be considered more invasive in some parts of the country than others. For instance, water hyacinth does not survive freezing winters but is invasive in the southern US. Some invasive plants can be controlled with time and effort.
The National Arboretum recommends following these steps to reduce the amount of non-native invasive species in the landscape:
Vermont - Invasive Plants Considered for Quarantine Rule