Executive Order 13112 defines an invasive plant as a non-native species “whose introduction does or is likely to cause economic or environmental harm or harm to human health.” The National Invasive Species Management Plan concentrates on non-native plants which do not provide a benefit to society which is equal to or greater than any negative impact.
Invasive plants may be introduced to the new environment deliberately through horticulture or agriculture, or through accidental plant transfer or seed dispersal. Plants which escape domestication are known as feral plants. If a feral plant successfully reproduces and starts to displace native plants and the ecosystems they support, it may be classified as invasive. Introduced plants which are kept under control are not considered invasive.
When determining whether to include a plant on the invasive plant list, the local environment and ecosystems are important. The federal noxious weed list includes both native and invasive species, but concentrates on plants which are widely distributed throughout the U.S. mainland. State invasive plant lists also include plants which are invasive in ecosystems specific to that state.
The specific criteria for including a plant on a state invasive plant list vary from state to state. For example, California rates its non-native plants based on 13 criteria which are divided into 3 sections: ecological impacts, invasive potential, and ecological distribution. States from the same geographical region may work together to establish common ratings across political boundaries.
Georgia, New Jersey, New York, and Rhode Island do not keep separate invasive plant lists. However, they do compile information and keep track of the status of invasive plants.
Although native plants can be a nuisance and even cause environmental damage, they are not normally classified as invasive. However, a native plant which is no longer kept in balance by its native ecosystem may become invasive. For example, a change in climate or the loss of a major grazing species may allow a native plant to escape its natural controls and spread to new ecosystems.
Even when a plant meets the criteria for being considered an invasive plant, it may not be included on a state’s invasive plant list. Feral generations of genetically modified canola have been found on uncultivated land in the United States, and have crossbred with wild species. This form of canola is highly likely to become an invasive plant, but it would be difficult to amend the existing noxious weed legislation to include it.