There are several characteristics that describe invasive species.

Escape from natural enemies

When a plant is introduced to a new area the fauna that kept the plant from growing uncontrollably in its place of origin are not present in the new environment. In some instances, the native species adapt quickly to a non-native plant and start to eat or otherwise use the plant in a way that keeps it under control. In other instances the non-native plant is not consumed or otherwise used, giving it a potential advantage over the native plants.

For example, the non-native chokecherry trees, European bird cherry and Canada red (Prunus padus and virginiana), are spreading throughout the forests in Anchorage, Fairbanks, and around other communities with planted trees. The plant is toxic to moose if too much is consumed. The lack of herbivore to consume the tree keeps it from experiencing the browsing pressure that results in a broomed architecture that other native tree species experience.

Graph ratio of trees broomed by moose

When moose consume trees they prefer to eat they can affect the architecture of the tree causing it to look like a broom. This shape is indicative of over browsing and is not good for tree health. Species that are less palatable to moose (alder and chokecherry) are able to spread more successfully than trees over browsed by moose.

Multiple Modes of Reproduction

Plants reproduce by seed (sexually and/or asexually), and vegetatively with clones that arise from creeping roots (rhizomes), above ground runners (stolons), and/or stem fragments. Species like orange hawkweed are in part successful because they can reproduce by many of these methods.

field with orange hawkweed

Orange hawkweed is able to reproduce by seed with or without sexual reproduction. Orange hawkweed also reproduces by stolons and rhizomes. Even a small fragment or rhizome that breaks off the rest of the roots can form a new plant. Shown here is a remote infestation of orange hawkweed near Karluk lake on the Kodiak archipelago. Photo courtesy Leslie Kerr US Fish and Wildlife Service.

Germinate prolifically

Some plants produce enormous amounts of seed, and may do so multiple times in the same growing season. High seed output makes for a seed bank in the soil that can overwhelm other species the next season.

a yellow flower a purplish flower with green leaves

Narrowleaf hawksbeard and hempnettle produce an abundance of seed, and can do so multiple times in the same season.

Able to disperse long distances

Whether by wind or barb stuck to an animal plants that can disperse for long distances spread rapidly.

High seed viability and persistence

Some plants produce seed that is highly viable, meaning nearly all their seeds produce new plants. When high seed viability is combined with dormancy that can last many years plants are able to germinate when the right conditions trigger the seed to sprout. These traits can make for a persistent population that lasts for many years even when actively controlled.