What Makes A Plant Invasive?

People have been moving plants across the North American landscape since the beginning of plant cultivation in pre-Columbian times. Non-native ornamental plants began appearing in nursery catalogs in the mid-nineteenth century. As humans became more mobile, and gardens became a sign of civilized life, introduced plants spread across the North American landscape. In most cases, introduced plants have been both useful and non-invasive. The term "invasive plant" is used to describe plants that aggressively compete with, and displace, locally adapted native plant communities.

A small percentage of introduced plants are responsible for the majority of invasive plant problems. In the United States, introduced species account for about 65% of the total weed flora (Westbrooks, 1998). However, only about 675 out of nearly 5000 introduced plant species now naturalized are estimated to be highly invasive and problematic. In New York State, about 3,000 plant species have been identified, with just over 1,100 being non-native (1)(2), and only a small sub-set of the 1,100 are considered invasive.

Invasive plants have many of the same aggressive growth attributes. When left unmanaged they can invade habitats and crowd out or displace native plants. Currently, the U.S. spends an estimated $4 billion dollars eradicating invasive species, and this figure does not quantify the loss of biodiversity and native habitat.

Common Characteristics of Invasive Plants

• Habitat generalists; able to invade a range of sites
• Primarily from temperate latitudes of Eurasia
• Early germination; leaf out before native plants
• Out-compete native plants through shading and nutrient competition
• Few natural predators here
• Adaptations such as toxins or thorns that render them unpalatable
• Reproduce both sexually and vegetatively
• Long flowering and fruiting periods
• Pollination by wind or by generalist pollinators
• High reproductive outputs
• Long-distance dispersal (e.g., birds, water, construction equipment)
• Ability to store large amounts of food in roots and rhizomes
• A tendency to form large monotypic stands

(1) "A Review of the Biology and Ecology of Three Invasive Perennials in New York State", Leslie Weston, Jacob Barney and Antonio DiTommaso, 2005

(2) Westbrooks R G Invasive plants, changing the landscape of America: Fact book. 1998. Federal Interagency Committee for the Management of Noxious and Exotic Weeds (FICMNEW). Washington DC. 109pp.