Toil and Trouble: Weed Risk Assessment

Contributors: Kerrie Kyde and Sylvan Kaufman
For more information, contact: MD Department of Agriculture- Plant Protection and Weed Management,

Toil and Trouble: Weed Risk Assessment

Weed Risk Assessors conferring over WRA answers. Source: Kerrie Kyde.

Over the course of two days in early March, 30 volunteers toiled, argued, and persevered through the process of assessing three different plant species for invasiveness. They dug up peer-reviewed papers, unearthed state government reports, scrolled through gardening website comments and downloaded maps of global species locations. From this material, they extracted the answers to numerous questions about their species’ establishment and spread, and its impacts, and examined what parts of the United States would be susceptible to invasion by their assigned species. This dedicated group was learning to use the US Department of Agriculture Plant Protection and Quarantine Weed Risk Assessment (WRA), which the Maryland Department of Agriculture uses to determine the likelihood that a given plant will be invasive in Maryland.

In 2012, the Maryland Legislature passed the Maryland Invasive Plant Prevention and Control law. The law established an 11-member body called IPAC, the Invasive Plant Advisory Committee, to assist the Secretary of Agriculture in assigning regulatory status to invasive ornamental terrestrial plants. The law required IPAC to adopt a risk assessment tool for use in this process; IPAC chose the WRA. Three IPAC members traveled to North Carolina in 2012 to learn to use the WRA. It soon became clear that more trained assessors were needed to handle the volume of plant assessments. This spring, Dr. Tony Koop, one of the originators of the WRA, and Dr. Sylvan Kaufman, an ecological consultant and author of a well-known book on invasive plants, created a streamlined training for new assessors, consisting of three webinars and a two-day workshop.

The WRA consists of 94 questions about a target plant species, divided into four risk factors: establishment and spread (how it grows and reproduces), impacts (what effects it has on natural, human and agricultural production systems), geographic potential (where it can live), and entry potential (how it can get here). As assessors answer each of these questions, they also note how sure (or not) they are of their answers, based on the quality and quantity of the evidence. A completed WRA gives a probability that a plant will be a major or minor invader, or not invasive at all, and whether there is a high or moderate risk of invasion.

Toil and Trouble: Weed Risk Assessment

One of the graphs produced in the assessment process, this shows how invasive the assessed species (black square) is in comparison with 204 species used to validate the original WRA. This example species appears in the High Risk section of the graph.

IPAC uses the WRA results to do further analysis to assign a regulatory rating to an assessed species of either Tier 1 or Tier 2. Tier 1 plants may not be purchased, sold, transferred, transported or introduced in Maryland. Tier 2 plants may still be sold, but must be clearly and brightly labeled as Tier 2 – invasive – plants. The sales restrictions and signage requirements are aimed to prevent or discourage further introductions of these invasive species through ornamental plantings. Currently, Tier 1 plants include fig buttercup, Amur honeysuckle, and shining geranium, among others. Some of the Tier 2 plants are burning bush, heavenly bamboo and wisteria. For the entire list, see

Prevention is the least expensive, most effective way of minimizing the ecological, economic and human health impacts of invasive plants. This new group of volunteers will speed the assessment and regulatory process, helping to prevent new introductions, and filling another gap in the fight against invasive species.

Toil and Trouble: Weed Risk Assessment

Sign that must accompany plants for sale with Tier 2 regulatory status.
Source: Maryland Department of Agriculture: Maryland Invasive Plants Prevention and Control.

For more information about Maryland’s Invasive Plant Regulations, Weed Risk Assessments, and the Invasive Plant Advisory Committee, visit: Maryland Department of Agriculture: Maryland Invasive Plants Prevention and Control.