United States 

Department of 

Agriculture

Animal and Plant 

Health Inspection 

Service

June 30, 2011

Weed Risk Assessment for NonHerbicide Tolerant and Herbicide 

Tolerant Types of Poa pratensis L., 

Kentucky Bluegrass

Kentucky bluegrass and clover by John D. Byrd, Mississippi State University

Agency Contact: 

Plant Epidemiology and Risk Analysis Laboratory

Center for Plant Health Science and Technology

Plant Protection and Quarantine

Animal and Plant Health Inspection Service

United States Department of Agriculture

1730 Varsity Drive, Suite 300

Raleigh, NC 27606
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 ii

Executive Summary

We assessed the weed risk potentials of non-herbicide tolerant (hereafter, ―nongenetically engineered‖, or non-GE) and herbicide tolerant (hereafter, ―GE‖) types 

of Poa pratensis L., Kentucky bluegrass, to determine whether or not the species

(which encompasses both types) is a candidate for listing. The Plant Protection 

Act (7 U.S.C. § 7701-7786, 2000) defines a noxious weed as ―any plant or plant 

product that can directly or indirectly injure or cause damage to crops...,

livestock, poultry, or other interests of agriculture, irrigation, navigation, the

natural resources of the United States, the public health, or the environment.‖ To 

determine that, we used a weed risk assessment tool that was validated using data 

on 204 plant species with known noxious behavior in the United States. The 

model predicts weed risk potential based on scores for Impact Potential (i.e., 

harm), and Establishment/ Spread Potential. The model was developed to assess 

basic invasive/weedy potential, however, and not to distinguish between plants

with different genotypes. Except for being herbicide tolerant, we have no evidence 

that the growth and behavior of the GE type will differ significantly from that of 

the non-GE type.

The model measures Impact Potential with 18 scored questions about the types of 

damage to crops or the environment that a species may cause (e.g., reducing crop 

yields, requiring control, changing community structure or function, adverse 

effects on human property). We assess Establishment/Spread potential using 27 

scored questions on species biology and history (e.g., invasiveness elsewhere, 

dispersal mechanisms). Our uncertainty about the risk scores discussed below was 

small because of the abundance of information about the biology, behavior, and 

performance of Kentucky bluegrass in the United States.

The Establishment/Spread Potential scores for the two types of Kentucky 

bluegrass were 23 for the non-GE type, and 24 for the GE type, which are high on 

the scale of -25 to 32. The one point difference, which is not significant, is due 

entirely to the herbicide tolerance of the GE type. 

For Impact Potential, both types scored 2.5 on a scale of 1 to 5.1. That score is 

greater than all scores for non-invasive species we evaluated during model 

validation, but is low or moderate when compared with High risk species: the 

most harmful species (e.g., Canada thistle, Cirsium arvense) have scores of 4 or 

greater. In this case, because the model is qualitative, the score for Impact 

Potential may overestimate the true damage this species can cause. For example, 

although Kentucky bluegrass can act as an agricultural weed, we found very few 

reports for that in U.S. crops. It is also reported to be a weed of production crops 

in only two other countries, despite a wide distribution through Europe and Asia. 

It seems most problematic in grassland systems, yet no states regulate it or have it 

prioritized for control.

Based on those scores, the predictive logistic regression model indicated that both 

the non-GE and GE types have High weed risk potentials. Therefore, the species 

Kentucky bluegrass, P. pratensis (including both types), can be considered for 

regulation as a Federally listed noxious weed. That decision is explained 

separately.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 iii

Table of Contents

Executive Summary........................................................................................ ii

1. Introduction to the PPQ Weed Risk Assessment Process....................... 1

1.1. Authority.............................................................................................. 1

1.2. Background.......................................................................................... 1

1.3. Terminology......................................................................................... 2

1.4. Risk assessment overview.................................................................... 2

2. Guide for Interpretation of WRA Results................................................ 2

3. Literature Cited .......................................................................................... 4

4. Weed Risk Assessment ............................................................................... 5

4.1. Non-GE Poa pratensis L., Kentucky Bluegrass.................................. 5

4.2. GE Poa pratensis L., Kentucky bluegrass......................................... 10

5. Appendices................................................................................................. 14

Appendix A. Logistic regression model formulas.................................... 14

Appendix B. Model cut-off scores............................................................ 15

Appendix C. Secondary screening system................................................ 16

Appendix D. Risk score reference dataset. ............................................... 17
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 1

1. Introduction to the PPQ Weed Risk Assessment Process

PPQ WRA is consistent 

with international 

guidelines

1.1. Authority

PPQ regulates noxious weeds under the authority of the Plant Protection Act (7 

U.S.C. § 7701-7786, 2000) and the Federal Seed Act (7 U.S.C. § 1581-1610, 

1939). A noxious weed is ―any plant or plant product that can directly or indirectly 

injure or cause damage to crops (including nursery stock or plant products), 

livestock, poultry, or other interests of agriculture, irrigation, navigation, the natural 

resources of the United States, the public health, or the environment‖ (7 U.S.C. § 

7701-7786, 2000). Our weed risk assessment guidelines (see below) were 

developed to determine whether or not species are candidates for listing as Federal 

noxious weeds. Such species are generally prohibited or restricted from entering 

the United States or moving through it (interstate). For transparency with 

stakeholders, these species are listed under the Federal Noxious Weed regulations 

(7 CFR § 360, 2010). Except for plant species unlikely to contaminate import or 

export pathways, most Federal noxious weeds are co-listed as noxious weed seeds 

(see 7 CFR § 361, 2010).

1.2. Background

In this document, we assess the weed risk potential of several plant species using 

Plant Protection and Quarantine’s (PPQ) weed risk assessment guidelines (PPQ, 

2009). The weed risk assessment (WRA) process and the predictive model utilized 

are consistent with the general guidance provided by international and North 

American standards for risk assessment (IPPC, 2009: ISPM Nos. 2 & 11; NAPPO, 

2008: RSPM No. 32). The weed risk assessment below contain information 

relevant for the initiation, species categorization, and risk assessment phases. These 

phases correspond to Stage 1 (initiation) and Stage 2 (risk assessment) of risk 

analysis (IPPC, 2009: ISPM No. 2). 

We combine pest 

categorization and pest 

risk assessment

A weed risk assessment can be initiated for any number of reasons, including, but 

not limited to, evaluation for listing or delisting Federal Noxious Weeds or plants 

for propagation which are designated as ―Not Allowed Pending Pest Risk 

Assessment‖ (NAPPRA) (APHIS, 2011). We note the reason for initiation, along 

with other background information, in each assessment. 

One of the phases of risk analysis is categorization of the species of interest, in 

which it is evaluated to determine whether it has the characteristics of a quarantine 

species or a regulated non-quarantine species (IPPC, 2009: ISPM No. 2). The intent 

of this phase is to identify (i.e., screen out) species that clearly do not meet these 

definitions before subjecting them to a potentially lengthy risk assessment process. 

However, because some plants that do not have evidence of spread or impact 

elsewhere later become weeds (IPPC, 2009: ISPM No. 2; Whitney and Gabler, 

2008), PPQ subjects most plants to the full weed risk assessment process to 

evaluate their weed potential based on their inherent biological traits (e.g., Mack, 

1996; Reichard, 2001). Essentially, we combine the species categorization and risk 

assessment phases, and use the risk assessment as a screening tool to categorize the 

potential risk and weed status of the plant. 
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 2

Confounded weed 

terminology

1.3. Terminology

Terminology in the weed/invasive plant literature is confounded, as words such as 

―weed‖ and ―invasive‖ have variable and subjective meanings (Richardson et al., 

2000). Development and validation of the PPQ model required some flexibility in 

terminology, particularly at different phases of the work. As with other studies that 

have developed and/or tested WRA systems (e.g., Gordon et al., 2008; Pheloung et 

al., 1999), we relied on information available in the literature to identify plants 

belonging to three categories of invasiveness: non-invaders, minor-invaders, and 

major-invaders. In this usage, invader broadly refers to a plant’s overall ability to 

spread and cause negative impacts, and reflects two components of risk (IPPC, 

2009: ISPM No. 11). 

In the PPQ WRA system, we evaluate the establishment/spread potential and 

impact potential of a species as two separate risk elements. Under establishment 

and spread we adopt a stricter definition of invasive that refers to a species’ 

capacity to establish and spread throughout a landscape (sensu Richardson et al., 

2000). However, at the end of the PPQ WRA process, we return to the broad usage 

of the term invader because we relate a species’ risk scores back to the dataset that 

was used to develop and test the WRA model. If introduced into the United States,

Low risk plants are likely to become non-invaders, while High risk plants are likely 

to become major invaders. 

Model based on U.S.

plants with known 

behavior

1.4. Risk assessment overview

We developed and validated the WRA process (Stage 2) using 204 plants with 

known weed/invasive (noxious) behavior in the United States (non-invaders, 

minor-invaders, and major-invaders) (manuscript in review). The process consists 

of a weed risk model as well as a secondary screening tool developed to further 

evaluate plants with intermediate risk scores.

WRA process does not 

make policy 

recommendations

We do not use the PPQ WRA process to make policy recommendations. Instead, 

we categorize weed risk and relate a species’ risk scores to the reference dataset of 

species with known invasiveness in the United States. This process results in one 

of three possible conclusions: ―Low risk,‖ ―Evaluate further,‖ and ―High risk.‖ 

While these conclusions are not official policy recommendations, the analytical and 

statistical methodologies behind them support management decisions of allowing 

entry for Low risk species, denying entry for High risk species, and evaluating 

further other species as appropriate. This yields results similar to outcomes reached 

using other weed risk assessment systems (e.g., Pheloung et al., 1999; Reichard and 

Hamilton, 1997). 

Agency does risk 

management separately

PPQ program managers use weed risk assessments to evaluate what Federal action 

may be appropriate. If regulatory action is prudent, program managers evaluate 

which risk mitigation options would reduce risk to an acceptable level. 

2. Guide for Interpretation of WRA Results

In this document, we summarize the results for one or more weed risk assessments. 

For a description of the WRA process and model, or a guide on answering 
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 3

questions used in the assessment, see the PPQ WRA Guidelines (PPQ, 2009). 

Establishment/spread 

and Impact risk elements

Below, we present risk scores for the establishment/spread and impact risk 

elements, along with their mean uncertainty. Risk scores can range from -25 to 32 

and 1.0 to 5.1, respectively, with greater scores indicating greater risk. Descriptions 

with each risk element highlight the risk factors that contributed to that score. We 

used the scores from these two risk elements to characterize the overall risk 

potential of the species and estimate the likelihood that it will be a non-invader, 

minor-invader, or major-invader (see below).

Geographic and entry 

potential are separate

Although we do not use the geographic and entry potentials of a species to estimate 

the overall invasive potential of a plant, these elements are none-the-less important 

components of risk. We report these elements separately so that regional and 

national managers can make appropriate decisions for their jurisdictions. Under 

geographic potential, we report the percent of the United States suitable for species 

establishment based on three climate variables: USDA cold plant hardiness zones, 

Köppen-Geiger climate classes, and ten-inch precipitation bands.. Under entry 

potential we evaluate the likelihood of species entry into the United States. All four 

scores can range from 0 to 1, with higher scores indicating higher risk.

Uncertainty For each of the risk scores described above, we report an index of uncertainty that 

describes the overall level of uncertainty associated with that risk element. The 

index ranges from zero to one, where a one corresponds to maximum uncertainty 

(i.e., all questions answered as unknown). The index considers the uncertainty 

rating given by the analyst to each question (negligible, low, moderate, high, or 

maximum) and the relative weight of each question in the risk element. 

WRA model In the next section of each assessment, we present the results from the WRA model 

and secondary screening (2° screening). The core of the WRA model is a logistic 

regression model (Appendix A) that uses the scores from the establishment/spread 

and impact risk elements to determine the probabilities that a species will be a 

major-, minor-, and non-invader (sensu lato). Because most management decisions

for plants will be to either allow or exclude entry, we used cutoff scores determined 

by Receiver Operating Characteristic (ROC) curve analysis (Appendix B) to 

categorize the overall risk of plant introduction (i.e., ―low risk‖ or ―high risk‖) and 

facilitate management decisions. ROC curve analysis is an analytical tool used in 

decision making that maximizes the predictive ability of a model while minimizing 

false-positive and false-negative errors (Caley and Kuhnert, 2006; Metz, 1978).

Secondary screening of 

species classified as 

“evaluate further”

Species classified as ―evaluate further‖ are species with intermediate risk scores, 

and are subjected to a secondary screening tool (Appendix C). With this tool, we 

examine specific traits that by our analysis were highly associated with plant 

invasive status in the United States. This approach is designed to help resolve the 

risk potential of the species. However, even after secondary screening, some 

species may remain in the ―evaluate further‖ category.

In the discussion section of each assessment below, we briefly review the available 

evidence and report our final conclusion. We also introduce additional information 

that may be relevant to decision-makers.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 4

3. Literature Cited

7 CFR § 360. 2010. Code of Federal Regulations, Title 7, Part 360, (7 CFR §360 -

Noxious Weed Regulations). United States Government.

7 CFR § 361. 2010. Code of Federal Regulations, Title 7, Part 361, (7 CFR §361 -

Importation of Seed and Screenings under the Federal Seed Act). United 

States Government.

7 U.S.C. § 1581-1610. 1939. The Federal Seed Act, Title 7 United States Code § 

1581-1610.

7 U.S.C. § 7701-7786. 2000. Plant Protection Act, Title 7 United States Code § 7701-

7786.

APHIS. 2011. Importation of plants for planting: Establishing a category of plants for 

planting Not Authorized for Importation Pending Pest Risk Analysis: Final 

rule. Federal Register Vol. 76(103):31172-31210. Last accessed June 16, 

2011, http://www.gpo.gov/fdsys/.

Caley, P., and P. M. Kuhnert. 2006. Application and evaluation of classification trees 

for screening unwanted plants. Austral Ecology 31(5):647-655.

Gordon, D. R., D. A. Onderdonk, A. M. Fox, R. K. Stocker, and C. Gantz. 2008. 

Predicting invasive plants in Florida using the Australian weed risk 

assessment. Invasive Plant Science and Management 1:178-195.

IPPC. 2009. International Standards For Phytosanitary Measures, 1 to 32 (2009 

edition). Food and Agriculture Organization of the United Nations, 

Secretariat of the International Plant Protection Convention (IPPC), Rome, 

Italy. 434 pp International standards for phytosanitary measures: 1-31. 

International Plant Protection Convention (IPPC) and the Food and 

Agriculture Organization of the United Nations, Rome.

Mack, R. N. 1996. Predicting the identity and fate of plant invaders: emergent and 

emerging approaches. Biological Conservation 78:107-121.

Metz, C. E. 1978. Basic principles of ROC analysis. Seminars in Nuclear Medicine 

8:283-298.

NAPPO. 2008. NAPPO regional standards for phytosanitary measures: RSPM#32: 

Pest risk assessment for plants for planting as quarantine pests. North 

American Plant Protection Organization (NAPPO), Ottawa, Canada. 16 pp.

Pheloung, P. C., P. A. Williams, and S. R. Halloy. 1999. A weed risk assessment 

model for use as a biosecurity tool evaluating plant introductions. Journal of 

Environmental Management 57:239-251.

PPQ. 2009. Weed-initiated pest risk assessment guidelines (v. 6.0). United States 

Department of Agriculture, Animal and Plant Health Inspection Service, 

Plant Protection and Quarantine (PPQ), Raleigh, NC, U.S.A.

Reichard, S. 2001. The search for patterns that enable prediction of invasion. Pages 

10-19 in R. H. Groves, F. D. Panetta, and J. G. Virtue, (eds.). Weed Risk 

Assessment. CSIRO, Collingwood, Australia.

Reichard, S. H., and C. W. Hamilton. 1997. Predicting invasions of woody plants 

introduced into North America. Conservation Biology 11(1):193-203.

Richardson, D. M., P. Pysek, M. Rejmanek, M. G. Barbour, F. D. Panetta, and C. J. 

West. 2000. Naturalization and invasion of alien plants: Concepts and 

definitions. Diversity and Distributions 6:93-107.

Whitney, K. D., and C. A. Gabler. 2008. Rapid evolution in introduced species, 

'invasive traits' and recipient communities: challenges for predicting invasive 

potential. Diversity and Distributions 14(4):569-580.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 5

4. Weed Risk Assessment

4.1. Non-GE Poa pratensis L., Kentucky Bluegrass

Background Family: Poaceae

Information Initiation: The International Center for Technology Assessment and the Center for 

Food Safety (ICTA/DFS) formally petitioned APHIS requesting that the 

Agency list turf grass types of creeping bentgrass, Agrostis stolonifera, and 

Kentucky bluegrass, Poa pratensis, that had been genetically engineered (GE) 

for tolerance to the herbicide, glyphosate, as Federal Noxious Weeds under the 

noxious weed provisions of the Plant Protection Act. APHIS denied this request 

for creeping bentgrass based on a 2003 WRA. APHIS determined that GE 

creeping bentgrass did not meet the criteria to be regulated as a Federal Noxious 

Weed. The decision was subsequently challenged in Federal court. 

The United States District Court for the District of Columbia vacated APHIS’ 

denial and remanded the matter back to the Agency. In response to the Court, 

APHIS is publishing a WRA for both the non-GE and GE types of Kentucky 

bluegrass. 

Foreign distribution: Poa pratensis is a common and widely cultivated species, and 

includes several infraspecific taxa (FNA Editorial Committee, 2007). Most 

infraspecific taxa are native to Europe and temperate Asia, although some also 

range south into northern Africa and northern India (NGRP, 2011). In general, 

P. pratensis is widely established throughout the temperate regions of the world 

(NGRP, 2001; FNA Editorial Committee, 2007).

U.S. distribution & status: Two subspecies of P. pratensis (subspp. alpigena and 

colpodea) have a circumpolar distribution and are native to North America. 

Subspecies alpigena ranges into the contiguous United States, but is not very 

common (FNA Editorial Committee, 2007). Most cultivated forms of P. 

pratensis resemble subsp. irrigata, pratensis, and angustifolia (FNA Editorial 

Committee, 2007). Thus, the naturalized populations recorded in several 

databases likely represent non-native taxa (Kartesz, 2010; Nature Serve, 2011; 

NGRP, 2011).

WRA area: The area considered here is the United States and its territories and 

possessions.

4.1.1.Analysis of Non-GE Kentucky Bluegrass

Establishment/Spread 

Potential

Risk score = 23 Mean uncertainty = 0.08

Kentucky bluegrass spreads aggressively (Nature Serve, 2011; Weber, 2003). It has 

escaped cultivation into native prairies (Grant et al., 2009). Still, we found very few 

reports of it invading managed agricultural systems (e.g., Bridges, 1992; see below).

Impact Potential Risk score = 2.5 Mean uncertainty = 0.17

That score is greater than all those for non-invasive species in the validation dataset, 

but is low or moderate when compared with High risk species. The most harmful 

species (e.g., Canada thistle, Cirsium arvense) have Impact Potential scores of 4 or 

greater. In this case, because the model is qualitative, the score for Impact Potential 

may overestimate the damage the species could cause, for the following reasons.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 6

Although Kentucky bluegrass can act a weed in U.S. agricultural systems, that is 

only reported by Bridges (1992) for a total of three crops—apples, spearmint, and 

peppermint—out of the hundreds of crops grown in the United States. In apples, we 

have found no evidence of it being a significant weed (e.g., listed but with no 

indication of severity in Rifai et al., 2002 [Nova Scotia, Canada]). Another report 

has volunteer bluegrass as a weed in bluegrass seed plots (Lee, 1978). Given both 

the long history of Kentucky bluegrass in the United States and the paucity of 

reports about weedy behavior in U.S. crops, it seems highly unlikely that it is a 

significant weed in managed agricultural systems in the United States.

Furthermore, it is reported to be a weed of production crops in only two other 

countries, despite a wide distribution through Europe and Asia. It is listed as an 

arable weed in Japan (Holm et al., 1979; Morita, 1997), but is not regulated or 

heavily controlled (NIES, 2011). In Australia, it is an agricultural weed, but was not 

ranked as either a noxious weed, or a serious, high impact weed (Randall, 2007).

Most of the evidence for invasiveness by Kentucky bluegrass comes from grassland

(esp. prairie) ecosystems, as mentioned above for Establishment/Spread Potential. It 

may become problematic particularly when such areas are not managed properly

(e.g., Grant et al., 2009). When Kentucky bluegrass invades native prairies, the 

recommended management is typically to apply general, adaptive (i.e., location 

specific) programs to control multiple species, not just Kentucky bluegrass (e.g., 

Grant et al., 2009; Hendrickson and Lund, 2010). Preferred methods of control are 

grazing and controlled burns (FEIS, 2009). Fire is usually a successful method of 

control for Kentucky bluegrass in these systems (Sather, 1996). When herbicides are 

used in such situations, invasive Kentucky bluegrass populations can be reduced to 

low levels in one or two seasons (Waller and Schmidt, 1983). 

Kentucky bluegrass has demonstrated value in some natural areas, however. For 

example, it can be an important part of the diet of wildlife (e.g., elk, bighorn sheep)

(FEIS, 2009), and Kentucky bluegrass is one of the most important forage species 

for cattle and sheep summering in mountain meadows in eastern Oregon (McInnis

and Vavra, 1986).

It is also important to note that Kentucky bluegrass is a recommended pasture grass 

in some eastern and western states (e.g., Tregoning, N/D; Wiedmeier et al., 2005), 

and of course is widely grown as a turf grass species in all 50 United States. It is a 

component of many grass mixes sold by major home improvement and gardening 

centers (pers. observation). Finally, no states currently regulate it, and we have 

found no evidence that any have prioritized it for control.

Geographic Potential Kentucky bluegrass is a cool-season grass with a high adaptive potential as evidenced 

by known occurrences spanning all USDA hardiness zones, all rainfall bands 0 to 

100+ inches per year, and all bioclimatic climate classes except polar icecaps (GBIF, 

2011; NAPPFAST, 2008) (Fig. 1). It has naturalized in all states of the United States 

(Kartesz, 2010) [Note: BONAP indicates it has been extirpated from Vermont, but 

we think this is unlikely]. Land conservationists assume that it has occupied most of 

its potential range in the United States (Nature Serve, 2011).
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 7

Entry Potential Kentucky bluegrass has already naturalized in all states of the United States 

(Kartesz, 2010). Therefore, assessment of its entry potential is not necessary.

Figure 1. Expected distribution of Poa pratensis in the entire United States. Alaska and 

Hawaii are shown on the left, and Puerto Rico on the right.

Figure 2. Risk score of non-herbicide tolerant Kentucky bluegrass, incorporating Monte 

Carlo simulation of uncertainty.

a

a

Central vertical and horizontal lines indicate the means of the simulated outcomes. The first 

(inner-most) box contains 50 percent of the outcomes, the second 95 percent, and the third 99 

percent.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 8

Figure 3. Risk score of non-herbicide tolerant Kentucky bluegrass relative to the 

validation dataset.

4.1.2. Results & Conclusion

Model Probabilities: P(Major Invader) = 0.942

P(Minor Invader) = 0.056

P(Non-Invader) = 0.002

Risk Result = High Risk

Secondary Screening = Not Applicable

The result of the weed risk assessment for non-GE Kentucky bluegrass is High risk.

4.1.3. Discussion

Non-GE Kentucky bluegrass is High risk (Figs. 2 and 3), and therefore the species, P. 

pratensis (including the GE type) is a candidate for regulation as a Federally listed 

noxious weed. When compared with other United States major-invaders examined in the 

validation study, however, its Impact Potential score was low to moderate (Fig. 3). We 

discussed above a few reasons why that score may overestimate its true damage 

potential. Most importantly, Kentucky bluegrass clearly does not behave like a 

significant weed in managed agricultural systems in the United States, and this is 

corroborated by the fact that no states regulate it. Finally, Kentucky bluegrass has wide 

use and economic status as a turf grass and pasture species, even in some natural areas. 

We believe these factors are significant.

4.1.4. Literature Cited

Bosy, J. L., and R. J. Reader. 1995. Mechanisms Underlying the Suppression of Forb 

Seedling Emergence by Grass (Poa pratensis) Litter. British Ecological Society 

9(1995):635-639.

Bridges, D.C. 1992. Crop Losses Due to Weeds in the United States – 1992. Weed 

Science Society of America, Champaign, Illinois, USA.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 9

FEIS. 2009. USDA Forest Service-Fire effects information center. USDA Forest Service. 

http://www.fs.fed.us/database/feis/. (Archived at PERAL).

FNA Editorial Committee (ed.). 2007. Flora of North America (V. 24): Magnoliophyta: 

Commelinidae (in part): Poaceae, part 1. Oxford University Press, New York, 

U.S.A. 908 pp.

GBIF. 2011. GBIF, Online Database. Global Biodiversity Information Facility (GBIF). 

gbif.org. http://data.gbif.org/welcome.htm. (Archived at PERAL).

Grant, T. A., B. Flanders-Wanner, T. L. Shaffer, R. K. Murphy, and G. A. Knutsen. 

2009. An Emerging Crisis across Northern Prairie Refuges: Prevalence of 

Invasive Plants and a Plan for Adaptive Management. Ecological Restoration 

27(1):58-65.

Halina, L., and W. Hardot. 2005. Allelopathic effects of Poa pratensis on other grassland 

spp. Allelopathy Journal (2005).

Hendrickson, J. R., and C. Lund. 2010. Plant Community and Target Species Affect 

Responses to Restoration Strategies. Rangeland Ecology & Management 63:435-

442.

Holm, L. G., J. V. Pancho, J. P. Herberger, and D. L. Plucknett. 1979. A Geographical 

Atlas of World Weeds. Krieger Publishing Company, Malabar, Florida, U.S.A. 

391 pp.

Huberty, A. F. 2011. Status of WRA for Kentucky bluegrass from Andrea Huberty, 

APHIS BRS, to Anthony L. Koop, PPQ CPHST 

Kartesz, J. 2010. Floristic Synthesis of North America by Biota of North America 

Program (BONAP), Version 1.0. Biota of North America Program. 

http://www.bonap.org/index.html. (Archived at PERAL).

Lee, W. O. 1978. Volunteer Kentucky bluegrass (Poa pratensis) control in Kentucky 

bluegrass seed fields. Weed Science 26(6):675-678.

McInnis, M.L., and M. Vavra. 1986. Summer diets of domestic sheep grazing mountain 

meadows in northeastern Oregon. Northwest Science. 60(4): 265-270.

Morita, H. 1997. Handbook of Arable Weeds in Japan. Niimura Printing Co., Ltd., 

Tokyo. 128 pp.

NAPPFAST. 2008. Databases used in the NAPPFAST system. NCSU APHIS Plant Pest 

Forecasting System (NAPPFAST). 

http://www.nappfast.org/databases/NAPPFAST%20Databases.htm. (Archived at 

PERAL).

Nature Serve. 2011. Nature Serve Explorer: An online encyclopedia of life [web 

application] Version 7.1. Nature Serve. http://natureserve.org. (Archived at 

PERAL).

NGRP. 2011. Germplasm Resources Information Network (GRIN). United States 

Department of Agriculture, Agricultural Resources Service, National Germplasm 

Resources Program (NGRP). http://www.ars-grin.gov. (Archived at PERAL).

NIES. Invasive species of Japan. Invasive Species Research Team, Center for 

Environmental Biology and Ecosystem Studies, National Institute for 

Environmental Studies, Ibaraki, Japan. 

http://www.nies.go.jp/biodiversity/invasive/DB/detail/80770e.html (Archived at 

PERAL)

Randall, J. M. 2007. The introduced flora of Australia and its weed status. CRC for 

Australian Weed Management, Department of Agriculture and Food, Western 

Australia, Australia.

Rifai, M.N., T. Astatkie, M. Lacko-Bartosova, and J. Gadus. 2002. Effect of two 

different thermal units and three types of mulch on weeds in apple orchards. 

Journal of Environmental Engineering and Science. 1: 331–338.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 10

Sather, N. 1996. Element Stewardship Abstract for Poa pratensis, Poa compressa. The 

Nature Conservancy, Minneapolis, MN. 21 pp.

Tregoning, D. No date.Pasture Renovation and Seeding. Montgomery County 

Cooperative Extension, Maryland. 

http://www.equinestudies.umd.edu/documents/UMSeminar/Tregoning.pdf

(Archived at PERAL)

Waller, S. S., D.K. Schmidt. 1983. Improvement of eastern Nebraska tallgrass range 

using atrazine or glyphosate. Journal of Range Management. 36(1): 87-90. 

Weber, E. 2003. Invasive Plant Species of the World: A Reference Guide to 

Environmental Weeds. CABI Publishing, Wallingford, UK. 548 pp.

Wiedmeier, R.D., D.R. ZoBell, A.C. Young, and P.R. Schmidt. 2005. Yearly Forage 

Production of Irrigated Pastures Grazed by Cow-Calf Pairs as Affected by the

Timing of Sprinkler Irrigation Application. Department of Animal, Dairy and 

Veterinary Sciences, Utah State University, Logan. 

4.2. GE Poa pratensis L., Kentucky bluegrass

Background

Information

Note: Nearly all of the information used in the analyses of both types of Kentucky 

bluegrass is exactly the same, regardless of type.

Family: Poaceae

Initiation: Please see above section for the non-GE type.

Foreign distribution: As discussed above, non-GE Poa pratensis is a common and 

widely cultivated species, and includes several infraspecific taxa (FNA Editorial 

Committee, 2007). Most infraspecific taxa are native to Europe and temperate 

Asia, although some also range south into northern Africa and northern India 

(NGRP, 2011). In general, P. pratensis is widely established throughout the 

temperate regions of the world (NGRP, 2001; FNA Editorial Committee, 2007).

U.S. distribution & status: The GE type of P. pratensis is not known to have 

established in the United States. For the distribution of the non-GE type, see 

above. 

WRA area: The weed risk assessment area considered here is the United States and 

its territories and possessions.

4.2.1.Analysis of GE Kentucky Bluegrass

Establishment/Spread 

Potential

Risk score = 24 Mean uncertainty = .08

Characteristics which contributed to a relatively high establishment and spread risk 

score included the same factors as for non-GE Kentucky bluegrass (4.1.1). The only 

difference between the two is that the GE type scored one more point because it is 

herbicide tolerant. We have no reason to expect these would be significantly 

different (greater or lesser) for the GE type.

Impact Potential Risk score = 2.5 Mean uncertainty = 0.17

Based on our assessment questions the GE type had the same score as the non-GE

type. Although one less herbicide would be available for the control of GE Kentucky 

bluegrass, in general it only has a low/moderate rating for difficulty of management 

(Nature Serve, 2011). We do not think tolerance to one herbicide would justify 

increasing that Nature Serve (2011) rating to high.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 11

As discussed above, the Impact Potential score may overestimate the true damage

that the species can cause. Kentucky bluegrass seems likely to be most problematic 

in natural grassland ecosystems, where typically only general management schemes 

are employed. In such systems, use of prescribed fires and grazing are often 

preferred to herbicides for management (e.g., FEIS, 2009; Grant et al., 2009; but see 

Hendrickson and Lund, 2010, for an example with the selective herbicide imazapic). 

Consequently, the GE type of Kentucky bluegrass might be no more difficult to 

control in those systems than the non-GE type.

Geographic Potential As discussed above, non-GE Kentucky bluegrass has naturalized in all states of the 

United States (Kartesz, 2010). Land conservationists assume that it has occupied 

most of its potential range in the United States (Nature Serve, 2011). We think the 

GE type could have a similar distribution potential.

Entry Potential If the GE type is approved for release, introduction is certain. 

Figure 4. Risk score of herbicide tolerant Kentucky bluegrass incorporating Monte 

Carlo simulation of uncertainty.a

a Central vertical and horizontal lines indicates means of the simulated outcomes. The first 

box contains 50 percent of the outcomes, the second 95 percent, and the third 99 percent.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 12

Figure 5. Risk score of herbicide tolerant Kentucky bluegrass relative to the 

validation dataset.

4.2.2. Results & Conclusion

Model Probabilities: P(Major Invader) = 0.954

P(Minor Invader) = 0.045

P(Non-Invader) = 0.001

Risk Result = High Risk

Secondary Screening = Not Applicable

The result of the weed risk assessment for GE Kentucky bluegrass is High Risk.

4.2.3. Discussion

The GE type of Kentucky bluegrass was High risk (Figs. 4 and 5). Compared to the 

analysis for the non-GE type above, the only change here was answering ―Yes‖ to 

the following question: ―Is tolerant to some herbicides or has potential to acquire 

herbicide resistance?‖ which increased the risk score by 1 point. While demonstrable 

(Figs. 2 and 4) the difference is not significant.

Therefore, as discussed for the non-GE type above, the species, P. pratensis

(including the non-GE type) is a candidate for regulation as a Federally listed 

noxious weed. We discussed above, for the non-GE type of Kentucky bluegrass,

some reasons why the Impact Potential score may not reflect its true damage 

potential: those significant factors also apply equally here to the GE type.

4.2.4. Literature Cited

Bosy, J. L., and R. J. Reader. 1995. Mechanisms Underlying the Suppression of Forb

Seedling Emergence by Grass (Poa pratensis) Litter. British Ecological 

Society 9(1995):635-639.

FNA Editorial Committee (ed.). 2007. Flora of North America (V. 24): 
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 13

Magnoliophyta: Commelinidae (in part): Poaceae, part 1. Oxford University 

Press, New York, U.S.A. 908 pp.

FEIS. 2009. USDA Forest Service-Fire effects information center. USDA Forest 

Service. http://www.fs.fed.us/database/feis/. (Archived at PERAL).

GBIF. 2011. GBIF, Online Database. Global Biodiversity Information Facility 

(GBIF). gbif.org. http://data.gbif.org/welcome.htm. (Archived at PERAL).

Grant, T. A., B. Flanders-Wanner, T. L. Shaffer, R. K. Murphy, and G. A. Knutsen. 

2009. An Emerging Crisis across Northern Prairie Refuges: Prevalence of 

Invasive Plants and a Plan for Adaptive Management. Ecological Restoration 

27(1):58-65.

Halina, L., and W. Hardot. 2005. Allelopathic effects of Poa pratensis on other 

grassland spp. Allelopathy Journal (2005).

Hendrickson, J. R., and C. Lund. 2010. Plant Community and Target Species Affect 

Responses to Restoration Strategies. Rangeland Ecology & Management 

63:435-442.

Holm, L. G., J. V. Pancho, J. P. Herberger, and D. L. Plucknett. 1979. A 

Geographical Atlas of World Weeds. Krieger Publishing Company, Malabar, 

Florida, U.S.A. 391 pp.

Huberty, A. F. 2011. Status of WRA for Kentucky bluegrass from Andrea Huberty, 

APHIS BRS, to Anthony L. Koop, PPQ CPHST 

Kartesz, J. 2010. Floristic Synthesis of North America by Biota of North America 

Program (BONAP), Version 1.0. Biota of North America Program. 

http://www.bonap.org/index.html. (Archived at PERAL).

Morita, H. 1997. Handbook of Arable Weeds in Japan. Niimura Printing Co., Ltd., 

Tokyo. 128 pp.

NAPPFAST. 2008. Databases used in the NAPPFAST system. NCSU APHIS Plant 

Pest Forecasting System (NAPPFAST). 

http://www.nappfast.org/databases/NAPPFAST%20Databases.htm. 

(Archived at PERAL).

Nature Serve. 2011. Nature Serve Explorer: An online encyclopedia of life [web 

application] Version 7.1. Nature Serve. http://natureserve.org. (Archived at 

PERAL).

NGRP. 2011. Germplasm Resources Information Network (GRIN). United States 

Department of Agriculture, Agricultural Resources Service, National 

Germplasm Resources Program (NGRP). http://www.ars-grin.gov. 

(Archived at PERAL).

Weber, E. 2003. Invasive Plant Species of the World: A Reference Guide to 

Environmental Weeds. CABI Publishing, Wallingford, UK. 548 pp.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 14

5. Appendices

Appendix A. Logistic regression model formulas

Below are the formulas for the logistic regression model of the probabilities of 

being a major-, minor-, and non-invader. E/S and Imp refer to the risk scores from 

the Establishment/Spread and Impact risk elements. All three probabilities sum to 1 

for each plant.
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 15

Appendix B. Model cut-off scores

In the diagram below we present the cut-off scores for the model probabilities for 

non-, minor-, and major-invaders. Composite Risk Score refers to a linear 

combination of the risk scores for the establishment/spread and impact risk 

elements. It is used in determining the probabilities and is calculated as (0.2356 × 

E/S) + (0.6019 × Imp). The cutoff scores below were determined by Receiver 

Operating Characteristic (ROC) curve analysis. This analysis maximizes the 

probabilities of accurately identifying non- and major-invaders, while minimizing 

errors. In the analysis, we assumed that the cost of a false-positive and falsenegative error were equal.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-5.2 -4.2 -3.2 -2.2 -1.2 -0.2 0.8 1.8 2.8 3.8 4.8 5.8 6.8 7.8

Probability

Composite Risk Score

P(Maj-I)

P(Min-I)

P(Non-I)

Low Risk

P(Non-I) ≥ 44.9

Evaluate 

Further

High Risk

P(Maj-I) ≥ 38.8
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 16

Appendix C. Secondary screening system.

This system uses key questions that were strongly associated with invasive status in 

the United States. The first is question E/S-1 from the WRA model, and refers to 

the species invasive status anywhere in the world, including in the United States if 

recently established. The first part of the questions in the next two diamonds 

represents choices from E/S-1. The secondary score is the sum of the scores for six 

questions from the WRA model: 1) prolific reproduction; 2) minimum generation 

time; 3) shade adapted; 4) commodity contaminant; 5) number of natural dispersal 

vectors; and 6) forms dense thickets.

What is its 

invasive status? 

High Risk Low Risk

Evaluate 

Further

Introduced > 75 years 

ago & never has 

escaped

Invasive (spreading or 

has spread throughout 

a region)

Naturalized 

somewhere & 

2° score > 0?

Neither

Introduced < 75 years 

ago, never escaped, & 

2° score < 0?

No

Yes

Yes

No
Weed Risk Assessment for Two Types of Poa pratensis

June 30, 2011 17

Appendix D. Risk score reference dataset.

Risk score distribution for the 204 species used to develop (N=102) and test 

(N=102) the PPQ WRA model. Marker color corresponds to the a priori 

classification for a species (major-, minor-, and non-invader). Marker type 

(triangle, circle, and x) corresponds to the conclusion following use of the model 

and secondary screening, if applicable.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-20 -15 -10 -5 0 5 10 15 20 25 30

Impact Potential

Establishment/Spread Potential

Major---Evaluate

Major---Likely Maj-I

Minor---Likely Non-I

Minor---Evaluate

Minor---Likely Maj-I

Non---Likely Non-I

Non---Evaluate

Non---Likely Maj-I