1

Forest Service

Northern Research Station

This publication is part of a series that provides an overview of the 

presence of invasive plant species monitored on an extensive systematic 

network of plots measured by the Forest Inventory and Analysis (FIA) 

program of the USDA Forest Service, Northern Research Station (NRS). 

Each research note features one of the invasive plants monitored 

on forested plots by NRS FIA in the 24 states of the Midwestern and 

Northeastern United States.

Background and Characteristics

Canada thistle (Cirsium arvense) is an herbaceous perennial in the Aster 

family. Since its arrival to North America from Europe in the early 1600s via 

contaminated crop seed, it has caused considerable impact by displacing 

native plants and reducing crop and pasture productivity (Czarapata 2005, 

Kaufman and Kaufman 2007).

Canada thistle is troublesome because each plant can produce 1,500 to 5,000 

seeds and the seeds remain viable up to 20 years (Kurtz 2013). Additionally 

the plants have sharp spines (Fig. 1) and send out extensive root systems. 

Birds, small animals, humans, and water are vectors of this species. While 

it is considered a problematic species due to its aggressive spread and 

agricultural impact, there are some beneficial qualities to note. Canada thistle 

offers nectar for a multitude of birds and insects (Fig. 2). It also is claimed to 

have medicinal value for dysentery, diarrhea, as a bowel tonic, dewormer, 

as well as for skin eruptions and ulcers, poison ivy rash, and tuberculosis 

treatment (Foster and Duke 2000). Despite these important traits, it is 

considered noxious in much of the United States where its spread threatens 

native species by competing for resources such as light and nutrients.

Description

Growth: stems to 5.0 feet long, often with hair along; dense clone growth; 

crinkly leaves up to approximately half a foot with abundant spines.

Reproduction: numerous purple (Fig. 3), lavender, to sometimes white 

flowers, around 0.5-inch diameter, that bloom in mid to late summer; seeds 

are small with a tuft of hair at the tip facilitating wind dispersal.

Habitat: agriculture fields, pastures, roadsides, disturbed areas, urban areas, 

along wetlands.

Growth conditions: prefers sun.

Control: herbicide can be effective but sensitivity of other plants is 

important; for small infestations, plants can be covered with black plastic and 

“cooked”; repeated mowing (Czarapata 2005, Kaufman and Kaufman 2007).

Growth Conditions and Range

Canada thistle is hardy throughout North America. Its northern limit 

corresponds with the January average temperature of -40.0 °F (Royer and 

Dickinson 1999). It is currently found throughout the United States and 

Canada (NRCS 2019).

Figure 2.—Canada thistle in flower. 

Photo by Ohio State Weed Lab, The Ohio State 

University, 1560037 from Bugwood.org.

Figure 3.—Comparison of bull thistle 

(left) to Canada thistle (right). Photo by 

Steve Dewey, Utah State University, 1459748 

from Bugwood.org.

United States Department of Agriculture

An Assessment of Canada Thistle in Northern U.S. Forests Research Note NRS-252

Figure 1.—Crinkled leaves and spines of 

Canada thistle. Photo by Leslie J. Mehrhoff, 

University of Connecticut, 5451395 from 

Bugwood.org.
2

FIA crews visited 4,462 forested1 Phase 2 (P2) invasive 

plots across the NRS region for the 2017 inventory. These 

P2 invasive plots are a subset of the standard P2 plots 

where 40 invasive plant species (IPS2) (39 species and one 

undifferentiated genus [nonnative bush honeysuckle]3) are 

monitored. Various attributes are collected including the 

occurrence and coverage of IPS as well as the standard forest 

variables measured on P2 plots (e.g., tree diameter, height). 

Overall, 53.1 percent of forested plots have one or more of 

the monitored invasives present with the number of species 

ranging from 1 to 12 per plot.

Canada thistle occurs on 177 plots (4.0 percent) across 14 of 

the 24 NRS states (Fig. 4). Though it was not observed on plots 

in 10 NRS states, this does not mean the species is absent 

from those states. Field crews did not record this IPS on plots 

in Delaware, Kansas, Massachusetts, Maryland, Missouri, New 

Hampshire, North Dakota, Rhode Island, Vermont, or West 

Virginia. Minnesota has the highest percentage of plots with 

Canada thistle (15.8 percent, based on 56 plots). It is important 

to remember only forested plots are monitored for IPS. 

Additionally the number of plots varies within states and must 

be considered when making conclusions. This number of plots 

per state and additional information is presented in Table 1.

For the 2017 inventory, Canada thistle is the twelfth most 

commonly observed invasive species after multiflora rose 

(30.7 percent), nonnative bush honeysuckles (20.3 percent), 

garlic mustard (12.2 percent), autumn olive (8.0 percent), 

Japanese honeysuckle (7.9 percent), Japanese stiltgrass (7.8 

percent), Japanese barberry (7.2 percent), common buckthorn 

(6.7 percent), black locust (6.4 percent), reed canarygrass (5.8 

percent), and Oriental bittersweet (5.0 percent). Additional 

information about the invasives monitored and county-level 

occurrence maps for the NRS region from 2005 through 2010 

can be found in Kurtz (2013).

Canada Thistle Presence on Phase 2 Invasive Plots, 2017

1 FIA defines forest land as land with at least 10 percent canopy cover of live tally tree species of any size or formerly having at least 

10 percent canopy cover of live tally species in the past. Additionally, the condition is not subject to nonforest use(s) intended to 

prevent normal tree regeneration and succession. Generally, the minimum area for classification as a forest is 1 acre in size and at 

least 120 feet in width. There are more specific criteria for defining forest land near streams, rights-of-way, and shelterbelt strips 

(USDA Forest Service 2018)

2 Hereafter IPS may also be referred to as “invasive species”, “invasive plants”, or “invasives”.

3 Autumn olive (Elaeagnus umbellata), black locust (Robinia pseudoacacia), Bohemian knotweed (Polygonum xbohemicum), bull 

thistle (Cirsium vulgare), Canada thistle (Cirsium arvense), Chinaberry (Melia azedarach), common barberry (Berberis vulgaris), 

common buckthorn (Rhamnus cathartica), common reed (Phragmites australis), creeping jenny (Lysimachia nummularia), 

dames rocket (Hesperis matronalis), English ivy (Hedera helix), European cranberrybush (Viburnum opulus), European privet 

(Ligustrum vulgare), European swallow-wort (Cynanchum rossicum), garlic mustard (Alliaria petiolata), giant knotweed (Polygonum 

sachalinense), glossy buckthorn (Frangula alnus), Japanese barberry (Berberis thunbergii), Japanese honeysuckle (Lonicera japonica), 

Japanese knotweed (Polygonum cuspidatum), Japanese meadowsweet (Spiraea japonica), leafy spurge (Euphorbia esula), Louise’s 

swallow-wort (Cynanchum louiseae), multiflora rose (Rosa multiflora), Japanese stiltgrass (Microstegium vimineum), nonnative bush 

honeysuckle (Lonicera spp.), Norway maple (Acer platanoides), Oriental bittersweet (Celastrus orbiculatus), princesstree (Paulownia 

tomentosa), punktree (Melaleuca quinquenervia), purple loosestrife (Lythrum salicaria), reed canarygrass (Phalaris arundinacea), 

Russian olive (Elaeagnus angustifolia), saltcedar (Tamarix ramosissima), Siberian elm (Ulmus pumila), silktree (Albizia julibrissin), 

spotted knapweed (Centaurea stoebe ssp. micranthos), tallow tree (Triadica sebifera), tree of heaven (Ailanthus altissima).

Figure 4.—Percentage of Phase 2 

invasive plots with Canada thistle, 

2017. Percentages are rounded to 

the nearest whole number.
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Along with assessing the distribution of invasive 

plants, it is also important to monitor cover. As cover 

increases, native plants face greater competition 

for space and resources like nutrients and water. 

Where the species is found, cover data helps to assess 

abundance. It is important for managers to monitor 

changes in distribution and abundance to determine 

how to most effectively and efficiently allocate funding 

for management. Watching for new occurrences as well 

as increases or decreases in presence can offer insight 

as to how management practices influence a species.

Average percent cover of Canada thistle varies 

throughout the region with higher cover on plots in 

the Midwestern part of the NRS region (Fig. 5). This 

portion of our region is also where Canada thistle is 

more common (Fig. 4). Used in tandem, figures 4 and 

5 reveal important information related to the presence 

and abundance of this noxious invader in the NRS 

states. Over time these maps will allow us to assess 

changes across the region.

Figure 5.—Average percent cover4 of Canada thistle on Phase 2 invasive plots, 2017. Percentages are rounded to the 

nearest tenth of a whole number.

4 Average percentage cover is calculated for plots based on subplot data for the portion of the plot that is forested. Each FIA plot 

consists of four circular 1/24-acre subplots located at the corners and center of an equilateral triangle that is 208 feet on a side.

Canada Thistle Cover on Phase 2 Invasive Plots, 2017
4

In the region monitored by NRS, Canada thistle was most 

abundant in the northern part of the Midwest (Table 1). 

Breaking the plot data down by percentage, Canada thistle 

increased in presence in ten states, decreased in six states, 

and two states showed no change. Additionally, Canada 

thistle remained absent from plots in six states. Across the 

region, Canada thistle presence increased by 0.6 percent in 

the last 5 years.

To assess if plots with Canada thistle differed from their 

counterparts, analyses were restricted to states where this 

species occurred on at least 10 percent of the plots. This 

resulted in 844 plots in Michigan, Minnesota, and South 

Dakota. Of these 844 plots, this invader was present on 114. 

For these states, the average number of seedlings per acre 

on plots with Canada thistle is 2,968 versus 3,213 seedlings 

per acre for those without Canada thistle. Despite differences 

in the number of seedlings per acre, the difference is not 

statistically significant (t-test; p>0.05). There is also no 

significant difference (t-test; p>0.05) in the percentage of the 

plot that was forested when Canada thistle was present.

Monitoring IPS offers insight on the status, trends, 

distribution, and population size, and helps to detect new 

populations. These preliminary investigations are important 

as they suggest there may be differences in plots with and 

without invasives. IPS can affect property and timber value, 

biodiversity, habitat quality, and sustainability. Future studies 

will help to derive important factors related to invasives 

presence and ecosystem impacts.

Characteristics of Plots with Canada Thistle, 2017

Table 1.—Canada thistle presence by state, 2012 and 2017

State

Plots with 

Canada 

thistle (2017)

Plots 

monitored 

for IPS (2017)

Plots with 

Canada thistle 

in 2017 (%)

Plots with 

Canada 

thistle (2012)

Plots 

monitored 

for IPS (2012)

Plots with 

Canada thistle 

in 2012 (%)

Change 

in % since 

2012

Connecticut 1 45 2.2 1 65 1.5 0.7

Delaware 0 16 0.0 0 60 0.0 0.0

Illinois 1 124 0.8 2 210 1.0 -0.2

Indiana 4 128 3.1 10 377 2.7 0.4

Iowa 2 80 2.5 1 114 0.9 1.6

Kansas 0 81 0.0 1 129 0.8 -0.8

Maine 8 383 2.1 7 578 1.2 0.9

Maryland 0 54 0.0 0 82 0.0 0.0

Massachusetts 0 72 0.0 0 100 0.0 0.0

Michigan 51 440 11.6 76 686 11.1 0.5

Minnesota 56 354 15.8 81 1,088 7.4 8.4

Missouri 0 370 0.0 2 573 0.4 -0.4

Nebraska 4 55 7.3 5 63 7.9 -0.6

New 

Hampshire

0 102 0.0 0 152 0.0 0.0

New Jersey 3 48 6.3 2 70 2.9 3.4

New York 3 408 0.7 4 606 0.7 0.0

North Dakota 0 13 0.0 5 26 19.2 -19.2

Ohio 9 203 4.4 11 315 3.5 0.9

Pennsylvania 9 667 1.3 8 596 1.3 0.0

Rhode Island 0 9 0.0 0 30 0.0 0.0

South Dakota 7 50 14.0 7 67 10.5 3.5

Vermont 0 99 0.0 0 139 0.0 0.0

West Virginia 0 221 0.0 2 350 0.6 -0.6

Wisconsin 19 440 4.3 36 1,103 3.3 1.0

Grand total 177 4,462 4.0 261 7,579 3.4 0.6
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Citation for this Publication

Kurtz, Cassandra M.; Hansen, Mark H. 2019. An assessment 

of Canada thistle in northern U.S. forests. Res. Note NRS252. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 5 p. https://

doi.org/10.2737/NRS-RN-252.

FIA Program Information

Bechtold, W.A.; Patterson, P.L., eds. 2005. The enhanced Forest 

Inventory and Analysis Program: national sampling design 

and estimation procedures. Gen. Tech. Rep. SRS-80. Asheville, 

NC: U.S. Department of Agriculture, Forest Service, Southern 

Research Station. 85 p. https://doi.org/10.2737/SRS-GTR-80.

Smith, W.B. 2002. Forest inventory and analysis: a national 

inventory and monitoring program. Environmental 

Pollution. 116: 233-242. https://doi.org/10.1016/S0269-

7491(01)00255-X.

References

Czarapata, E.J. 2005. Invasive plants of the upper Midwest: 

an illustrated guide to their identification and control.

Madison, WI: University of Wisconsin Press. 215 p.

Foster, S.; Duke, J.A. 2000. A field guide to medicinal plants 

and herbs of eastern and central North America. 2nd ed.

New York: Houghton Mifflin Company.

Kaufman, S.R.; Kaufman, W. 2007. Invasive plants: a guide 

to identification and the impacts and control of common 

North American species. Mechanicsburg, PA: Stackpole 

Books. 458 p.

Kurtz, C.M. 2013. An assessment of invasive plant species 

monitored by the Northern Research Station Forest 

Inventory and Analysis Program, 2005 through 2010. Gen. 

Tech. Rep. NRS-109. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research Station. 70 p. 

https://doi.org/10.2737/NRS-GTR-109.

Natural Resources Conservation Service (NRCS). 2019. The 

PLANTS database. Greensboro, NC: U.S Department of 

Agriculture, Natural Resources Conservation Service, National 

Plant Data Team. https://plants.usda.gov (accessed March 14, 

2019).

Royer, F.; Dickinson, R. 1999. Weeds of the northern US and 

Canada. Edmonton, Alberta: The University of Alberta Press. 

434 p.

USDA Forest Service. 2018. Forest inventory and analysis 

national core field guide, vol. 1: field data collection 

procedures for phase 2 plots, ver. 8.0. 439 p. www.fia.fs.fed.

us/library/field-guides-methods-proc/ (accessed April 26, 

2019).

Additional Invasive Plant Information

Invasive and Exotic Plants: 

http://www.invasive.org/species/weeds.cfm

Invasive Plant Atlas of New England: 

http://www.eddmaps.org/ipane/

Invasive Plant Atlas of the United States: 

http://www.invasiveplantatlas.org/index.html

Midwest Invasive Plant Network: http://mipn.org/

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Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American 

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To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at http://www.

ascr.usda.gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information 

requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail: U.S. 

Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: 

(202) 690-7442; or (3) email: program.intake@usda.gov.

Contact

Analyst: Cassandra Kurtz, (651)649-5149; cassandra.m.kurtz@usda.gov

Page 1 and 5 header: Canada thistle infestation. Photo by Alec McClay, McClay Ecoscience, 1929056 from Bugwood.org.