Noxious or not? A continuance of the Canada thistle discussion

Ray Eckhart, Master Gardener and loyal blog reader, wrote a long response to Alan’s request for research for the ongoing debate on Canada thistle started a few weeks ago by Jeff. Because he has a lot of links to research in his response, I thought it should have its own posting. So here’s Ray:

Here is a brief summary of the results of a google search of .edu and .gov or .us sites on the subject of Canada thistle or Cirsium arvense as a noxious weed, examining the “whys” by a mostly volunteer* Master Gardener reliant on published literature by reputable sources and charged with fulfilling the Land Grant University charter to bring science based information to the local level.

(* about $6500 of my annual salary and benefits comes from fulfilling Master Gardener responsibilities.)

From the Minnesota pdf referenced above:
“Noxious weeds are difficult to control and injurious to public health, the environment, roads, crops, livestock and property. By law, these weeds must be controlled on all public and private lands.”

From Montana:
“Canada thistle threatens productivity in both crop and non-croplands. In cropland, Canada thistle causes extensive yield losses through competition for light, nutrients, and moisture. It also increases harvesting problems due to seed and forage contamination. In Montana, it is estimated that two shoots per square yard can reduce wheat yield by 15 percent and 25 shoots per square yard can reduce wheat yield by 60 percent. Other Montana crops seriously threatened by Canada thistle include peas, corn, beans, alfalfa and sugar beets. Heavy infestations are also commonly found in overgrazed pastures and ranges and may crowd-out and replace native grasses and forbs, decreasing species diversity in an area.

“By 1795, Vermont enacted noxious weed legislation against Canada thistle and, in the early 1900’s, the currently named Noxious Weed Act gave a person the right to eradicate this species wherever they found it without fear of trespassing.

“In alfalfa stands grown for seed production, Canada thistle can reduce yield by 48 percent. An extra ten percent yield reduction can occur in alfalfa seed production due to seed cleaning. In pastures, Canada thistle reduces productivity by crowding out forage species with spiny leaves that deter cattle from grazing. In non-cropland ecosystems, Canada thistle can crowd out and replace native grasses and forbs limiting land’s recreational use. In gardens, flower beds, and lawns, Canada thistle’s extensive root system makes it a hassle to control. Mowing or pulling this weed is not effective because it grows again from vegetative buds on the roots. In fact, improper cultivation can even worsen Canada thistle problems!”

From Pennsylvania:
“In the Northeast, several weeds including bull and musk thistle, Canada thistle, purple loosestrife (Lythrum salicaria), mile-a-minute (Polygonum perfoliatum), and garlic mustard (Allaria petifolia) are receiving attention [for biological control efforts – ed.] because of their invasive nature.”

2nd Cite for Pennsylvania:
ECOLOGICAL THREAT: Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants.

“Canada thistle is declared a “noxious weed” throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention.”

“Some noxious or invasive weeds are highly toxic to equines, however, and can cause tremendous problems if allowed to invade horse pastures. This may be partially due to the extensive taproot in many broadleaf weeds that allow them to remain green longer into the dry season, thereby appearing potentially attractive to horses grazing in poor pastures. This list includes tansy ragwort, yellow starthistle, Russian knapweed, Canada thistle (Cirsium arvense), poison hemlocks, field bindweed, houndstongue, Scotchbroom (Cytisus scoparius), horsetails, leafy spurge, black henbane (Hyoscyamus niger), Klamath weed or St. Johnswort, kochia, yellow toadflax or butter-and-eggs (Linaria vulgaris), silverleaf nightshade (Solanum elaeagnifolium), and puncture vine.”

Colorado: “Canada thistle (Cirsium arvense) is an aggressive, creeping perennial weed that infests crops, pastures, rangeland, roadsides and noncrop areas. Generally, infestations start on disturbed ground, including ditch banks, overgrazed pastures, tilled fields or abandoned sites. Canada thistle reduces forage consumption in pastures and rangeland because cattle typically will not graze near infestations. In 2002, the Colorado Department of Agriculture surveyed counties and while incomplete, the results showed more than 100,000 acres infested with Canada thistle (Figure 1).”

2nd cite Colorado:
“Impacts Agricultural: Canada thistle is an aggressive, creeping, perennial weed. It infests crops, pastures, rangelands, roadsides, and riparian areas (Beck 1996).

“Ecological: Canada thistle spreads rapidly through horizontal roots, which give rise to shoots (Moore 1975). Its root system can be extensive, growing horizontally as much as 18 feet in one season (Nuzzo 1998). Most Canada thistle patches spread at a rate of 3-6 feet/year, crowding out more desirable species and creating thistle monocultures.

“Human: Spiny thickets of Canada thistle can restrict recreational access to infested areas.”

South Dakota:
“Noxious weeds are found in range and pasture as well as noncrop areas and cropland. Troublesome statewide noxious weeds like Canada thistle, leafy spurge, perennial sow thistle, Russian knapweed, and hoary cress can be serious problems in pasture and rangeland.”

“Weeds can reduce the quantity and the stand life of desirable forage plants in pastures and hayfields. These unwanted plants are often more aggressive than existing or desired forage species and compete for light, water, and nutrients. Weeds can also diminish the quality and palatability of the forage available for livestock grazing, and certain weed species are potentially poisonous to grazing animals. The aesthetic value of a pasture is also impacted by weeds.

“The state regulations of the Kentucky Seed Law classify certain plants such as Canada thistle, johnsongrass, and quackgrass as noxious weeds and prohibit their presence in commercial seed sold in Kentucky.”

National Park Service:
“Thistles are pioneer species and are most often found in sites where the ground cover has been disturbed by grazing, erosion, traffic, or other means. Thistles reduce the use of an area for grazing or recreational purposes because of the prominent spines on leaves, stalks, and blooms. Livestock do not eat thistles and will not graze between thistle plants on more desirable forage (Batra 1982).” (linked from .gov sites):
“THREATS POSED BY THIS SPECIES: Natural areas invaded by Cirsium arvense include prairies and other grasslands in the midwest and Great Plains and riparian areas in the intermountain west. Cirsium arvense threatens natural communities by directly competing with and displacing native vegetation, decreasing species diversity, and changing the structure and composition of some habitats. Species diversity in an “undisturbed” Colorado grassland was inversely proportional to the relative frequency of Canada thistle (Stachion and Zimdahl 1980). Canada thistle invades natural communities primarily through vegetative expansion, and secondarily through seedling establishment. Cirsium arvense presents an economic threat to farmers and ranchers. Infestations reduce crop yield through competition for water, nutrients and minerals (Malicki and Berbeciowa 1986) and interfere with harvest (Boldt 1981). In Canada, the major impact of Cirsium arvense is in agricultural land, and in natural areas that have been disturbed or are undergoing restoration (White et al. 1993). In the U.S., it is a host for bean aphid and stalk borer, insects that affect corn and tomatoes (Moore 1975), and for sod-web worm (Crampus sp.) which damages corn (Detmers 1927). In Bulgaria Cirsium arvense is a host for the cucumber mosaic virus (Dikova 1989). In addition to reducing forage and pasture production, Canada thistle may scratch grazing animals, resulting in small infections (Moore 1975).”

Washington State:
“Why is it a noxious weed? Once established, it spreads quickly replacing native plants. It grows in circular patches, spreading vegetatively through roots which can spread 10 -12′ in one season. It poses an economic threat to the agriculture industry by reducing crop yields.”

“Threats: Canada thistle’s rapid growth aggressively competes with native plants and crops for nutrients, moisture and light. It releases chemicals toxic to other plants. The result is a loss of natural diversity. It is known to harbor other pest species, e.g., insects, and has long been recognized as an agricultural est. Both natural and human caused disturbances can create the opportunity for Canada thistle to become established in natural communities.”

“PROBLEM: The extensive root system of Canada thistle allows it to out-compete and displace many native species, especially in degraded prairies where native species are not well established. Spreading both by seed and rhizome, Canada thistle can create monocultures covering large areas. The wind-dispersed seeds may remain viable for 20 years or more, allowing it to spread quickly and making it difficult to eradicate.”

There are more, but I stopped on page 3 of the 120 page result of the google .edu search. I’ll leave it to others more qualified than I am to further debate the relative merits of why or why not a more cavalier (heh!) approach other than current government regulatory action is or is not warranted.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

7 thoughts on “Noxious or not? A continuance of the Canada thistle discussion”

  1. Nope, I’m still on the cavalier side — at least as Canadian thistle compares to other weeds. Many of these references simply state what noxious is supposed to mean and provide no evidence that Canadian thistle actually fits the definition — that’s not helpful and doesn’t prove anything. As an analogy, You can call yourself a race car driver, but that doesn’t make you one, and I wouldn’t believe you are one without some kind of proof — like an article showing how well you’d done in a race last week. One reference points out that Canadian thistle is toxic to horses — that’s wrong. In some cases it may lead to cuts and sometimes infections — but it’s not toxic. The reference from Montana points out that two thistles per square yard can have a significant effect on the growth of crops — I certainly believe that, but then many other weeds will do the same thing so I don’t see why that information should make this weed significantly worse than others. The points from are well made, particularly because they cite research that shows exactly what Canadian thistle is capable of — but in my opinion they over interpret the results. Basically the citations above point out that this stuff is a weed — which we already knew — I just don’t see this information raising Canadian thistle to the level of noxious weed.

  2. So what is the objective, neutral, criteria that qualifies one weed as noxious, and another not? Of the 11 on the Minnesota list, can you rank them, and tell us why or why not they should be on there? Can we ignore these lists, based on what you say here? If the political process is not the way that legal definitions are made, what do you propose as an alternative? An Oligarchcal Committee of Horticulturists unswayed and uswayable by political interests? If Dr. Chalker-Scott can use noxious weed lists as a standard against which to judge permaculture practices, on what basis should folks like me be more or less cavalier about them?

  3. In Minnesota we do have a process by which weeds are listed — and it as objective a process as I’ve seen for these weeds (it is a bit oligarchical at that!). That said, Canadian thistle is listed because of political pressure (I do sit on the Minnesota Department of Agriculture’s committee which reviews these weeds). Should you ignore the lists? That’s up to you, but if you’re serious about invasive plants then you shouldn’t follow them blindly. We (the people of this country) only have so much in the way of resources to combat invasive plants and noxious weeds, so we need to do our best to focus our efforts where it really matters. In my opinion Canadian thistle isn’t where it’s at. Others will disagree. It’s commonly accepted that the time to stop a weed is when it first appears. Once it’s here efforts to stop them aren’t usually that effective (sometimes biological controls will work). On what basis should folks like you be more or less cavalier? Ask yourself what weeds you can target where it will make the most difference. I think you’ll find that the answer is emerging weed problems rather than weeds which have been here for almost 400 years.

  4. This is an interesting discussion but I’m very confused. Many states list more than a hundred noxious weeds and others only list a few. Virginia is a prime example in that there are only two weeds listed yet neighboring North Carolina, with very similar climate and growing conditions, lists over a hundred. Obviously the selection process varies widely from state to state. So, to go to Ray’s question about a cavalier attitude, my opinion is it varies just as widely as the state decision making process. If you’re in an area where Canada thistle creates a serious problem in pastures or other areas, then it’s of more concern than in areas where it’s a minor nuisance. Serious problem should equal active efforts to limit the spread. Personally, I say destroy every one of the nasty things you see. They are not on our list but I’ve had enough pain from them to consider them Devil’s spawn! (Of course, the goldfinches would disagree with me!)

  5. Sorry, Jeff. Still not convinced. I make a distinction between noxious and invasive, first off. Jimsonweed and poison ivy are native and on many state lists. Garlic mustard has been here since colonial times and is on many lists, and makes a fine pesto. Deer tend to avoid it, though. “Edibility” thus becomes a non-factor, since it ignores the obvious “You can led a horse to thistle, but you can’t make him eat” factor, and the converse of a fine forage food (and erosion control conservation benefits) in the case of kudzu doesn’t change its undesirability. Field bindweed has also been around since colonial times. Purple loosestrife has been established for over 200 years, and is quite beautiful, with medicinal properties. You emphasize the economic costs of control, while downplaying the economic costs of non-control to the Ag industry. I’ll leave cannabis out the discussion, for obvious reasons, but I don’t think it should be “up to me” to decide these things. Isn’t that the point of these lists to begin with? The horse people I have only a nodding acquaintance with, are the Point-to-Point Steeplechase folks of the DC exurbs in Virginia. Add livestock and dairy interests, and I think you’re tilting at windmills with your fellows on the committee. Were I in your position, I’d spend my “Political Capital”, on eliminating all the native, beneficial species on the Minnesota secondary noxious weed list bannable at the county level by petition. And although I greatly value your opinion, my skepticism dictates more facts and figures before I’d sign a petition to remove canada thistle from the noxious weed list.

  6. I have to agree with Jeff’s first comment. How can one differentiate between fact and baseless opinion when no actual research is presented as is the case with the danger stated of thistle to pasture. Even research can be suspect if, as Jeff points out, the thistle isn’t being compared with other weeds as opposed to pristine growing conditions. We need a comparison to rate the relative harm of CT. This isn’t intended to be a defense of Canadian thistle- only a statement of a desire for a more scientific evaluation.

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