Thistle Weed Seeds

Canada thistle Canada thistle ( Cirsium arvense ), is a spreading perennial weed of crops, pastures, and disturbed sites. It is most troublesome in perennial crops, rangeland, and areas where The Weed Science Program’s goal at MSU is to provide science-based research and extension information on integrated weed management in field crops. Common Name: Canada Thistle

Canada thistle

Canada thistle (Cirsium arvense), is a spreading perennial weed of crops, pastures, and disturbed sites. It is most troublesome in perennial crops, rangeland, and areas where reduced tillage is practiced. Canada thistle spreads through both seeds and budding from underground roots. Its root system can extend more than fifteen feet out and six feet down from the plant, making management difficult. It is found throughout the northern half of the United States and southern Canada.

Identification

Seedlings: When sprouting from seed, cotyledons are dull green, relatively thick, rounded-oval to oblong (Fig.1). Many new plants sprout from root (rhizome) fragments, and lack cotyledons. Young leaves are thick, egg-shaped to spear-shaped with wavy-lobed, spiny edges; the whole leaf is covered with short, bristly hairs (Fig. 2). Seedlings form a basal rosette and then elongate later in the season.

Fig. 1. Canada thistle cotyledons and early leaves. Photo from “Weed Identification, Biology and Management”, by Alan Watson and Antonio DiTommaso.

Mature plant: Stems are upright and 0.3-1.2 m (1-4′) tall, grooved, and with no hairs or just a few hairs. The stems are also slender and branched at the top. The mature plant grows in groups/patches.

Leaves: On mature plants, leaves are alternate and are egg- to lance-shaped with irregular lobes, no petiole, and spiny edges. The base of each leaf surrounds the stem.

Fig. 2. Canada thistle seedlings. Photo from “Weed Identification, Biology and Management”, by Alan Watson and Antonio DiTommaso.

Flowers/Seeds: Flowers are present from June through August, with multiple flowers per stem. Flower heads are about an inch wide, and have pink to purple (or rarely white) flowers surrounded by spineless bracts (Fig. 3). Male and female flowers are produced on separate plants. Seeds are produced singly in a flat, brown fruit about 1/10 of an inch long. Seeds are curved or straight, with the tip abruptly cut off and a bump in the center (Fig. 4). Seeds produce a feathery pappus (similar to dandelion ‘seeds’) which help disperse the seeds (Fig. 4).

Fig. 4. Canada thistle seeds. Photo from “Weed Identification, Biology and Management”, by Alan Watson and Antonio DiTommaso.

Fig. 3. Canada thistle flowers. Photo from “Weed Identification, Biology and Management”, by Alan Watson and Antonio DiTommaso.

Herbicide Resistance: Currently there are no reports of herbicide resistance of Canada thistle in the US.

Management

Several factors make Canada thistle especially hard to manage. Vegetative reproduction from the extensive root system can occur whenever it is stressed as well as in the spring and fall. Seeds can persist over twenty years in the soil. Reproduction from the root system means any treatment that does not affect the roots is largely ineffective. Any treatment, chemical or mechanical, will need to be repeated for several years. Canada thistle can be suppressed by crops that create substantial shade in the spring, such as alfalfa and winter wheat. A mixture of cultural, mechanical, and chemical control methods is often most effective.

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Chemical control

Cornell University’s Turfgrass and Landscape Weed ID app offers suggestions for conventional and alternative chemical control options. Generally, treatments between June and September have been most effective. Pennsylvania State University has useful management advice focused for the Conservation Reserve Management Program. Techline has published the results of a study comparing Milestone rates and Milestone or Tordon applications at different times in North Dakota.

In apple orchards, post-emergent clopyralid applications can help control Canada thistle. Some pre-emergent herbicides (dichlobenil and rimsulfuron) and post-emergent burndown herbicides (glufosinate, pyraflufen-ethyl and saflufenacil) can be used against thistle as well. See this ENYCHP Tree Fruit News article for details.

Use this tool to look up the efficacy of herbicides on a particular weed species. For general guidance on weed control, get the latest edition of the Cornell Crop and Pest Management Guidelines.

Non-chemical control

Cornell University’s Weed Ecology and Management website provides ecological control options for Canada Thistle. Persistent cultivation can be effective over several years, as can competition from dense crop cover or cover crops. The University of Nebraska also has a page devoted to organic Canada thistle management, as does North Dakota State University.

See A Grower’s Guide to Organic Apples from Cornell for non-chemical weed control options in apple orchards.

Uva R H, Neal J C, DiTomaso J M. 1997. Weeds of the Northeast. Book published by Cornell University, Ithaca NY. The go-to for weed ID in the Northeast; look for a new edition sometime in 2019.

Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2020 or 2021.

Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed.

Colorado State University Extension Canada thistle fact sheet. This is an excellent review of effective management, but please keep in mind that New York’s pesticide regulations are much stricter than Colorado’s, and some of these treatments may not be legal in New York.

Techline Invasive Species Management’s Canada thistle management reports from studies in North Dakota, here and here. please keep in mind that New York’s pesticide regulations are much stricter than North Dakota’s, and some of these treatments may not be legal in New York.

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University of Nebraska Cooperative Extension: Managing Canada Thistle.

Pennsylvania State University: Managing Canada Thistle.

Identification profile for Canada Thistle by the University of California Statewide Integrated Pest Management Program.

Basedow, M and L Sosnoskie. 2020. Strategies for Dealing with Pesky Perennial Weeds. Article published in the April 2020 issue of Tree Fruit News. Covers control of yellow nutsedge, Canada thistle, field bindweed, and dandelion in apple orchards.

Peck, G M and I A Merwin. A Grower’s Guide to Organic Apples. Covers organic weed control methods for organic apple orchards.

Breth, D I and E Tee. 2016. Herbicide AI by Weed Species. This tool allows you to look up the efficacy of an herbicide active ingredient on a particular weed species.

Weeds

Perennial. Emerges in spring and flowers when days are the longest. Plants die after the first killing frost.

Emergence:

Seedlings produced from seeds emerge from soil depths of 1/4- to 1/2-inch. However, seeds have been found to germinate from 3-inch soil depths. Adventitious shoots (vegetative propagules) from creeping roots can come up from greater depths.

Reproduction:

Mode(s) of Reproduction: Most local reproduction is from creeping roots. Seed production allows for local and long distance reproduction.

Production Range: Seed production ranges from 1,500 to 5,300 seeds per plant.

Dispersal Mechanisms: Creeping roots can be moved from field to field on tillage equipment. Each seed has an attached pappus which allows for wind dispersal.

Longevity: Low to moderate persistence – when buried 1 to 3-inches in the soil 45 to 60% of seed germinates the first year and less than 1% survives after 3 to 5 years. When buried at greater depths (7-inches or more) and left undisturbed seeds have been found to be viable for up to 30 years.

Dormancy: Though most seed is capable of germinating upon dispersal in the fall it enters secondary dormancy during the winter months.

Competitiveness:

Moderate shoot densities have been shown to reduce spring wheat yield and alfalfa seed yield by up to 50%.

Preferred Soil/Field Conditions:

Prefers perennial and no-till cropping systems and rangelands.

Management:

Biological

Predation/grazing: When present Orellia ruficauda (i.e. a seed-head fly found in Canada and the United States) can be responsible for 20 to 80% seed predation. Other agents have been studied, but eliminated for various reasons. Some livestock have been known to graze on Canada thistle at different life stages (see Chapter 5).

Decay: No information.

Mechanical

Tillage: Tillage, mowing and other forms of mechanical control have been deemed ineffective for control. Tillage can increase the problem by spreading vegetative propagules.

Rotary Hoeing: Not effective.

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Flaming: Not effective

Cultural

Crop rotation: Canada thistle populations have been shown to be reduced by the use of a summer annual cover crop such as sudangrass (See the cover crop chapter in IWM: Fine Tuning the System).

Planting date: Most likely will not affect Canada thistle infestations.

Chemical

Application timing and effectiveness: Most susceptible to herbicides between the bud and flower stages of Canada thistle. Sequential herbicide applications may be necessary for control. Please refer to E-434, “MSU Weed Control Guide for Field Crops,” for herbicide recommendations.

Canada Thistle

Efforts must be made to prevent seed maturation and dispersal of plants into new areas. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality. Minnesota Noxious Weed Law

Background

Canada thistle is native to Europe. It was introduced to North America in the 1600s, probably in agricultural seed shipments and is now widespread throughout the United States and Canada.

Description

  • An aggressive perennial with a vigorous root system that continually produces new shoots, invading new areas and outcompeting other vegetation types.
  • Grows 2 – 5 feet tall.
  • Leaves are alternate, lance shaped, irregularly lobed, and have wavy spiny/toothed margins.
  • Stems are usually smooth, but sometimes have short hairs and are slightly grooved.
  • Flowers are purple and pink, occasionally white, and are borne at the end of the stems in clusters. Buds are 1/2 inch wide by 3/4 -1 inch long, have a tear-dropped shape, and lack spines.
  • This plant is a prolific seed producer and also spreads by roots.
  • Seedlings emerge as small rosettes in the fall or early spring, eventually bolting into erect branched flowering stems. Flowers begin to develop in late June, blooming between July and August.
  • This plant is most recognizable in mid-July when flowers change to seedheads with obvious white fluffy tops. Seeds are attached to the “fluff” and can become airborne and spread to new areas.

Habitat

Found growing in a wide range of habitats. Typically infests a variety of disturbed landscapes and is commonly found along roadsides, trails, natural areas, pastures, forest and field margins, mining locations, waste areas and unmaintained gravel pits. This plant establishes quickly after new road construction, housing and development projects, overgrazing of pastures, forestry clear-cuts, and destructive flooding events.

Means of spread and distribution

Spreads primarily by rhizomes and seeds. Found commonly throughout Minnesota.

Impact

This plant is highly invasive, severely reduces pasture capacity and desirable forages, degrades wildlife habitat, and can hinder reforestation and landscape restoration efforts. Once a population gets established, it begins to quickly displace native vegetation, including desirable pollinator habitat, creating large stands with little biological diversity and low habitat value.