Cirsium arvense

Canada thistle produces flowers that are pink to purple to white.

Photo: Rob Routledge, Sault College, Bugwood.org

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Description

Native to eastern Europe, Canada thistle is a rhizomatous perennial herb belonging to the daisy family. A denizen of old farm fields and waste places, the species grows 1-5 feet tall, and forms a basal rosette of variably shaped, prickly leaves. The species is most conspicuous in midsummer when small, classic ‘thistle’ flowers bloom pale lavender on tall, hairless stems. (2)

Leaves
Leaves are alternately arranged and generally sessile (or stemless) on the plant’s main stem. Leaf margins are variable- from entire to deeply pinnate. Leaves spiny. (2)

Flowers
Flowers bloom towards midsummer in the Hudson Valley, and are approximately ½ inch wide, looking like classic ‘thistle’ flowers. Plants are dioecious, having either all male or all female flowers. There are 1-5 flower heads per branchlet, and each flower is typically lavender in color but can be pinkish to white. (4)

Introduction History

Canada thistle was introduced to North America in the 1600s as a contaminant of agricultural seed. Prior to its arrival in the colonies the species was a notorious weed of farm fields across Europe. The species is now extant across North America and considered one of the world’s most troublesome agricultural invasives. (3)

Ecology and Habitat

Canada thistle is widespread across temperate North America and will invade any open, disturbed habitat, from parking lots to old fields to cropped fields to roadsides. It can be found in sunny, open natural habitats, such as river banks and forest edges. Trails present a major route of invasion into natural areas for this species as its seeds may be transported by mammals such as deer or horses. However, the species does not compete well in low light environments. (7)

Reproduction and Phenology

Canada thistle is capable of producing thousands of viable seed per plant, although its main, and perhaps most problematic, method of reproduction is asexual, via vegetative propagation of the root system. (6) Roots can withstand freezing, drying and cultivation: the fragmentation of Canada thistle’s rhizomes gives rise to more plants. Seeds can remain viable in the soil for up to 20 years. (3) Vectors include wind, water and mammals. (4)

Impacts of this species

An aggressive, rhizomatous perennial, Canada thistle is one of the most widespread agricultural weeds in the world. A single seedling is capable of forming dense, homogenous stands via vegetative propagation of the root system, quickly altering community composition and displacing native plants. Beyond reducing biodiversity, this species has important implications for the agricultural industry as it can decrease available forage and crop productivity. (3,4)

Management Methods

Biological Control
While there are over 10 different biological control agents in use against Canada thistle, none have proven very effective. (7)

Manual or Mechanical Control
Pulling / Digging Up: Consistent pulling by hand is effective against Canada thistle, so long as all of the species’ extensive taproot is removed. This is a very time and labor-intensive effort and useful only for very small populations. All the species’ underground biomass must be removed in order for this treatment to ultimately be effective. (8)

Mowing: Although consistent mowing prior to or at flowering onset does diminish flowering, seed set, and below ground biomass, it must be performed at least three times a year to achieve any positive results. (8)

Girdling: Not applicable

Prescribed Fire: Canada thistle is largely resistant to controlled burns due to its rhizomatous below ground biomass. Furthermore, evidence suggests this species may change fire regimes, making the habitat it invades more likely to burn at greater intensity. (7)

Prescribed Grazing: Not applicable. Rejected by most grazers perhaps due to the species abundant spines. Goats and sheep may graze this species if given few other options but overgrazing of subsequently unmanaged pastures will create ideal, low-competition conditions for Canada thistle re-invasion. (7)

Soil Tilling: Tilling will cause root fragmentation and encourage sprouting of new plants. (3)

Mulching: Mulching with black plastic may be effective at controlling very small areas of Canada thistle. (9)

Solarization: Not applicable

Hot Foam Spray: Not applicable

Chemical Control
The pesticide application rates and usage herein are recommendations based on research and interviews with land managers. When considering the use of pesticides, it is your responsibility to fully understand the laws, regulations and best practices required to apply pesticides in a responsible manner. At times, the pest you seek to treat may not be on a pesticide label, requiring a 2ee exemption from NYSDEC. Always thoroughly read the label of any pesticide and consult the NYSDEC or a licensed pesticide applicator with questions.

Foliar Spray: 2-4D, Aminopyralid, Dicamba, picloram, and glyphosate are all effective against Canada thistle when applied during active growth. Glyphosate is not selective: the slender form of Canada thistle makes it very difficult to avoid non-target plants when performing management via foliar spray. Repeat applications of all herbicides will be necessary to control emerging root sprouts. Always read and follow all directions on the label and to mix appropriate concentrations of each herbicide. Follow up treatment will be necessary the following season. (7, 8)

Cut Stump: Not applicable

Basal Bark: Not applicable

Hack-And-Squirt

Stem Injection: Not applicable

Pre-Emergent Spray: Not applicable

Summary of Best Managment Practices

General management overview and recommendation
Sequential herbicide treatments will provide the highest rate of control of Canada thistle. Two applications may be necessary during the first year of control, and all managed sites will need to be retreated in the following season— possibly for retreatment of mature plants and to target newly emerged seedlings and root sprouts. Given the highly aggressive nature of this extremely common, difficult to control invasive it may be more fruitful to simply keep it out of areas where it threatens vulnerable or rare native species. (9)

Post treatment monitoring
Depending on the management method employed, controlled populations should be revisited throughout the growing season for repeat management activities in approximately 2-3 weeks post treatment. Due to the species long-lived seed bank, all managed sites should be visited for the foreseeable future to prevent germination of viable seed and to prevent reinvasion.

Disposal Methods
Waste material must be thoroughly dried and can then be composted, burned, or disposed of in a landfill. Rhizomes have the potential to re-sprout. All viable seed must be bagged and disposed of. (9)

Additional Information

REFERENCES

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