Invasive Species Spotlight: Canada Thistle

Guest post by Jacqueline Hyman 
Though many types of thistle exist, both nonnative and native, a largely invasive plant in the greater Washington, DC-area is Canada Thistle.

    Canada Thistle (Circium arvense) is an invasive that “is designated a noxious weed in 43 states,” according to Plant Invaders of Mid-Atlantic Natural Areas. It was introduced accidentally to North America in the 1600s, according to the book. 

    The plant blooms from late June through August with purple to white flowers and “strongly-prickled” leaves. This perennial invasive can grow up to 4 feet tall.

    “Canada Thistle is an aggressive competitor,” wrote Rochelle Bartolomei, the native plant specialist at Montgomery County Parks, in an email. “It crowds out native plants and changes the composition of the plant community. It can create monocultures preventing other native species from thriving and thereby reducing food sources for native insects and birds.”

    Bartolomei said the plant thrives in disturbed soil, but that she finds it in many environments, such as sunny meadows, roadsides, cropland, and gardens. According to Plant Invaders, the thistle is not very tolerant of shade.

    The book notes that the seeds are dispersed through the wind, and that it “expands locally by vegetative means through lateral roots and root fragments.” Canada Thistle is dioecious, meaning it requires both male and female plants to produce seed. Additionally, even a small part of the plant left in the soil can produce a new plant, said Bartolomei.

    Canada Thistle is a tough plant to remove and is not very susceptible to chemical treatment. I suggest cutting the plant to the base and painting a bit of Roundup on the root and basal leaves in summer. The plant will try to store resources at that time and will draw the herbicide into the root,” Bartolomei wrote. “You can also just keep cutting it to the base and eventually it will die off and definitely don't let it go to seed.”

    Because the plants are intolerant of shade, Bartolomei said growing taller plants in the same area may be helpful, but “since it grows and flowers in the cool season, these taller plants may not be effective at shading it out so repeated cutting back is the safest course.”

    Bartolomei added, “To help reduce the spread of the plant, collect seed heads and dispose of in a sealed bag, cut plants to the ground and remain vigilant.”
    Canada Thistle is very common and is found in many parks and other natural areas such as gardens and farms. It is important to deal with this invasive species properly and ensure that it is being eradicated whenever possible.

The  "Invasive Species Spotlight" is a summer blog series focusing on a different plant each week that is a problem for Mid-Atlantic home gardeners.

About the author:
Jacqueline Hyman is a junior journalism and English major at the University of Maryland. She is the editor-in-chief of the Mitzpeh, an independent Jewish newspaper at UMD. In addition, Jacqueline enjoys musical theater, and teaches piano and voice at Guitar Center. She is excited to be interning this summer for the Washington Gardener.

Image credit: 
Photograph by Jim Kennedy,
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