By Sophia Cameron, Invasive Plant Management Intern and Jesse Wheeler, Vegetation Program Manager
Most can recognize a thistle without much previous knowledge. These invasive plants have distinctive wavy leaves with “prickles”, or thorns growing out of the edge of them. Often, hikers and other outdoor recreationists feel these vicious spikes before they see the plant. Canada thistle (Cirsium arvense) and bull thistle (Cirsium vulgare) can be difficult to differentiate to the untrained eye, as they both have those notorious spiked leaves. If the thistle has rhizomes—or roots that extend out from one plant to grow another plant nearby—then it is likely a Canada thistle. If it does not have rhizomes but has a taproot and has spiny wings up the length of its stem that is needle-shaped, then it is a bull thistle. Bull thistles have also been described to be larger and “meaner-looking” than the Canada thistle.
Both species mature to be about four feet tall and can grow in just about any environment. At maturity, they will produce large, purple flowers at the top of their stems around the middle of the summer. Towards the end of the summer and into the fall these flowers will become seed heads, with dandelion-like seeds. These seeds are dispersed primarily by the wind but may also attach to animals passing by. While bull thistle lives for just two years (biennial), usually flowering the second year, Canada thistles are perennial weeds, surviving more than two years without external interference. This means that the same mature plant can produce millions of seeds in their lifetime.
How did it get here?
Canada thistle is thought to have been brought over by the French to Canada during the 1800s as an alternative feed option of pigs and other livestock. It was initially native to eastern Europe and western Asia. It can most commonly be found in agricultural areas and along roadways where there are high levels of disturbance. In these areas, native thistles are often mistaken for invasive thistles and removed, which opens up a niche for the invasive thistles to inhabit. Canada thistle prefers a colder climate with traditional seasons, so it does not do as well in the southern states. Bull thistle has a similar history to Canada thistle, however, its introduction to the U.S. is more ambiguous. It likely wasn’t used as feed stock, because while it is edible as well, its larger thorns can injure the mouths of livestock, leading to infection.
Why is it invasive?
Both species have been found in all 50 states and Canada. Their success can mainly be attributed to its diverse methods of dispersal, using both wind and animal transportation. The seeds can travel miles on the wind due to their light-weight nature and sail-like heads. They are also a food-source for small birds and can be ingested by herbivorous mammals while they are feeding on the leaves. Canada thistle was also found to be allelopathic, which means that it secrets chemicals from its roots that are toxic to surrounding plants, giving it an advantage. Thistles are notorious for draining nutrients from the soil around them, so they can be very damaging to pastures and crops. They also use large amounts of water, so they can make soil dry and increase erosion potential.
Global climate change has created a unique opportunity for thistles. A study conducted in 2011 found that the higher temperatures in the northern hemisphere during the summer months and much colder temperatures in the winter months caused an increase in growth and dispersal of many species of thistle. As we continue forward, unfortunately, we may begin to see much greater numbers of thistles due to this.
Management at Acadia
The Invasive Plant Management Team (IPMT) at Acadia National Park works throughout the summers to remove thistles within the park. Major areas of interest in the past have been the Seawall picnic area, and all along the Seawall beach to Ship Harbor, as well as in open areas, such as the Great Meadow. To manage these invasive plants the team uses a combination of mechanical and chemical removal techniques. All treatment is set to be accomplished before the thistles go to seed at the end of the summer and into the fall, so it is prioritized in the beginning and middle of the summer. Most often Canada thistles are treated with herbicides because if the entire plant and roots are not removed by hand-pulling, it can regrow. Hand-pulling Canada thistles is made more difficult by their large taproots. Bull thistles can be hand-pulled when they are mature and flowering, but since they grow very large, they may require herbicide or machinery to be effectively removed.
One of the best ways to manage thistles at home, especially in lawns or pastures, is by routine mowing. While this may not eradicate the thistles, it will prevent them from going to seed, preventing any further spread. Ideally, they should be removed fully from the ground and discarded before they reach maturity, however they can sometimes be tricky to spot until they are very large. Canada thistles specifically should be dug up with a spade, as pulling them may leave a rhizome behind. Be sure to look under large, low-hanging evergreen trees, as this is a favorite spot for thistles. Be careful when removing thistles by hand, and wear thick, protective gloves to avoid any injury from the plant. If you see thistles on or near park land, please contact the head of the Acadia IPMT at the email or phone number listed below.
Andersen, Chad P, and Svata M Louda. 2006. “Abundance of and Floral Herbivory on Exotic Bull Thistle versus Native Tall Thistle in Western Tallgrass Prairie.” Faculty Publications in Biological Sciences: University of Nebraska-Lincoln. 19.
Hodgson, Jesse M. 1927. The Nature, Ecology, and Control of Canada Thistle. Agricultural Research service, U.S. Department of Agriculture.
Moore, R. J. 1975. “THE BIOLOGY OF CANADIAN WEEDS.: 13. Cirsium Arvense (L.) Scop.” Canadian Journal of Plant Science 55 (4): 1033–48. https://doi.org/10.4141/cjps75-163.
Zhang, Rui, Eelke Jongejans, and Katriona Shea. 2011. “Warming Increases the Spread of an Invasive Thistle.” PLOS ONE 6 (6): e21725. https://doi.org/10.1371/journal.pone.0021725.