Mario Bautista
Thistles are a genus of plants that get a bad rap. They fall within the aster family, Asteraceae, and are characterized by their very sharp, prickly marginalized leaves that will make you regret touching them or, at the very worst, accidentally stepping on one while barefoot. They are biennial, starting as leaves and roots and not flowering until their second year.
In Illinois, there are several species of invasive thistles, including Canada thistle, Cirsium arvense; bull thistle, Cirsium vulgare; and musk thistle, Carduus nutans. These are all considered noxious species, meaning not only are they an aggressive spreader, but they cause harm in one or many ways. In the case of these invasive thistles, they are guilty of harming productive land, posing harm to humans, resisting chemical controls, clogging waterways and ponds, displacing native plants and altering soil composition. In Illinois, controlling these plants is required by law, thanks to the Noxious Weed Law.
How could we ever love a thistle?
Let’s look at our native thistles. Yes, they still have spines sharper than needles, but what they offer is an incredible source of food and habitat for wildlife of many different classes of animals. Our native thistles include tall thistle, Cirsium altissimum; field thistle, Cirsium discolor; and several others that have unfortunately become very rare in the state and are listed as endangered or threatened.
Native thistle seeds are a food source for songbirds including goldfinches and indigo buntings, as well as mammals like the 13-lined ground squirrel, Ictidomys tridecemlineatus. Over 200 pollinator species enjoy the nectar, while some are hosts to endangered species, such as the swamp thistle, Cirsium muticum, to the swamp metalmark butterfly, Calephelis muticum. Other insects use the foliage of the plant in their diet, with some preferring to only eat thistle leaves and nothing else. Thistles also provide nesting cavities for native bees and are found to be preferable to other nest options. Thistles are also a favorite nectar source for our beloved Ruby-throated hummingbird, Archilochus colubris.
I came across a patch of field thistle on a walk the other day at Heritage Park in Champaign and was pleasantly greeted by well over 10 species of insects on one plant. Monarch butterflies, black swallowtail butterflies, bumble bees, honeybees, tiger beetles, fritillaries and more were happy in each other’s company in the bounty of this thistle’s high-quality nectar. There was even a sleepy moth resting on the stalk.
Native thistles are declining for many reasons. In addition to habitat loss, fire prevention and biocontrol insects, invasive thistle species can take over the niches of our native species. Climate change will only exacerbate this. Rising temperatures increase the growth rate and population size of invasive thistles. Extreme weather events are also harder on our native species than their hardier invasive counterparts. Being able to identify our native species from invasive species is important when it comes to eradicating thistles from any landscape.
I hope you get to know your Illinois thistles before you judge them — native species are an asset to our ecosystems and worthy of our admiration.
Upcoming event
The East Central Illinois Master Naturalists’ annual native tree and shrub sale is open through Sept. 20 with pickup Oct. 4 and 5 at the Urbana Park District. More information is available at go.illinois.edu/nativesale.
