When most Americans think of thistles (Cirsium spp.) they picture the evil, invasive plants that have been imported from Europe and Asia, and think that the only good thistle is a dead thistle. However, there are a number of thistles native to North America that aren’t invasive, stay in place, and play a niche role in biodiversity.
Why Native Thistles Are Not Invasive
Three fairly common native thistles that have an overlap in range between the northern United States and southern Canada are Drummond's thistle (Cirsium drummondi), Flodman’s thistle (Cirsium flodmanii), and wavy-leaf thistle (Cirsium undulatum). These species evolved through time with a number of native insects to keep them in balance with other plant species. Any native plant specialist who has tried to collect seed from native thistles can attest that it is often very hard to find large amounts of viable seed that's not had insect predation. Much of the time the seed heads have been totally gutted by insects. Thus, the reproductive capacity through seed is limited.
Some of the most invasive and noxious plant species that we fight today were originally brought in from other continents as benign garden plants. After being introduced into their new locations, some are thought to have hybridized with similar species, and thus become problematic due to having no natural predators. Wavy-leaf and Flodman’s thistles have also been reported to hybridize with each other, but the seed produced has been found to be highly sterile and won’t germinate.
Most native thistle species are either biennial or perennial. Their root structure is either a tap root or horizontal runner roots that produce root sprouts. Some invasive thistles, like Canada thistle (Cirsium arvense), have rhizomes that readily produce new stems from each joint in the root. The rhizomes give Canada thistle the advantage to spread agressively on disturbed sites. In situations of ground disturbance and lack of competition native thistles will also spread, but not as aggressively as their invasive cousins. Once the disturbed site is again vegetated, the native thistles will fall back into their natural ecological balance.
The Value of Native Thistles
Many insects depend on native thistles for sustenance. This is a direct relationship where the insects are being nourished by the plant. Some insects are actually known as specialists, which means they only prey on certain plants that they have evolved with through time.
Some researchers believe that the insects that are dedicated to native thistles could cross over to invasive thistles if the natives were gone. This is unproven, however. Other plant ecologists worry that the insects introduced for biological control of invasive thistles will move to the natives once they get the invasive thistles under control. Good point, but also not scientifically proven. In spite of all the debate, one of the author’s observations is that in healthy stands of native plants that contain native thistles, a much more diverse group of insects can be seen than in monocultures of invasive thistles.
Along with thistles, insects also often have a bad reputation with people, but we must be realize that most life on earth is either directly or indirectly dependent on bugs. Native thistles have a co-dependency with insects because they need pollination to produce seed, and as mentioned above, predation of seed is thought to keep populations in balance.
Birds, particularly goldfinches, are partial to native thistle and utilize the the plants by eating the seed and using the down from the seed heads to line their nests.
Native Americans and white settlers ate the fleshy roots of some of native thistles as a vegetable, but today’s human population benefits more indirectly from them. The honey produced by bees that feed on the thistle is known to be particularly good. Insects that feed on native thistle are a food source for mammals higher on the food chain, some of which could end up on the human table.
Native thistles hold a niche in the plant world that is often only appreciated by botanists. Hopefully, through increased awareness of their value native thistles will someday be more appreciated by land managers. As a result, they should learn to distinguish the natives from the aliens, and keep the value of natives in mind during herbicide applications.
References
ThistleDefense, accessed November 5, 2011
Thistles of North Dakota, accessed November 5, 2011
Native Thistles: Expendable or Integral to Ecosystem Resistance to Invasion, accessed November 5, 2011
Douglas W. Tallamy, Bringing Nature Home: How You Can Sustain Wildlife with Native Plants, Timber Press, Portland/London, 2009.
Helen McGranahan - Helen McGranahan has worked her entire career dedicated to wise and sustainable management of natural resources.
