Thistles often get the worst reputation, because many are invasive in pastures and lawns. But did you know there are several native thistles which are important parts of the native ecosystem? We will go over the various thistles, both natives and nonnatives. It is always important to distinguish between the two, since there are insects which rely on them for food and habitat.
There are approximately 64 native species of thistles in the United States. However, for some reason, the state of Iowa has considered 3 of them to be noxious or invasive, despite being native to the prairies and savannahs of Great Plains. As for introduced invasive species, there are only 3 in the Genus Cirsium. The other invasive thistles in the US are from different genera. However, all are often grouped as bad by farmers, ranchers, and pasture managers.
About Thistles
Thistles are members of the Asteraceae Family, and have broad flower heads with ray and disc flowers. Members of the genus Carduus lack ray flowers and are often called plumeless thistles. The flowers are usually pink, white, or purple-pink in color and are attractive to a wide range of pollinating insects. Birds, such as goldfinches eat the seeds.
Altogether, there are around 200 species worldwide. Most of the native thistles of North America have a woolly, white underside on the leaves. This helps distinguish them from the invasive nonnatives. Although, Canada or creeping thistle has a whitish film underneath the leaves. In the field, knowing which is which can help determine whether or not to treat with a harsh chemical or dig out. In my own pasture, I dig out the invasive musk thistle, but leave the native tall thistle.
There are 6 species of native thistles in the Central Great Plains, and 1 hybrid. All the thistles have spines on the leaves and/or the stems. This makes them fairly unpalatable to most herbivores, though goats and donkeys are known for eating them. If you recall Winnie the Pooh, Eeyore was always eating thistles.
Invasive Thistles
All the invasive thistles are native to Europe and Asia. Many of them came over to the United States in boats or with people bringing them for their cottage gardens. Most of the thistle species have various medicinal uses, and therefore were brought over by people who did not know any better. They wanted their own thistle species with them.
Now we have a problem though, as many of these invasives have spread throughout the United States. Treatment is usually done with a hand sprayer, and the applicator riding a 4-wheeler or side-by-side machine through the area of concern. While spraying before or after flowering does usually kill the plant, sprayers do not usually limit their application to the plant only, but also spraying native grasses, forbs, and other plants at the same time. Thistles can also be chopped down and dug out, as long as you get as much of the roots as possible when doing so.
Species
The following list of thistle species are considered widely invasive. Most are on the noxious weeds lists in multiple states.
- Musk Thistle (Carduus nutans)
- Creeping or Canada Thistle (Cirsium arvense)
- Bull Thistle (Cirsium vulgare)
- Scotch Thistle (Onopordum acanthium)
- Yellow Star Thistle (Centaurea solstitialis)
- Russian Knapweed (Rhaponticum repens)
Natives
There are more native thistles in the United States than invasives. Therefore, it can be quite confusing for uneducated spray applicators or farmers who just see all thistles as bad. But there are many faunal associations worth protecting for the thistle. In the Central Great Plains the following 6 species can be found.
- Tall thistle (Cirsium altissimum)
- Wavy-leaf thistle (Cirsium undulatum)
- Platte thistle (Cirsium canescens)
- Field thistle (Cirsium discolor)
- Flodmann’s thistle (Cirsium flodmannii)
- Yellow-spine thistle (Cirsium ochrocentrum)
One of my favorites, the wavy-leaf thistle has light pink blossoms above silvery-gray foliage. When hiking in the Tallgrass National Prairie Preserve by Strong City, KS, you can see this thistle (in June) and all the butterflies which flock to it.
Insect Associations
Thistles are attractive as both a host plant for moths and butterflies, as well as a nectar source for many different kinds of insects. There are at least 17 moths and 2 butterflies which use it as a host plant.
Using Thistles in the Landscape
Now, you would probably not intentionally add thistles to most landscapes and gardens. But you could if you had a Monarch Waystation, Meadow Garden, or Butterfly Garden with a lot of other species already there. I have purposely left seedlings of tall thistle in different parts of my landscape just for the blooms. Also, I had wavy-leaf thistle pop up one year in my backyard lawn, so I let it grow and did not mow it down. It was worth it to see the butterflies using it.
But thistles are not the plant for the tidy landscape, nor the foundation planting. Just like most natives, they grow better when surrounded by other native and adapted plants in a garden setting that is more wild.
Conclusion
Native thistles have been misrepresented by the culture which concludes that because this thistle (invasive musk) is bad, all must be bad. But many thistles have important roles within the landscape, including as a host plant and pollinator nectar source. So learn and let live.
Happy planting!
