How to ID Thistles

In Pennsylvania, there are 7 species in the thistle genus Cirsium. Surprisingly, only two of the seven species of Cirsium are non-native to Pennsylvania, Cirsium vulgare and Cirsium arvense. In this post we will look at several tools on how to ID thistles. Often you will hear the single name “thistle” used to name any of the species in the genus Cirsium that is growing in an open field, pasture or meadow habitat. In these conditions you may certainly run into the noxious C. vulgare and C. arvense, but at the same time, you will certainly run into valuable native species such as C. discolor.

It is incredibly important to understand which thistles to keep and which to manage for, as many of the native species have massive ecological benefits. Killing all the thistles on your landscape may be a way to prevent two invasive species from being present, but you will ultimately lose the incredible value of the other 5 native species that you may be fortunate enough to have!

The mature seed heads of our native powerhouse, Cirsium discolor.

Impressive late season pollinator action on Cirsium discolor.

All of the native species in the genus Cirsium play an important role in their respective ecological conditions. This makes the ability to ID one species from another very valuable, especially for land managers. Sometimes it can be tricky to remember how to discern species in a genus, especially when they can look so similar in appearance. First, being able to identify the two pesky non-native species, which may be most prevalent on the landscape, makes for a great starting point. 

Fortunately, one can ID the species within Cirsium rather easily. The thistles are a part of the plant family Asteraceae, also known as the sunflower family. One key characteristic of flowers within Asteraceae are their involucres. Involucres are a whorled set of bracts that hold the flower in place. If you've ever admired the underside of a big yellow sunflower and noticed the impressive green bracts, you're looking at the involucres .Each individual bracts are known as phyllaries or involucral bracts. 

Underneath the pink flowers of Cirsium vulgare, you can see the sharp pointed bracts that make up the involucre

To separate C. arvense from the rest of the genus we can start with the involucre. Texture, size and shape are all characteristics of these bracts that provide a species level ID. In the case of Cirsium arvense, does the involucral bract measure 1-2 cm high? If so, this is Cirsium arvense, also known as Canada thistle. This can serve as a great starting tool for an ID. Once familiar with the other species of Cirsium you will notice that the inflorescence on C.arvense is noticeably smaller than the other species in the genus. This size difference will allow you to spot C.arvense from a distance.

Canada thistle is also a strong colony builder, often spreading from rhizomes. This makes for an interesting challenge when attempting to control its growth in the landscape! This rhizomatous growth habit is another helpful ID characteristic. As a tip, never cut this species without plans to fully remove or treat with herbicides. You may want to cut this to prevent it from going to seed, but you will ultimately create a growth response that will only send out more stems! In one growing season you could cut back a small patch in the spring and by the end of the summer you could have a patch double or triple the size. Even worse, this new expanded patch can and will flower and set seed that late into the season!

Cirsium arvense with fluffy mature seed heads. Notice the colony forming growth habit! The ability for C.arvense to grow from rhizomes is one of the many reasons it has become so widespread.

To separate the other non-native Cirsium, C. vulgare, we use another tool. This time we can single out the species by looking at the plant's stem. Unlike any of the other species found in the genus, Cirsium vulgare has a sort of winged stem. This is a result of the leaf extending down the stem where attached. 

Here you can see the characteristic winged stem of C.vulgare. Notice how each leaf blade look as though it rides down the edge of the stem?

Unlike Cirsium arvense, with its ability to dominate the composition of a disturbed area, it is unlikely to see blankets of Cirsium vulgare on the landscape. Unfortunately, it is often found growing among Cirsium discolor and the other native thistles. Even though one single C.vulgare can be hiding in a patch of Cirsium discolor, you can always get your ID by finding the winged stem. With time you will notice small differences in the shapes and sizes of the plant and inflorescence as well.

There are several other ways to identify these two species among their native counterparts, but these two methods tend to be the quickest and most reliable. With time you can strongly familiarize with the species of Cirsium that are found around you and use location, time of flower and general appearance to recognize one from the other.