Those of us in the Wyoming Native Plant Society are interested not only in natives, but invasives, for they can be destructive, as we know. However, it seems that many people believe that any thistle is by definition a weed. Not so. We have both native thistles and noxious weeds here, and this post intended to help everyone know the difference.
Plant keys and guides help identify thistles with technical details, but a broad-brush way to tell the so-called good from bad thistles involves things you can see immediately. All the species of native thistles are covered in white hairs that give them a gray or silvery appearance. Elk thistle (Cirsium scariosum) is distinguished by having a single leafy stem 3 feet tall and a cluster of pale flowers near the top.
Our other mid-elevation native species, Teton thistle (Cirsium subniveum), is also grayish in appearance, but it has branches coming off the main stem, each with one to several showy flowers that range from pale to a more intense violet. They are usually around 2-3 feet tall. If you look at the base of the leaves, especially at the lower part of the stem, they are strongly decurrent, as if pasted to the stem for as much as an inch.
Tweedy’s thistle (Cirsium tweedyii or C. eatonii), an alpine species, is similar in appearance to Teton thistle, and also has branches and decurrent stem leaves, but is only seen above around 9,000 feet. Look for it above Tower 3 on the Rendezvous Mountain road.
These native thistle species are found with other forbs and grasses and rarely dominate. In newly disturbed sites, where their airborne seeds can take hold, they can be pioneer species, but as other plants colonize the disturbance, they take their place as an occasional plant among the wide variety of natives. Deer and elk favor the young growth and flowers, and pollinators of all kinds visit the many disk flowers. Seeds are eaten by birds and small rodents.
The most common non-native thistles do not appear gray or silvery. The plants have hairs, but they still look mostly dark green. Canada thistle (Cirsium arvense) grows in clumps, has narrow stems and leaves, and clusters of small purple flowers. Usually found in moist places, it can reach 3 feet. This patch, in a recently vacated sheep grazing allotment, is the only green thing left after the sheep have left in fall. A few years later, mountain brome and bluebell are becoming lush and keeping the thistle contained.
Musk thistle (Carduus nutans) is the largest of the weeds, with flowering stems reaching 6 – 8 feet. It has a thick-leaved, robust base and a stout stem that branches near the top. Its deep-magenta flowers can be more than an inch wide.
Bull thistle (Cirsium vulgare) is less widespread, and more silvery looking, than other weedy thistles. It is branching, kind of like Teton thistle, but the habit is more upright and it has stiff bristly stem leaves that are not decurrent. The calyx is tall and vase-shaped, whereas the calyx of the Teton thistle appears more rounded.
The photo below shows a patch of bull thistle gone to seed – an impressive number of parachute seeds that can take off in the wind and colonize disturbed areas deep into the backcountry.
A few recommended do’s and don’ts for anyone interested in helping reduce these weeds:
• Let Teton County Weed and Pest know if you find an infestation, especially of musk thistle. UTM (GPS) coordinates nice to have if in the backcountry. Weed and Pest will send crews to spray if needed.
• If you see someone pulling native thistles, let them know these aren’t weeds.
• Do pull musk thistle if so inclined (thick gloves recommended!). They have tap roots so they can be pulled. However, if they are in bloom it does no good to leave them lying there. The blooms will mature and go to seed. Before they bloom they can be pulled and left.
• Don’t pull thistles unless you’re sure of the identity.
• Don’t try to pull Canada thistle. It has underground rhizomes and cutting them will only stimulate growth.
TC Weed and Pest (www.tcweed.org) is a good resource for identifying weeds, thistles or otherwise.
Susan Marsh, Jackson, WY