Musk, Scotch, & Bull Thistle
Musk (Carduus nutans), Scotch (Onopordum acanthium), and Bull (Cirsium vulgare) thistles are defined as non-native biennial forbs. They can also be described as short-lived perennials or annuals. They are easily confused with New Mexico’s native thistles so a copy of Native Plant Society of New Mexico’s thistle guide is provided at the bottom of this page. The reason Musk, Scotch, and Bull thistles are grouped together here is that they are often confused with each other and management techniques for all three are the same. Basically, to manage this weed, you need to keep it from seeding. Each plant reproduces from seed only. The easiest time to manage these plants is when the rosettes are forming. Simply take a shovel, cut at least four inches of the root and turn the rosette over on the ground. You may need to check back once or twice to make sure the effort was successful. Herbicides can be effective while these weeds are in the rosette stage.
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Musk Thistle Rosette:  |
Scotch Thistle Rosette:  |
Bull Thistle Rosette:  |
If it’s too late to catch it at the rosette stage, it takes a little more work to keep it from seeding. By spring (following rosette development), the weeds begin to bolt (grow stems). Once this happens, they are much more resistant to herbicides because their energy is focused outward, instead of toward their root system.
| Musk Thistle Bolting | Scotch Thistle Bolting | Bull Thistle Bolting |
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If the weed has bolted but flower heads have not yet been produced, you can simply leave the uprooted plant in the field to die. If the plant has already produced flowers, you will need to remove the plant (or at least the flowers) because there is still enough stored energy in the stalks and flower to continue it’s mission to take over the world! The flower heads look similar so here is a guide to identify the weed using the flower:
| Musk Thistle Flower: Can produce up to 20,000 seeds per plant, one third of which are viable in the soil for at least 10 years! Below are images of different Musk thistle flower stages: | |
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| Scotch Thistle Flower: Can produce up to 40,000 seeds per plant, most of which are viable in the soil for 7 to 39 years! Below are images of different Scotch thistle flower stages: | |
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| Bull Thistle Flower: Can produce up to 4,000 seeds per plant, most of which are viable in the soil for up to 10 years! Below are images of different Bull thistle flower stages: | |
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The full-grown plants can vary in size, density, and number of flower heads produced. After the weed bolts, it may try to grow flowers and set seeds multiple times before it dies. With each of these weeds prolific ability to produce seeds and the heartiness of seeds produced, diligence is critical when trying to control this weed.
Below is an educational video with more information about how to identify these plants and various control methods. This video features New Mexico State University Extension Weed Specialist Dr. Leslie Beck and was produced in partnership with Taos Soil & Water Conservation District.
More information on Musk, Scotch, & Bull thistle can be found at: https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5410130.pdf
Not to be confused with New Mexico’s native thistles!
To help differentiate between native and non-native thistles, here is an excellent Native Plant Society of NM Thistle Guide which includes identification of native thistles: NM Thistle ID Guide – NPSNM
For more information about New Mexico’s native plants, visit NPSNM’s website: https://www.npsnm.org/