Scotch Thistle

Onopordum acanthium

Class A Noxious Weed

This plant is new to the state and has been designated a Class A Noxious Weed by the New Mexico Department of Agriculture.

Commercially Available

Despite being classified as a noxious weed, this plant is still widely sold in stores! Keep an eye out, and remember that it is illegal to plant and transport noxious weeds in the state of New Mexico.

Agricultural Threat

This plant threatens our state's food security and economy by reducing agriculture yields and/or degrading soil resources.
Read more >>>

QUICK FACTS

Scotch thistle is a large biennial plant that can reach heights of up to 8 feet. Its sharp, spiny leaves and fluffy flower heads bloom in midsummer, typically in shades of dark pink to lavender.
It thrives in varied environments, particularly in moist areas like riparian zones, irrigated fields, or acequias. However, it can also establish itself in disturbed, drier regions and is often found alongside other invasive species.
Scotch thistle is a competitive, noxious weed with sharp spines that deter grazers. It also affects the health of surrounding vegetation and poses a risk of injury to children and pets.

1. Overview

family	Asteraceae – daisies & sunflowers	origin	Europe
life cycle	Biennial	other names	Cotton thistle, Horse thistle

The Monster Thistle.

If you live in the West, you will likely notice this gigantic grey-green plant with its formidable (and painful) spines, especially if it appears in your garden, pasture, or croplands. Brought to America for floral arrangements, Scotch thistle or cotton thistle is a large invasive plant that is thankfully easy to spot but poses a significant threat to pastures and livestock. It also only reproduces through seeds, which can be prevented by removing and disposing of flower heads early in their development. This is easy to do in small areas but may prove difficult for larger infestations. Scotch thistle prefers access to water but can be established anywhere.

History of Scotch Thistle

Scotch thistle was brought to North America in the late 1800s as an ornamental plant. It quickly gained popularity for its dramatic appearance in gardens and floral arrangements. Gardeners and florists used it for its aesthetics and resilience. The flowers remain viable long after being cut, allowing for prolonged enjoyment in floral displays. [1]

Today, Scotch thistle is a widespread invasive species found in 37 states in the U.S. and six Canadian territories. Its ability to survive in various soil types and environments has made it prolific in many regions across North America.. [6]

The Scotch thistle has a fascinating role in Scottish history during the Viking invasions of the tenth century. One of the most famous stories comes from the reign of King Malcolm I when Vikings attempted to capture Staines Castle. As they drew near, they unexpectedly found themselves slowed down by the thick thistles that had taken over the dry moat around the castle. The Northmen cried out in pain when their bare feet were pierced by the thistle’s sharp spines. These shouts were heard by the defenders, who launched a counterattack against the distracted invaders, and the Scots secured a decisive victory. [1,3]

Though the Scotch thistle is steeped in Scottish lore and symbolism, it is not native to Scotland. It was likely introduced from mainland Europe before the 16th century. Early depictions on Scottish coins show an image resembling Cirsium vulgare, also known as spear thistle or bull thistle, which may have been the true Scottish thistle. [7]

2. ID Guide

What does it look like?

Scotch thistle is a gigantic plant that can reach heights of 8 feet or more and achieve up to a 6-foot wide diameter, with main stems that measure up to 4 inches in diameter at the base. The stems feature prominent vertical rows of spiny, ribbon-like “wings” that extend up to the flower heads. A variant of Scotch thistle, O. tauricum has a very similar appearance to O. acanthium but lacks the dense, wooly hairs. [1]

Having trouble identifying a weed? Contact us.

Key Features

Roots: Scotch thistles possess a large fleshy taproot. The roots are not regenerative and will generally die if severed below the surface. [1]
Leaves: Its leaves, which can grow up to 2 feet long and 1 foot wide, are armed with sharp, green-white spines and are covered with a thick mat of cotton-like or woolly hairs, contributing to its alternate name, cotton thistle. This unique foliage often gives the plant a distinctive gray-green appearance. [1]
Flowers: The flowers bloom in midsummer. They are fluffy, globe-shaped heads that cluster in groups of 2-3 at the tips of the stems. These flower heads can reach up to 2 inches in diameter and are adorned with long, stiff, needle-like bracts at their base. The flowers’ colors vary, typically from dark pink to lavender. [1]
Seeds: Each flower head produces 110-140 black-brown oblong seeds that are wrinkled and about 3/16 inches long. Each seed is tipped with a bristly pappus, which allows for wind dispersal. [1]

5273045

The flowers bloom in midsummer. They are fluffy, globe-shaped heads that cluster in groups of 2-3 at the tips of the stems.

5448097

In its first year, Scotch thistle forms a rosette with a deep taproot extending more than a foot into the soil. The plant bolts as early as the second year, building a formidable structure.

1459800

Scotch thistle roots are not rhizomatous, and early removal of the rosette and root crown will kill the plant.

5405758

The leaves, which can grow up to 2 feet long and 1 foot wide, are armed with sharp, green-white spines and are covered with a thick mat of cotton-like or woolly hairs.

5448071

Scotch thistle is a gigantic plant that can reach heights of 8 feet or more and achieve up to a 6-foot wide diameter!

5448076

Each flower head produces 110-140 black-brown oblong seeds that are wrinkled and about 3/16 inches long. Each seed is tipped with a bristly pappus, which allows for wind dispersal.

3. Infestation Basics

“Onopordum Acanthium…A single, villainous looking shrub, 6 or 7 feet tall and nearly as wide, existed, a few years ago, in this town. Apparently an introduction of rare and local occurrence.”

-The New England Botanical Club, 1925

Where does it grow, and how does it spread?

Scotch thistle thrives in moist areas like riparian zones and irrigation ditches but can also invade drier, disturbed habitats such as construction sites, overgrazed pastures, and roadsides. Its adaptability and tendency to coexist with other invasive species, like cheatgrass, make it a persistent weed in degraded landscapes.

It is a robust biennial that typically lives for two growing seasons, although it can sometimes behave like an annual or even a short-lived perennial. This is believed to give the plant a distinct competitive advantage despite being competitively weak in its native range. During the late fall, seeds usually germinate, but they can also sprout at other times of the year. The seeds contain a water-soluble growth inhibitor and thus germinate more easily when more water is present. Seedlings that emerge in late autumn develop as true biennials, while those that appear in late summer or early autumn act more like annuals. [1,2]

In its first year, Scotch thistle forms a rosette with a deep taproot extending more than a foot into the soil. The plant bolts as early as the second year, building a formidable structure. Flowering from July to September, the plant is known to produce an impressive number of seeds—ranging from 8,400 to 40,000. Remarkably, these seeds can remain viable in the soil for over 20 years. Unlike other thistles, which can spread through creeping rhizomes, Scotch thistles can only reproduce by seed. [1,2]

Scotch thistle likes to grow in areas with high soil moisture, such as riparian zones or irrigation ditches and acequias. Its adaptability allows it to invade agricultural fields throughout the western United States. Beyond these moist habitats, Scotch thistle can also be observed in drier regions, particularly where the soil has been disturbed or degraded due to human activity, such as construction sites, overgrazed pastures, or uncared-for roadside landscapes. Additionally, it often occurs in conjunction with other invasive species like cheatgrass (downy brome), which can create conditions that further support its spread. Scotch thistle will thrive in areas frequently exposed to nitrogen fertilization, such as pastures or surface runoff areas from fertilized fields. [1]

Key takeaway: Scotch thistle thrives in moist and dry disturbed habitats, making it a resilient and aggressive invader in areas with compromised native vegetation.

Why is it so invasive?

Scotch thistle overwhelms desirable plants and crops by forming large stands that monopolize sunlight, water, and nutrients. Its spiny foliage creates physical barriers, deterring livestock and wildlife from grazing and further allowing it to dominate an area.

Scotch thistle produces thousands of seeds per plant, which can remain viable in the soil for years and are easily spread by wind, water, or human activity. Its adaptability enables it to thrive in damp riparian zones, irrigation ditches, and dry, disturbed habitats such as overgrazed pastures, roadsides, and construction sites. [1]

Scotch thistle is particularly adept at deterring both agricultural and wild grazers. Livestock may graze on the plant in its early stages, but later on, Its sharp spines often act like a barbed wire fence, preventing access to nearby forage or even to entire grazing areas. The enormous basal area of the Scotch thistle plays a crucial role in its competitive advantage. The plant’s large leaves can quickly overshadow and crowd out other, more desirable plants, reducing nearby forage plant’s health and productivity. These large thistle plants can pose a significant risk. The sharp, jagged spines can cause painful injuries, making them particularly dangerous in areas with children or pets. [1]

An unfortunate trait of Scotch thistle is its robust seed development. After the flower head is cut, the seeds can mature and disperse. Many landowners use cutting the flower heads as a control method, but this often leads to unintended consequences, as flower shop patrons or uninformed landowners may further spread the seeds. [1]

Key takeaway: Scotch thistle outcompetes desirable plants, reduces forage accessibility for livestock and wildlife, and poses a risk of harm to children and animals.

Common risk factors for invasion

Disturbed Soils: Scotch thistle thrives in areas where the soil has been disturbed, making these sites ideal for its establishment. Overgrazing, construction, or plowing create open spaces where the weeds can quickly take hold. These disturbed areas often lack the competition needed to suppress Scotch thistle.
Reduced Competition from Desirable Plants: Poor land management, such as overgrazing or inadequate cover crops, creates an environment where Scotch thistle can dominate. Native plants are weakened or absent, giving the weed a competitive edge. Without intervention, Scotch thistle can form dense stands that continually crowd out native species and reduce biodiversity.
Seed Dispersal Opportunities: The lightweight seeds of Scotch thistle are easily spread by wind, water, and human or animal activity. Vehicles, livestock, and even water runoff can transport the seeds to new locations, facilitating their spread. Once established, the plant produces abundant seeds, creating a reinfestation cycle if not controlled. Flat, windy areas are particularly at risk for infestation.

Impacts

Agricultural

Scotch thistle takes over open fields when other crops are not present. Allowing the plant to take over will guarantee severe soil nutrient depletion, making it challenging to re-introduce crops or establish a healthy pasture. The thistle’s sharp spines and tall height often form a natural barrier, preventing livestock from accessing forage areas. [1,8]

Health and Safety

The plant’s dense, spiny foliage can physically injure children, workers, livestock, and wildlife, causing puncture wounds, skin irritation, or infections. The obstruction of firebreaks and pathways by large thistle infestations can exacerbate fire risks in dry regions, threatening property and personal safety. [1,8]

4. Management Strategies

Scotch thistle only reproduces from seeds, so preventing seed head production is most critical. This can be difficult, however, as the seeds become fertile even after the flower is removed. The most effective management measures involve the prevention of flowerheads forming in the first place. Sheep and goats (not cattle) will readily consume thistle rosettes when young. This can have a significant impact on the plant’s growth as they will not be able to produce stems and, subsequently flowers and seeds. [1,8]

Prevention is the first step in the management of Scotch thistle. If using equipment to remove already seeded thistle stands, clean the equipment before moving it to another site. Thistle seeds easily contaminate crop seeds, and the seed purchased should be well reviewed and tested to prevent infestation of scotch thistle (among other weeds). Frequent checks should be performed on private and public lands to prevent the spread of new infestations. Scotch thistle is relatively simple to eradicate if caught early and before seed production. [1]

This weed thrives in frequently disturbed areas with little competitive vegetation. Biennial thistles suffer most when competing with perennial grasses. A successful culture of healthy grass species will inhibit the growth of young thistle plants and potentially slow the invasion’s spread. [1]

DO’s

Identify the plant when it is young; young rosettes can be easily removed by hand
Be wary that this plant is currently sold in flower arrangements and as an ornamental, so it could appear anywhere
Out-compete the plant with well-established crops and native grass communities.

DON’Ts

Let it go to seed. Scotch thistle produces thousands of seeds per plant, which can remain viable in the soil for years.
Ignore early infestations Waiting until an infestation is extensive before taking action makes control significantly harder.
Let the plant become established; management becomes more difficult with time, especially after its first year of flowering.

** The following information is provided courtesy of the UC Weed Research and Information Center. The Taos Soil and Water Conservation District does not endorse the use of any particular product, brand, or application thereof. **

Scotch thistle can be managed through a combination of mechanical control like hand-pulling, mowing before flowering, and cultivation, as well as grazing with sheep or goats on young plants. Supporting these efforts with good pasture management and minimizing soil disturbance helps prevent regrowth and spread.

Hand-pulling	Small infestations can be removed by manual methods. Digging is effective and the preferred manual removal method. When digging, sever the root below the soil surface.
Mowing	Mowing in the late bolting or bud stage can reduce seed production. Mow before flowering to prevent seeds from developing in severed flowerheads. On sites with high soil moisture, plants may need to be remowed to prevent flowering on regrowth. When mowing is conducted too early, it only delays flowering. Similarly, when plants are cut too late in the flowering process, viable seed may still develop in the seedhead. Because there can be wide variation in plant maturity, a single mowing is unlikely to provide satisfactory control, but repeated mowing throughout the entire growing season can be successful. For total kill, plants must be cut off below the soil surface with no leaves remaining.
Tilling	Tillage will control emerged plants but often stimulates germination. Land managers using tillage for seedbed preparation for reseeding efforts should prepare for a flush of seedlings when soils become saturated.

Sheep, goats and horses, but not cattle, have a significant effect on thistles in the early stages of infestation when they eat young thistle plants. Sheep may graze small rosettes. Goats ignored the leaves of Scotch thistle, but they ate all the flower heads and prevented seed production and dispersal. Cattle avoid grazing Scotch thistle. Overgrazing promotes Scotch thistle.

Fire is not an effective control.

Promoting competitive vegetation can slow spread and help prevent establishment. Perennial grass plantings have been shown to inhibit Scotch thistle seedling establishment and can reduce Scotch thistle populations. Perennial grass stand density and vigor should be managed to minimize bare ground exposure.

No biological controls are currently available in the United States. Australia has released biocontrol insects, but some of them have failed host specificity tests in the U.S. Insects are being evaluated by USDA for release in the U.S.

The following specific use information is based on published papers and reports by researchers and land managers. Other trade names may be available, and other compounds also are labeled for this weed. Directions for use may vary between brands; see label before use. Herbicides are listed by mode of action and then alphabetically. The order of herbicide listing is not reflective of the order of efficacy or preference.

2,4-D

Several names

Rate: 1 to 2 qt product/acre (0.95 to 1.9 lb a.e./acre)

Timing: Postemergence from rosette to beginning of bolting, or fall rosette. Most effective on small rosettes.

Remarks: Often tank-mixed with chlorsulfuron or dicamba for quicker burndown. Does not control large bolting plants. Broadleaf-selective and safe on most grasses. 2,4-D has minimal soil activity. Do not apply ester formulation when outside temperatures exceed 80°F. Amine forms are as effective as ester forms for small rosettes, and amine forms reduce the chance of off-target movement.

Aminocyclopyrachlor + chlorsulfuron

Perspective

Rate: 4.75 to 8 oz product (Perspective)/acre

Timing: Postemergence and preemergence. Postemergence applications are most effective when applied to plants from the seedling to the bolting stage.

Remarks: Aminocyclopyrachlor provides excellent control of Scotch thistle at most growth stages. One of the few herbicides that provides soil residual control 1 year after application. Perspective provides broad-spectrum control of many broadleaf species. Although generally safe to grasses, it may suppress or injure certain annual and perennial grass species. Do not treat in the root zone of desirable trees and shrubs. Do not apply more than 11 oz product/acre per year. At this high rate, cool-season grasses will be damaged, including bluebunch wheatgrass. Not yet labeled for grazing lands. Add an adjuvant to the spray solution. This product is not approved for use in California and some counties of Colorado (San Luis Valley).

Aminopyralid

Milestone

Rate: 5 to 7 ounces product/acre (1.25 to 1.75 oz a.e./acre)

Timing: Postemergence from the rosette to young bolting stage.

Remarks: Longer soil residual than clopyralid. Safe on most grasses, although preemergence application at high rates can greatly suppress some annual grasses, such as medusahead.

Applications can decrease seed production in some annual and perennial grass species. Provides over 90% control when applied to rosettes. For postemergence applications, adding a non-ionic surfactant (0.25 to 0.5% v/v spray solution) enhances control under adverse environmental conditions; however, this is not normally necessary.

Other premix formulations of aminopyralid can also be used for Scotch thistle control. These include

Opensight (aminopyralid + metsulfuron; 1.5 to 2.5 oz product/acre) and Forefront HL (aminopyralid

+ 2,4-D; 1.5 to 2.1 pt product/acre); apply at the rosette to bolting stage.

Clopyralid

Transline

Rate: 0.67 to 1.33 pt product/acre (4 to 8 oz a.e./acre). Use higher rate for older plants or dense stands.

Timing: Postemergence from the rosette to young bolting stage. Results are best if applied to rapidly growing weeds.

Remarks: Most effective for young plants. Shorter soil residual than aminopyralid or aminocyclopyrachlor. Controls or injures plants in the Asteraceae and Fabaceae but safe on most other broadleaf species and all grasses. For postemergence applications, a non-ionic surfactant (0.25 to 0.5% v/v spray solution) enhances control under adverse environmental conditions; however, this is not normally necessary.

Clopyralid + 2,4-D

Curtail

Rate: 2 to 4 qt Curtail/acre

Timing: Same as for clopyralid.

Remarks: Add a non-ionic surfactant.

Dicamba

Banvel, Clarity

Rate: 0.5 to 2 pt product/acre (0.25 to 1 lb a.e./acre). Use higher rates for large rosettes and bolting plants.

Timing: Postemergence from rosette to beginning of bolting, or fall rosette.

Remarks: A broadleaf-selective herbicide often combined with other active ingredients. Not typically used alone to control Scotch thistle. Dicamba can also be mixed with 2,4-D (0.5 to 1 pt dicamba + 2 pt 2,4-D/acre) from rosette to bolting stage.

Picloram

Tordon 22K

Rate: 0.5 to 0.75 pt product/acre (2 to 3 oz a.e./acre).

Timing: Preemergence and postemergence. With postemergence application, best time is rosette to early bolting stage, when plants are growing rapidly.

Remarks: Picloram controls a wide range of broadleaf species and has relatively long soil residual activity. Lower rates in rate range may require annual spot treatments. Although well-developed grasses are not usually injured by labeled use rates, some applicators have noted that young grass seedlings with fewer than four leaves may be killed. Picloram is a restricted use herbicide. Picloram is not registered for use in California. Do not apply near trees, or where soil is highly permeable and where water table is high.

Control with lower rates may be improved by tank mixing with dicamba or 2,4-D: picloram + dicamba (0.25 to 0.5 pt picloram + 0.125 to 0.25 pt dicamba/acre), or picloram + 2,4-D (0.5 to 1 pt picloram + 1 to 2 pt 2,4-D/acre).

Glyphosate

Roundup, Accord XRT II, and others

Rate: 1 to 2 qt product (Roundup ProMax)/acre (1.1 to 2.25 lb a.e./acre). Spot treatment: 1.5% v/v solution.

Timing: Postemergence to rapidly growing plants from the rosette to early bolting stage. Remarks: Glyphosate will only provide control in the year of application, and will not kill seeds or inhibit germination the following season. Glyphosate has no soil activity and is nonselective. It can

create bare ground conditions that can make an area susceptible to weed recruitment. In areas with

desirable vegetation, use spot treatment. Glyphosate is a good control option if reseeding is planned shortly after application, as it will not injure seedlings emerging after application. Add a surfactant when using a formulation where it is not already included (e.g., Rodeo, Aquamaster).

Chlorsulfuron

Telar

Rate: 1 to 2.6 oz product/acre (0.75 to 1.95 oz a.i./acre)

Timing: Postemergence from the rosette to flower bud stage.

Remarks: One of the more effective treatments for large bolting plants. Always use a surfactant. Can be tank-mixed with 2,4-D for quicker burndown.

Metsulfuron

Escort

Rate: 1 to 2 oz product/acre (0.6 to 1.2 oz a.i./acre)

Timing: Postemergence from the rosette up until flower-bud stage.

Remarks: Similar activity compared to chlorsulfuron. Always use a surfactant. Metsulfuron can be tank-mixed with 2,4-D for quicker burndown. Other premix formulations of metsulfuron can be used at similar application timing. These include Cimarron Max (metsulfuron + dicamba + 2,4-D), Opensight (metsulfuron + aminopyralid), and Cimarron X-tra (metsulfuron + chlorsulfuron).

Metsulfuron and premixes containing metsulfuron are not labeled or use in California.

5. Citations

[1] Sheley, Roger L., and Janet K. Petroff. Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, 1999.

[2] Burges, N. A., et al. Flora Europaea. 4: Plantaginaceae to Compositae (and Rubiaceae). Cambridge University Press, 1976.

[3] Johnson, Ben. “The Thistle – National Emblem of Scotland.” Historic UK, 15 Nov. 2023, www.historic-uk.com/HistoryUK/HistoryofScotland/The-Thistle-National-Emblem-of-Scotland/

[4] Garsiya, Ekaterina Robertovna, et al. “Traditional Medicine Plant, Onopordum Acanthium L. (Asteraceae): Chemical composition and pharmacological research.” Plants, vol. 8, no. 2, 12 Feb. 2019, p. 40, https://doi.org/10.3390/plants8020040

[5] Bergo, Alan. “Foraging and Cooking Thistles.” Forager, 4 June 2023, foragerchef.com/edible-thistles/

[6] USDA. “Onopordum Acanthium L.” USDA Plants Database, 2024, plants.usda.gov/home/plantProfile?symbol=ONACA

[7] NRS. “Scotch Thistle.” National Records of Scotland, National Records of Scotland, 31 May 2013,www.nrscotland.gov.uk/research/archivists-garden/index-by-plant-name/scotch-thistle#:~:text=The plant we know as,accepted as a national emblem

[8] Anderson, Lars W., et al. Weed Control in Natural Areas in the Western United States. Davis, California, University Of California Weed Research And Information Center, 2013.

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Fun Facts

For many years, Scotch thistle has been traditionally valued for its anti-inflammatory, anti-tumor, and heart-supporting properties. The plant’s flowers contain a unique group of enzymes called “onopordosin,” which can assist in cheese-making by promoting milk curdling. Scotch thistle is rich in natural substances, including flavonoids and other compounds known for their potential health benefits. [4]

Simón Pla García, inaturalist.org

“…Abounding on waste ground, from the towering cliffs, where it rears its head among the crevices, down to the lowliest valley or the brambled nook, where grasses, docks, and nettles tangle about it, and where its purple flowers rise above them all…a curse to toiling man.”

-Anne Pratt, 1899

John M. Randall, The Nature Conservancy, Bugwood.org

Fun Facts

Scotch thistle extracts may support heart health and slow the growth of cancers. Various clinical tests and research back these findings. While invasive and troublesome to landowners, the productive use of these plants may raise awareness and inhibit their spread. Scotch thistle is worth further study for its health benefits and use in traditional medicine. [4]

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Fun Facts

European cuisine frequently embraces Scotch thistle as a delightful and flavorful substitute for artichokes. This plant adds a unique flavor to dishes, offering a slightly nutty and earthy flavor that can enhance salads, pasta, and hearty entrees. [5]