Scotch thistle (Onopordum acanthium)

Synonyms: Onopordon acanthium, Carduus acanthium

Common Names: Cotton thistle, Scotch cotton thistle, cottony thistle, heraldic thistle

Description:

Scotch thistle (Onopordum acanthium) is a huge, spiny, gray-woolly thistle in the aster family, native to Europe and western Asia, that was brought to North America as an ornamental for its bold silvery foliage and large flowers [1][3]. It has escaped onto rangeland, pastures, roadsides, and waste ground, where it grows head-high and forms dense, impassable, spiny thickets [1][2]. MISIN tracks it because those thickets shut livestock, wildlife, and people out of the land, crowd out useful plants, and are backed by a huge, long-lived seed bank that makes the plant very hard to clear [1][2].

Habit:

Scotch thistle is unmistakable once it is full-grown: a coarse, much-branched biennial that commonly stands 3 to 8 feet tall and can reach 12 feet, with a single plant sometimes bearing more than a hundred flower heads [1][2]. The whole plant is covered in a thick mat of cottony white hairs that give it a distinctive gray-green to bluish-white color [1]. Its stems carry broad, spiny, ribbon-like "wings" that run down from the flower heads to the base ? a key feature [1]. The leaves are large, up to 2 feet long, lobed, and armed with sharp yellow spines [1].

It lives two years. In its first year it is only a rosette ? a low, flat cluster of woolly gray leaves that can spread up to 5 feet across, sitting over a stout taproot [2]. In its second year it bolts upward, flowers, sets seed, and dies. The flowers are globe-shaped heads about 1 to 2 inches across, dark pink to purple, ringed with stiff spiny bracts, and ? unlike nodding musk thistle ? they stand upright [1][2]. It blooms from July into fall [stored].

Look for the flowering plants from mid to late summer, when the tall gray spiny stalks and purple heads tower over surrounding vegetation, and watch for the big woolly first-year rosettes in spring and fall [2].

At-a-glance cues:

1. Very tall (3 to 12 ft), coarse, branched biennial thistle covered in cottony hairs that make it look gray-green to bluish-white [1][2]
2. Broad, spiny, ribbon-like wings running down the stems [1]
3. Large spiny lobed leaves up to 2 feet long with sharp yellow spines [1]
4. Upright globe-shaped flower heads about 1 to 2 inches across, dark pink to purple, ringed with spines [1][2]

Leaves:

1. Arrangement: first-year leaves in a basal rosette; second-year leaves alternate up the stem, mostly stalkless [1]
2. Shape and size: oblong, lobed or toothed, large ? up to about 2 feet (60 cm) long and a foot wide [1]
3. Margins: armed with sharp yellow spines [1]
4. Surface: both sides covered with a thick mat of cottony white hairs, giving a gray-green color [1]

Stems:

1. Habit: a coarse, erect, much-branched biennial, commonly 3 to 8 feet tall and up to about 12 feet [1][2]
2. Wings: broad, spiny, ribbon-like wings run down the stems from the flower heads to the base [1]
3. Surface: densely woolly, giving a bluish-white to gray appearance [1]
4. Root: a stout, fleshy taproot reaching a foot or more deep [2]

Flowers:

1. Color: dark pink to purple or lavender [1]
2. Shape and size: globe-shaped heads about 1 to 2 inches (2.5?5 cm) across, ringed with stiff, needle-like spiny bracts [1]
3. Arrangement: borne in groups of two or three at the branch tips, held upright (not nodding) [1][2]
4. Bloom time: mid summer into fall [stored]

Fruit and seeds:

1. Seeds: smooth, slender, gray, about 4?5 mm long, each tipped with a plume of hairs (pappus) that can be twice the seed's length [stored][1]
2. Output: a single plant can produce roughly 8,400 to 40,000 seeds [2]
3. Longevity: seeds stay viable in the soil for many years ? commonly cited at up to about 20 years [stored][2]
4. Dispersal: seeds move on the wind and are also spread by water, livestock, wildlife, people, and equipment [1]

Habitat:

Scotch thistle is native to Europe and western Asia and has been introduced across temperate North America and Australia [1]. In North America it is most troublesome on western rangeland, but it turns up in scattered populations across many regions, including the Midwest [1][3].

It is a plant of open, sunny, disturbed ground. It grows in pastures, rangeland, fields, waste places, fence lines, gravel pits, and along roadsides and railways, and it often follows disturbance around old buildings and livestock areas [1]. It favors deep, moist, fertile soils and disturbed sites but tolerates a wide range of conditions [2]. Because it needs open ground to establish, it takes strongest hold on overgrazed or bare land [1][2].

Reproduction:

Scotch thistle spreads only by seed, but it is a prolific seed producer [1][2]. For a homeowner or land manager, the ways it arrives are worth knowing: the plumed seeds catch the wind, and they also travel in mud, in water, on livestock and wildlife, and on vehicles and equipment moving through an infested area [1]. A patch on a neighbor's overgrazed field or a nearby roadside can seed its way onto your ground.

The output is large and long-lasting. A single plant can set 8,400 to 40,000 seeds, and those seeds can lie dormant in the soil for many years ? commonly cited at up to about 20 years ? so the buried seed bank keeps producing new plants long after the visible thistles are gone [2].

It is a biennial, and its life cycle is the key to controlling it. In the first year it is only a low rosette over a taproot; in the second year it bolts, flowers, and sets seed, then dies [2]. The control message follows directly: kill the plants before they flower and set seed in summer ? and because the seed bank is so long-lived, plan on returning for several years to catch each new crop of rosettes [1][2].

Impact and Damage:

Scotch thistle's main harm is physical, and it is severe. It grows into dense, head-high stands of coarse, spine-covered plants that form an impassable barrier ? walls of thistle that keep livestock, wildlife, and people from reaching forage, water, and open ground [1][2]. On a property, a stand can take over a pasture or field, make a yard or campsite unusable, and block trails and fence lines; the spines make it painful to handle without heavy gloves [2]. It is not known to be poisonous ? the danger is the spines and the sheer density, not toxicity.

Ecologically, it crowds out and replaces desirable and native plants on the ground it invades, lowering the value of pasture and rangeland and reducing the forage available to both livestock and wildlife [1][2]. Because it establishes fastest on bare and overgrazed ground, it both follows land degradation and worsens it, and its enormous, long-lived seed bank lets an infestation rebound quickly if control lapses [1][2].

The economic weight falls on ranching and land management. Scotch thistle causes significant economic losses for ranchers on western rangeland, both through lost forage and the cost of controlling a weed that resists quick fixes, which is why it is treated as a serious, widely regulated rangeland invader [1][2].

Similar species:

Scotch thistle is one of several large, spiny thistles that get confused with one another. What sets it apart is its combination of great size, the dense cottony hairs that make the whole plant look gray-blue, and the broad spiny wings running down its stems.

Bull thistle (Cirsium vulgare) is the big spiny thistle most often mistaken for it, and it also has spiny-winged stems [1]. Bull thistle is greener and much less woolly, though, its leaves are bristly-hairy rather than gray-felted, and it is generally shorter ? it lacks Scotch thistle's overall gray-blue, cottony look.

Nodding thistle (Carduus nutans, also called musk thistle) is another large, showy thistle with purple heads [1]. Its flower heads are larger and characteristically nod or droop to one side, while Scotch thistle's smaller heads stand upright, and nodding thistle is smooth and green rather than densely woolly.

Spiny plumeless thistle (Carduus acanthoides) shares the spiny-winged stems [1]. It is distinguished by its green, hairless-to-thinly-hairy foliage and smaller flower heads, lacking the thick gray wool and the great size of Scotch thistle.

Italian plumeless thistle (Carduus pycnocephalus) is a smaller winged thistle with clustered pink heads [1]. It too is green rather than gray-woolly, and it stays far smaller than the towering Scotch thistle.

Monitoring and rapid response:

Signs to watch for:

1. From mid to late summer, very tall gray-woolly spiny plants with upright purple flower heads towering over pasture, roadsides, or waste ground [1][2]
2. In spring and fall, big woolly gray-green first-year rosettes, sometimes several feet across, on bare or disturbed soil [2]
3. New plants near overgrazed areas, fence lines, livestock spots, and old buildings [1]

What you can do:

1. Wear thick gloves and long sleeves ? the spines are sharp. Dig or cut plants a few inches below the soil surface to sever the taproot below the crown, so the plant cannot regrow [2][stored].
2. Time it right: treat first-year rosettes in spring or fall, and cut second-year plants before they flower and set seed in summer ? once a plant seeds, the work is undone [2].
3. Bag and remove any plants that already have flower heads, since heads can still ripen seed after cutting; do not leave them on the ground [2].
4. Repeated mowing or cutting can knock back a bolting stand, but plan on follow-up because plants resprout and the seed bank keeps germinating [2].
5. Commit to several years. Because buried seeds stay alive for many years, clearing the visible plants is only the start ? return each spring and fall to remove new rosettes until the seed bank runs down [1][2].
6. For a natural area or ranch crew, prioritize small, new, outlying patches before they seed, and improve ground cover and grazing management so bare soil does not invite reinfestation [1][2].

When to call a professional:

1. Large, established infestations across pasture or rangeland, where the scale and the long-lived seed bank call for a coordinated, multi-year plan [1][2].
2. Situations where hand and mechanical removal cannot keep up with regrowth and reseeding.
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. IUCN/SSC Invasive Species Specialist Group. Global Invasive Species Database: Onopordum acanthium. iucngisd.org
2. Utah State University Extension. Identification and Management of Scotch Thistle (Onopordum acanthium) in Pastures, Rangelands, and Non-Crop Settings. extension.usu.edu
3. USDA NRCS Plants Database. Onopordum acanthium L. (Scotch cottonthistle), symbol ONAC. plants.usda.gov

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