Spiny plumeless thistle (Carduus acanthoides)

Synonyms: Carduus acanthoides subsp. acanthoides

Common Names: Plumeless thistle, welted thistle

Description:

Spiny plumeless thistle (Carduus acanthoides), also called plumeless thistle, is a spiny biennial thistle native to Europe and Asia, first found in North America in Camden, New Jersey, in 1878 [1]. MISIN tracks it because it forms dense, spiny stands that push out pasture grasses and other plants, and because it's easily confused with other invasive thistles already established in the region [2][3].

Habit:

Spiny plumeless thistle is an upright, spiny biennial (occasionally acting as a winter annual) that spends its first year as a low, spiny-leaved rosette, then bolts into a branching, spiny-winged flower stalk up to about 4 feet tall in its second year [1][2]. Its stems carry narrow, spine-edged "wings" of leaf tissue running down from the leaf bases, so the whole stem looks bristly and ridged [2]. The deeply lobed leaves have wavy, spiny margins and are hairy on both sides, especially underneath [2][1]. From late spring through fall it produces small, rounded flower heads ? purple to pink, up to about an inch across, made up entirely of tube-shaped disk flowers ? with bracts that each taper to a single sharp spine [2]. Look for it in summer, when the purple flower heads top the spiny stems along roadsides and in pastures; the rest of the year, watch disturbed ground for the low, spiny rosette [1].

At-a-glance cues:

1. Narrow, spiny "wings" of leaf tissue running down the stem from the leaf bases [2]
2. Deeply lobed, spiny-edged leaves in a low rosette the first year [2][1]
3. Small (about an inch or less), purple to pink flower heads made up only of disk flowers [2]
4. Each flower-head bract tapers to a single sharp spine [2]

Leaves:

1. Arrangement: a basal rosette the first year; alternate along the stem the second year [2]
2. Shape: deeply lobed or pinnately divided, roughly 4 to 12 inches long [2][1]
3. Margin: wavy and spiny-edged [1][2]
4. Surface: hairy on both sides, especially the underside [2]

Stems:

1. Habit: herbaceous, erect, freely branched above, growing from a stout taproot [1]
2. Wings: narrow, spiny wings of leaf tissue run down the stem from the leaf bases, extending up to the flower heads [1]
3. Surface: short, upward-pointing hairs between the leaf nodes [2]
4. Size: typically 1 to 4 feet tall, occasionally taller [1]

Flowers:

1. Color: purple to pink, occasionally white [2]
2. Shape: small, rounded heads about ? to 1 inch across, made up only of tube-shaped disk flowers with no daisy-like rays, the bracts each tapering to a single sharp spine [2]
3. Arrangement: at the ends of branches, singly or in small clusters [1]
4. Bloom time: June through October [1]

Fruit and seeds:

1. Fruit type: a small, dry, one-seeded fruit (achene) [2]
2. Seeds: topped with a pappus (tuft of bristles) about ? inch long that is not feathery ? unlike the branched, feathery pappus of Cirsium thistles [2]
3. Dispersal: the pappus is hooked or barbed, which catches the wind as well as fur, feathers, clothing, and equipment [2]

Habitat:

Spiny plumeless thistle is native to Europe and Asia and was first recorded in North America in Camden, New Jersey, in 1878 [1]. It's now found across most of the northeastern, midwestern, and western United States, though it's absent or uncommon in parts of the Southwest and Southeast [1][7]. It grows in full sun on disturbed, open ground ? pastures, roadsides, old fields, waste areas, and annual grasslands ? and spreads fastest where existing vegetation is thin or disturbed [3][1].

Reproduction:

Spiny plumeless thistle spreads only by seed ? it has no creeping roots ? and its seeds are topped with hooked, bristly tufts that catch the wind as well as fur, feathers, clothing, and equipment [2]. That's the main way it reaches a new pasture, roadside, or yard: seed blown in on the wind, or carried in on mowing equipment, contaminated hay, or an animal's coat. It's a biennial, spending its first year as a low, spiny rosette and its second year bolting, flowering, setting seed, and dying [1][2]. Because everything depends on that second-year seed crop, cutting or pulling plants before the flowers open is the single most effective timing for control.

Impact and Damage:

For homeowners, the main issue with spiny plumeless thistle is its spines: the plant's spiny leaves and stem wings make it unpleasant to handle and unwelcome in yards, paddocks, and pastures, where dense, prickly patches take over open ground and crowd out grass and other plants [3]. In a grazed pasture, thistle patches reduce the area livestock will use and the amount of usable forage, since livestock generally avoid grazing near the spines [5].

For natural area stewards, spiny plumeless thistle competes for space with more useful vegetation rather than invading intact natural areas outright. A field study comparing plots with and without Carduus thistles found that invaded areas consistently supported different plant communities than nearby uninvaded ground within the same fields ? white clover, a valuable pasture species, showed up reliably in thistle-free areas but rarely alongside thistle patches ? suggesting the thistles either out-compete surrounding vegetation or actively suppress it [5].

Spiny plumeless thistle also readily hybridizes with musk thistle (Carduus nutans), another invasive thistle already established in the region, producing a fertile hybrid; where the two species grow together, this can complicate both identification and management [4].

Similar species:

A few other thistles get confused with spiny plumeless thistle, especially before the flowers open.

Musk thistle (Carduus nutans) is the closest match and the one it most often crosses with in the wild, producing a fertile hybrid where the two grow together. Musk thistle is larger overall, with flower heads 1.5 to 3 inches across ? noticeably bigger than spiny plumeless thistle's roughly 1-inch heads ? and spiny wings running nearly the full length of its stems [4].

Curly plumeless thistle (Carduus crispus) looks almost identical at a glance, since both are spiny-winged Carduus biennials with purple disk-only flower heads. The most reliable difference is on the underside of the leaf: spiny plumeless thistle has one uniform type of curling hair there, while curly plumeless thistle has two distinct hair types mixed together [2].

Canada thistle (Cirsium arvense) is a common mix-up for beginners, but it lacks the spiny stem wings that give plumeless thistles their name ? its stems are smooth between the leaves. Its flower heads are also smaller, its seed tufts are feathery and branched rather than simple bristles, and it spreads by creeping roots rather than by seed alone, so a "patch" is often one plant with many connected stems [6].

Monitoring and rapid response:

Signs to watch for:

1. Upright, spiny plants with narrow, spiny wings running down the stems, on roadsides, in pastures, and on disturbed ground [1]
2. Deeply lobed, spiny-edged leaves ? a low rosette in the first year [2]
3. Small purple to pink flower heads, made up only of disk flowers, from late spring through fall [2]

What you can do:

1. Dig or cut plants in spring of their second year, before the flower heads open and set seed ? wear thick gloves for the spines [1]
2. If digging, sever the root a couple of inches below the soil surface so the rosette can't regrow [1]
3. Bag and remove plants that are already flowering or in seed rather than composting them, since seed is the only way this thistle spreads [2]
4. Watch the spot for a few seasons and pull new rosettes, since wind-blown seed can keep arriving from elsewhere [2]
5. Natural area crews can map and cut small outlier patches first, time repeat cutting to just before flowering, and clean seed off equipment before moving between sites [1]
6. Two weevils (Rhinocyllus conicus and Trichosirocalus horridus) have been introduced as biological control agents for thistles in this genus, but both are documented to also feed on and reduce seed production in native North American thistles ? natural area managers should weigh that risk before relying on them, especially near populations of native thistles [8]

When to call a professional:

1. Large or well-established stands, especially across pastures or rights-of-way, where hand removal can't keep up
2. Sites where new plants keep appearing each year from wind-blown seed despite repeated removal
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. National Park Service. Exotic Species: Spiny Plumeless Thistle. nps.gov
2. Native Plant Trust, Go Botany. Carduus acanthoides (spiny plumeless-thistle). gobotany.nativeplanttrust.org
3. California Invasive Plant Council. Carduus acanthoides Profile. cal-ipc.org
4. USDA Forest Service, Fire Effects Information System (FEIS). Carduus nutans, musk thistle, species review. research.fs.usda.gov
5. Rauschert ESJ, Shea K, Goslee S (2015). "Plant community associations of two invasive thistles." AoB Plants 7:plv065. pmc.ncbi.nlm.nih.gov
6. Minnesota Department of Natural Resources. Canada thistle (Cirsium arvense). dnr.state.mn.us
7. USDA NRCS PLANTS Database. Carduus acanthoides L. plants.usda.gov
8. Wiggins GJ, Grant JF, Lambdin PL, Ranney JW, Wilkerson JB, Reed A, Follum RA (2010). "Host utilization of field-caged native and introduced thistle species by Rhinocyllus conicus." Environmental Entomology 39(6):1858?1865. pubmed.ncbi.nlm.nih.gov

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