Arctium minus
Arctium
bourrier, burweed, button-bur, chou bourache, cibourroche, common burdock, cuckoo-button, lesser burdock, lesser burrdock, louse-bur, petite bardane, wild rhubarb
bardane, burdock, clotbur
erect, openly branched, branches ascending.
basal and cauline; long-petiolate; gradually smaller distally;
blade margins entire or dentate (pinnately lobed or dissected), faces abaxially resin-gland-dotted, adaxially often tomentose.
petioles hollow (sometimes only at base), 15–50 cm, thinly to densely cobwebby;
blades 30–60 × 15–35 cm, coarsely dentate to subentire (rarely deeply dissected), abaxially ± thinly gray-tomentose, adaxially green, sparsely short-hairy.
0–9.5 cm.
15–40 mm diam.
± flat, epaleate, bearing subulate scales.
30+;
corollas purple, pink, or white, 7.5–12 mm, glabrous or limb glandular-puberulent.
(5–)20–40+;
corollas pink to ± purple, glabrous or glandular-puberulent, tubes elongate, throats campanulate, lobes narrowly triangular, ± equal;
anther bases tailed, apical appendages ovate, obtuse to acute;
style branches: fused portions distally hairy-ringed, distinct portions oblong, acute or obtuse.
linear to linear-lanceolate, glabrous to densely cobwebby, inner often purplish tinged, margins often minutely serrate with fine teeth, puberulent with glandular and or eglandular hairs.
many in 9–17 series, outer and mid narrowly linear, bases appressed, margins entire, apices stiffly radiating, hooked-spiny tipped, inner linear, ascending or erect, straight tipped.
in racemiform or paniculiform clusters, sessile to pedunculate.
discoid, in leafy-bracted racemiform to paniculiform or corymbiform arrays. (Peduncles 0 or 1–9 cm.) Involucres spheric to ovoid.
dark brown or with darker spots, 5–8 mm;
pappus bristles 1–3.5 mm.
obovoid, ± compressed, rough or ribbed, glabrous, attachment scars basal;
pappi falling, of many bristles in 2–4 series.
= 18.
= 32 (Germany), 36 (as A. nemorosum).
Arctium minus
Arctium
Arctium minus has been reported from Delaware and Texas; I have not seen specimens.
Arctium minus is a complex species with many variants that have been recognized at ranks ranging from forma to species (J. Arènes 1950). Some North American workers (e.g., R. J. Moore and C. Frankton 1974) have often distinguished plants with involucres more than 3 cm diameter that equal or overtop the corollas as A. nemorosum. Arènes treated those plants as a subspecies of A. minus. Arctium nemorosum was recognized as a species distinct from A. minus (H. Duistermaat 1996), with a different and more restricted circumscription than that used by North American workers. Although most of the characters that Duistermaat used to separate those A. nemorosum from A. minus overlap extensively, the consistently wider mid phyllaries of A. nemorosum (1.7–2.5 mm wide versus 0.6–1.6 mm in A. minus) supposedly distinguish the species. None of the North American specimens examined in preparation of this treatment had the wide phyllaries of A. nemorosum in the sense of Duistermaat, who stated that she had seen no material of this taxon from the American continent. Some American authors have taken up the name Arctium vulgare in place of A. nemorosum and applied A. vulgare (dubbed woodland burdock) to the larger-headed North American plants. Duistermaat considers A. vulgare to be a synonym of A. lappa.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Species 10 (3 in the flora).
At maturity the dry heads of Arctium species are readily caducous with the enclosed cypselae, and the hooked phyllary tips cling easily to fur or fabrics. Animal dispersal is a major factor in the spread of burdock species across North America. The burs are a major problem when they become entangled in the wool of sheep and fur of dogs and other animals.
Published chromosome reports for Arctium other than n = 18 are probably in error because of difficulty in interpretation of somatic chromosomes (R. J. Moore and C. Frankton 1974).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Heads usually sessile to short-pedunculate in racemiform or paniculiform clusters. | A. minus |
1. Heads usually long-pedunculate in corymbiform clusters | → 2 |
2. Involucre 2.5–4 cm diam.; phyllary apices glabrous or loosely cobwebby; corollas glabrous | A. lappa |
2. Involucre 1.5–2.5 mm diam.; phyllary apices densely cobwebby; corollas minutely glandular-puberulent | A. tomentosum |
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