Hieracium is a difficult genus, with many problems in both native and introduced species. The latter, particularly have had hundreds of variants dignified with scientific names. More broadly defined species, as here, are understandably quite variable morphologically, especially in regard to leaf shape and arrangement and to pubescence. Exceptions may occur to almost any character, whether due to mere variability or introgression, and a good deal of judgment is often needed to place puzzling specimens even in the broadly defined species or species complexes recognized below. Most hairs, especially on the stems, are pale when fresh but after a few years in the herbarium turn a rich orange-brown or tawny shade.
Some authors separate the introduced species with leaves all basal into the genus Pilosella; our treatment of these is tentative. Unusual individuals in this group that have the heads, or many of them, individually long-pedunculate, but with the peduncles arising from near the top of a tall stem, are probably hybrids of species with compact inflorescences, like Hieracium caespitosum or H. piloselloides, with the shorter species H. pilosella and H. flagellare, with one or few long-pedunculate heads.
1. Leaves of erect flowering stems all or mostly in a basal rosette, or densely crowded very near the base of the stem (leafy stolons may also be present).
2. Heads 1–3; leaves with numerous to dense stellate hairs beneath.
3. Leaves beneath with numerous but not densely felted stellate hairs; involucre ca. 9–12 mm long; heads mostly 2–3.
3. Leaves beneath with a dense felt of stellate hairs (white when fresh, reddish brown after several years); involucre ca. 7–8 (–9.5) mm long; heads solitary.
2. Heads several to many; leaves with sparse stellate hairs (or none) beneath (rarely such hairs numerous in H. caespitosum).
4. Ligules red-orange; plants stoloniferous (or with shallow rhizomes).
4. Ligules yellow; plants stoloniferous or not.
5. Achenes 1.5–2 mm long; heads ± crowded in a corymbiform inflorescence; well-developed cauline leaves 0–3 (and smaller than rosette leaves); rosette leaves ca. (3–) 6–20 times as long as broad.
6. Leaves hairy above (many hairs as close together as 1 mm) and not glaucous; mature involucres ca. (6.5–) 7–8 (–9) mm long; pedicels densely stellate-pubescent.
6. Leaves glabrous or unevenly and sparsely hairy above, ± glaucous; mature involucres usually ca. (4–) 5–7 (–7.5) mm long; pedicels with stellate hairs none or usually sparse (occasionally dense).
5. Achenes over 2 mm long; heads usually in a ± wide open or elongate inflorescence; well-developed cauline leaves usually more than 3 (and equal to rosette leaves) (usually none in H. venosum, with rosette leaves up to 3–4 times as long as wide).
7. Basal leaves with soft minutely barbellate hairs, these at least partly curled and/or tangled; principal leaves usually coarsely toothed; achenes truncate, not narrowed toward apex.
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7. Basal leaves with prominent long bristly hairs at least on margin (and often upper surface), these mostly straight and not tangled; principal leaves entire (or minutely denticulate); achenes usually at least slightly constricted or narrowed toward apex at maturity.
8. Principal leaves all basal or nearly so (at most a few small cauline bracts), ca. 3 (–4) times as long as broad (or shorter), usually with veins outlined in reddish purple; stem glabrous or at most finely pubescent toward base; mature achenes (2.2–) 2.5–3 mm long.
8. Principal leaves partly cauline (as large as basal leaves), 3–8 times as long as broad, the veins not outlined; stem long-hairy at least toward base; mature achenes 3–4 mm long.
9. Longest hairs ca. 4–6 mm; involucre 6.5–9 mm long.
9. Longest hairs of stem and leaves at least 8 mm; involucre 8.5–10.5 (–11.5) mm long.
1. Leaves of erect flowering stem all or mostly cauline (not basal).
10. Longest hairs toward base of stem and on lower leaves 3–10 mm (or even more).
11. Flowering heads large (ca. 1.5–2.5 cm long including ligules); leaf blades elliptic, usually coarsely toothed, at least the lowest on clearly defined petioles.
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11. Flowering heads smaller; leaf blades elliptic to oblong or oblanceolate, usually sessile or tapering into indistinct petioles.
12. Leaves distinctly paler beneath than above (glaucous when fresh); heads with fewer than 20 florets, on very slender (ca. 0.2–0.3 mm, almost filiform) elongate pedicels nearly or quite glabrous (or often with sparse stalked glands).
12. Leaves not glaucous, little if at all paler beneath; heads many-flowered on pedicels stout, ± densely pubescent and/or stipitate-glandular pedicels.
13. Achenes truncate (not narrowed at apex) at maturity; pedicels with ± dense dark gland-tipped hairs; longest hairs at base of plant less than 7 mm.
H. scabrum (in part)
13. Achenes narrowed toward apex; pedicels only sparsely if at all with stalked glands or longest hairs at least 8 mm long.
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10. Longest hairs toward base of stem and on lower leaves less than 3 mm (or none).
14. Leaf blades elliptic, usually coarsely toothed, at least the lowest clearly petioled; flowering heads large (ca. 15–25 mm long, including ligules).
15. Leaves streaked and blotched with red-purple.
15. Leaves green throughout.
16. Cauline leaves (2–) 4–7; leaf blades tapering narrowly into the petiole.
16. Cauline leaves 0–1 (or 2 near base); leaf blades broadly rounded to subcordate at base.
14. Leaf blades oblong to elliptic or oblanceolate, toothed or entire, sessile (or if the lower ones clearly petioled, the heads less than 1.5 cm long); flowering heads 8–25 mm long (including ligules).
17. Involucres and pedicels with ± dense dark gland-tipped hairs; leaves entire or barely and remotely denticulate, tapered more gradually to the base than to the apex (lower leaves even into petioles); flowering heads 8–13 mm long.
H. scabrum (in part)
17. Involucres and pedicels without dark gland-tipped hairs; leaves almost always with evident but remote teeth, all sessile or nearly so, tapered about equally to base and apex or more abruptly to the base; flowering heads ca. (12–) 15–25 mm long.
18. Leaves without prickle-like “hairs” on the surface (at most slightly roughened to the touch); involucre often at least sparsely strigose and/or stellate-pubescent.
18. Leaves strongly roughened with dense, stout, broad-based, conical (but very short) prickle-like “hairs,” especially above and toward the margins; involucre glabrous (at most slightly pubescent at the very base).