This is a difficult genus; the characters are sometimes subtle, and pressed specimens may lack the distinctive aspects of fresh material. Several of the smaller species can be especially troublesome to identify and, indeed, even more than in some groups it is necessary to use a good, well-illuminated lens and to consider a balance of characters rather than relying on a single one to separate similar species.
Most Galium species are ± erect, but some are mat formers (e.g. G. brevipes or G. triflorum), and others may sprawl over vegetation. The only species that could be truly considered vining is G. asprellum, which clings to other vegetation (and clothes) and may climb to the tops of shrubs in wetlands.
References in the key to the pedicel are strictly to the stalk of an individual flower, not to any portion of the stem or inflorescence from which that (unbranched) stalk arises.
1. Ovary and fruit spreading-bristly, or leaves with 3 prominent longitudinal veins, or (usually) plant with both these conditions (ovary smooth in some plants of G. boreale; leaves with only the midrib prominent at base of the blade in some others).
2. Leaves all or mostly in whorls of 6–8 (–9) with only one longitudinal vein (the midrib) prominent at base of the blade; plant ± reclining or trailing.
3. Stem and leaf margin rough-scabrous with retrorse (or at least divergent) broad-based barbs (may also have slender hairs at or above the nodes); leaves linear-oblanceolate (usually broadest beyond the middle, tapering more abruptly to apex than to base); annual.
3. Stem smooth or nearly so (occasionally with some spreading hairs) and leaf margin antrorse-scabrous; leaves narrowly (rarely broadly) elliptic (tapered ± equally to apex and base); perennial.
4. Inflorescence terminal; corolla funnel-shaped, the tube at least half as long as the lobes; rarely escape from cultivation.
4. Inflorescences axillary as well as terminal; corolla flat, the tube less than a third as long as the lobes; common native species.
2. Leaves all in whorls of 4 (or fewer), often with a pair of weaker longitudinal veins besides the midrib at base of the blade; plant erect.
5. Branches of the inflorescence with lateral flowers (and fruits) mostly sessile or nearly so (pedicels if any less than 1 mm long).
6. Principal leaves broadly ovate-elliptic, obtuse, mostly less than 2.5 times as long as broad; internodes and outside of corolla usually pubescent; corolla greenish.
6. Principal leaves (not the lowest) broadly lanceolate (widest below the middle), acute, (2–) 2.6–4.2 (–5.5) times as long as broad; internodes and outside of corolla glabrous; corolla maroon.
5. Branches of the inflorescence with all flowers (and fruits) distinctly pediceled.
7. Leaves linear-lanceolate, the principal ones mostly 4–14 times as long as broad, with 1–2 pairs of longitudinal veins (besides the midrib) prominent about half or more the length of the leaf; fruit with bristles none or if present not hooked at the tip (at most curved).
7. Leaves broadly ovate-elliptic, less than 3 times as long as broad, with the lateral pair of longitudinal veins very weak or absent (conspicuous only in the rare northern G. kamtschaticum); fruit (and ovary) with bristles hooked at the tip.
8. Plants low, with only (2–) 3–5 whorls of leaves, the upper ones clearly the largest; stems glabrous; leaves not gland-dotted; corolla white to whitish green.
8. Plants taller, with numerous whorls of leaves, the upper ones smaller (or at least no larger) than the middle and lower ones; stems ± pubescent; leaves obscurely to distinctly gland-dotted beneath; corolla maroon on the upper surface.
1. Ovary and fruit smooth or at most roughened (with tubercles or a few appressed spicules) and leaves with only one prominent longitudinal vein (i.e., the midrib).
9. Leaves sharply cuspidate or awned at apex, (5–) 6–12 in a whorl.
10. Stems smooth, glabrous or puberulent, erect or ascending; leaves at main nodes 8–12 per whorl.
11. Corolla bright yellow, with acute but scarcely apiculate lobes; internodes puberulent (at least in inflorescence); leaves narrowly linear, strongly revolute, more densely pubescent on the nearly hidden lower surface than on the upper surface.
11. Corolla white, greenish or greenish-yellow with at least some lobes often apiculate with prolonged tips; internodes glabrous or scabrous (rarely puberulent); leaves usually ± oblanceolate, flat or very slightly revolute, the surfaces nearly or quite glabrous (except in G. pedemontanum).
12. Largest leaves less than 2 (–2.5) cm long, stiff, densely antrorse-scabrous on the margins.
12. Largest leaves 2.5–4 cm long, thin and membranous, sparsely scabrous on the margins.
10. Stems smooth or retrorse-scabrous (not puberulent), reclining on or supported by other plants; leaves at main nodes (5–) 6 in a whorl.
13. Margins of leaves smooth or weakly antrorse-scabrous; midrib smooth; leaves linear to very narrowly elliptic-oblong.
13. Margins of leaves and midrib beneath strongly retrorse-scabrous (antrorse in the rare G. verrucosum); leaves narrowly elliptic to obovate.
14. Mature fruit less than 2 mm long, smooth or nearly so, on slender, smooth, straight pedicels; plant a much-branched perennial, native throughout Michigan.
14. Mature fruit ca. 3–4 mm long, densely tuberculate or papillose, on stout, scabrous, strongly recurved pedicels; plant an annual, sparsely if at all branched, a rare waif.
9. Leaves merely acute to rounded (not cuspidate or awned) at apex, not over 4–6 in a whorl.
15. Leaves pubescent on both surfaces; flowers yellowish, on long-hairy pedicles in the leaf axils; weedy annual of upland sites.
15. Leaves glabrous; flowers white, pedicels smooth or retrorse-scabrous; perennials of wetlands.
16. Flowers all or mostly (7–) 10–25 per inflorescence (i.e., beyond the distal reduced but leaf-like bract); corolla ca. 2–4 mm broad, 4-lobed; nodes glabrous or nearly so.
16. Flowers 1–6 (–8) per inflorescence; corolla less than 2 mm broad (3- or 4-lobed) or nodes pubescent.
17. Corolla 2–3 (–4) mm broad (i.e., ca. 1 mm or more long), 4-lobed, the lobes longer than broad; nodes ± bearded with short hairs; leaf margins scabrous-ciliate with short, conical (broad-based, symmetrical) ± spreading hairs (or tiny prickles).
18. Leaves less than 2.5 mm broad (to 3.5 mm in shade forms), ± linear with revolute margins, all or mostly reflexed, the midrib beneath smooth or nearly so; mature fruit ca. 1–1.8 mm long, on pedicels 1–4 mm long; wet peatlands.
18. Leaves (at least the broadest) 2.5–6 (–7) mm broad, narrowly elliptic, the midrib beneath usually nearly or quite as bristly-ciliate as the margins; mature fruit ca. 2–2.8 mm long, the longest pedicels 3.5–7 (–12) mm long; swamps and mesic forests, especially on floodplains.
17. Corolla not over 2 mm broad (i.e., less than 1 mm long), usually 3-lobed (some flowers sometimes 4-lobed), the lobes no longer than wide; nodes glabrous; leaf margins smooth or usually scabrous with short retrorse deltoid barbs.
19. Pedicels ± densely (but minutely) retrorse-scabrous, the longer ones (6–) 7–17 (–18) mm long, curved toward the tip at maturity, usually 2 or 3 subtended by a whorl of leaves at ends of branchlets, without a common peduncle.
19. Pedicels smooth, not exceeding 8 mm long, and usually straight at maturity, often 2 or 3 on a peduncle (which may be scabrous).
20. Pedicels 0.5–3.5 (–4.5) mm long and often curved at maturity, solitary or in pairs in leaf axils or at ends of branches but not on a common peduncle; corolla not over 1 mm wide; mature fruit 0.8–1.2 mm long; leaves mostly 2.5–7 mm long.
20. Pedicels (at least the longest) 3–8 mm long and nearly always straight at maturity, often on a peduncle; corolla ca. 1–1.8 mm wide; mature fruit 1–1.8 (–2) mm long; leaves mostly 5.5–14 (–22) mm long.