1. Plant woody, a bushy, often small, but much-branched shrub; styles coherent at anthesis. splitting in fruit; stamens numerous (over 100).

2. Carpels (styles, apparent locules) 3; inflorescences axillary as well as terminal.

H. prolificum

2. Carpels 5 (rarely 4 or 6 in a few flowers); inflorescences all terminal.

3. Larger leaves 4–9 (–10) mm wide; longest mature sepals (on young fruits) 4–10.2 mm long; larger inflorescences with 1–7 flowers, rarely more.

H. kalmianum

3. Larger leaves 10.5–15 mm wide; longest mature sepals 10.5–13.2 mm long; larger inflorescences mostly with 15 flowers, occasionally more.

H. swinkianum

1. Plant herbaceous, with a central stem and in some species few if any lateral branches; styles distinct at anthesis (except in H. ellipticum and H. sphaerocarpum); stamens fewer than 100 (except in H. ascyron).

4. Carpels (styles and locules) 5; flowers ca. 4–6.5 cm broad; stamens numerous; capsules 12–19 mm long.

H. ascyron

4. Carpels 3 (styles 3; locules 1 or 3); flowers less than 3 cm broad; stamens fewer than 100; capsules less than 10 mm long.

5. Leaves minute, scale-like, linear-subulate, ± appressed, less than 3 mm long.

H. gentianoides

5. Leaves not so reduced, linear to elliptic or lanceolate.

6. Black spots (streaks and/or dots) clearly visible on petals (sometimes also on sepals, anthers, and/or leaves).

7. Styles ca. 4–6.5 mm long, longer than the ovary; black spots on petals largely restricted to margins and apical portion, on sepals and leaves few or none.

H. perforatum

7. Styles ca. 1–2 (–2.5) mm long, shorter than the ovary; black spots on petals scattered all across them, on sepals usually similar (rarely absent), and on lower surface of leaves likewise throughout.

H. punctatum

6. Black spots none.

8. Leaves definitely pinnately veined (one central longitudinal vein plus weaker lateral veins), elliptic-oblong; petals (5–) 6.5–10 mm long, deciduous after anthesis; stamens more than 20; styles (1.5–) 2–3.5 (–4.5) mm long, coherent into a beak (until capsule dehisces, rarely separate).

9. Plant 10–32 cm tall, from slender easily collected superficial rhizomes; principal leaves 1.5–2.8 (–3.4) cm long; Upper Peninsula.

H. ellipticum

9. Plant ca. (30–) 40–75 cm tall, from a deeper rhizomes; principal leaves 3.5–7 cm long; southern Lower Peninsula.

10. Sepals revolute; seeds distinctly less than 1 mm long.

H. adpressum

10. Sepals ± flat; seeds at least 2 mm long.

H. sphaerocarpum

8. Leaves with 3–5 strong longitudinal veins from the base, or if only 1 then narrowly linear, the lateral veins obscure or not visible; petals less than 6.5 mm long, persistent (but withered) at base of fruit; stamens fewer than 20; styles not over 1 mm long, separate.

11. Sepals and outline of fruit oblong to elliptic, broadest about the middle and tapering (if at all) scarcely more toward the apex than toward the base, their tips rounded or obtuse; branches of inflorescence ± divergent at maturity; leaves elliptic-oblong.

12. Ultimate bractlets of inflorescence leaf-like, at least 0.5–2 mm wide.

H. boreale

12. Ultimate bractlets of inflorescence greatly reduced, ca. 0.2 mm wide (or less).

H. mutilum

11. Sepals and often outline of young fruit lanceolate, broadest below the middle and more strongly tapering to acute tip; branches of inflorescence strongly ascending; leaves lanceolate or linear-oblanceolate.

13. Cauline leaves with 1 (–3) nerves, linear or slightly oblanceolate, tapering to the base; sepals mostly 2.5–4 (–4.8) mm long.

H. canadense

13. Cauline leaves with 3–5 (–7) nerves, usually lanceolate, broadest below the middle; sepals (at least the largest, as around fruit) mostly (4–) 4.5–6 (–7) mm long.

H. majus