Viburnums are readily recognized, but individual specimens in some species pairs or complexes can be difficult to place. The fruit of most species is popular with wildlife. The flowers (including large sterile ones in a few species), colorful autumn foliage, and red or purple-black fruit make several species, American as well as Asian, well known in cultivation.
A single vegetative specimen exists of Viburnum lantanoides Michx. (V. alnifolium of many older works, not Marshall) alleged to be from “Pentwater, Mich.” (Youngken in 1938, BRIT). Though not inconceivable, this would be a striking disjunction from the central part of southern Ontario for this distinctive and showy shrub, but the generic label and scanty data militate against accepting this without additional corroboration. It resembles V. lantana somewhat, but has generally more orbicular leaves and enlarged sterile marginal flowers in the inflorescence.
1. Leaves lobed (except sometimes the distal pair or two on elongate branches), ± palmately veined with 3 (–5) main veins (each bearing regular lateral veins) arising at base of blade.
2. Marginal flowers of inflorescence sterile and much larger than flowers toward the center; petioles with prominent glands toward the summit and stipules at the base; undersides of leaves not dotted.
3. Larger petiolar glands 0.4–0.8 mm long (rarely absent), usually ± stalked and flat topped.
3. Larger petiolar glands (0.8–) 0.9–1.5 (–2) mm long, usually sessile and with the apex ± concave and indented.
2. Marginal flowers similar to the others; petioles without large glands, and stipules sometimes absent; undersides of leaves ± sprinkled with red-orange dots.
4. Inflorescences only on lateral shoots bearing one pair of leaves (primary shoots ending in leaves, not cymes); mature fruit red or yellowish; leaves glabrous beneath or with only simple hairs.
4. Inflorescences all or at least partly terminal on long leafy branches; mature fruit purple-black; leaves ± densely pubescent beneath with mostly stellate or tufted hairs.
1. Leaves not lobed, pinnately veined (only the midrib with regular lateral veins).
5. Leaf veins anastomosing extensively before reaching the closely crenulate-toothed margin.
6. Teeth of leaf margin low and obtuse to broadly rounded or reduced to mere gland-like swellings; inflorescence on a single terminal peduncle (6–) 8–17 (–26) mm long.
6. Teeth of leaf margins acute; inflorescence sessile (at most a peduncle 2.5–4 mm long), several branches of it arising from the axils of a terminal pair of leaves.
7. Petioles with a green often revolute margin ± undulate, or even appearing toothed; apex of most if not all leaves with a distinctly prolonged acuminate tip usually ca. 1–1.5 cm long; shrub occurring throughout the state.
7. Petioles scarcely if at all margined, the margins if any on petioles of non-flowering branchlets not undulate, revolute, or appearing toothed (may be broader and reddened on leaves beneath inflorescences on fertile branchlets); apex of leaf rounded or acute to very short-acuminate; shrub of southernmost Lower Peninsula.
5. Leaf veins (and their branches if any) each extending to the tip of a prominent large marginal tooth; leaves truncate to cordate at base (except in the rare escape V. sieboldii, with leaves clearly widest above the middle).
8. Leaves oblanceolate to obovate, widest well above the middle and cuneately tapering to the base from about the middle of the blade.
8. Leaves elliptic or ovate, broadly tapered, truncate, or cordate at base.
9. New growth and leaves (margins, lower surfaces, petioles) all ± strongly stellate-pubescent, the young twigs whitened with pubescence; winter buds naked (consisting of reduced leaves covered by no additional scales).
9. New growth and leaves bearing only scattered, if any, stellate hairs (± dense in young V. plicatum, but not completely coating twigs, which still therefore appear dark); winter buds covered with 1 or more scales that do not develop into leaves.
10. Marginal (or all) flowers of an inflorescence sterile and much enlarged; new growth, especially youngest shoots, with ± dense stellate (or tufted) hairs; plant a rare escape from cultivation.
10. Marginal flowers similar to the others; new growth with few if any stellate hairs; plants widespread native species.
11. Enlarged base of style glabrous; petioles up to 8 (–12) mm long, usually pubescent and usually stipulate; leaf margins ciliate.
11. Enlarged base of style (persistent on fruit) antrorse-hairy; petioles 13–22 mm long, glabrous or nearly so, without stipules at base; leaf margins usually not ciliate.