Flora Pittsburghensis

Category: Adoxaceae

Japanese Snowball (Viburnum plicatum)

Photographed May 2.

This spectacular bush comes in two varieties. The one that has become common in our woods is the fertile one, for obvious reasons, where the small inner flowers are surrounded by showy sterile flowers that attract the pollinators. In that form the plant is commonly called “Doublefile Viburnum.” Every so often, though, a plant pops up that has all sterile flowers, which form these white snowballs. These plants were blooming along the Trillium Trail in Fox Chapel, where the standard fertile form is very common.

Oddly, European botanists encountered this form of the plant first, so it was given the species name Viburnum plicatum, and the normal fertile form was called Viburnum plicatum var. tomentosum. We still see that variety name used often to describe the Doublefile Viburnum.

The usual fertile form, with small fertile flowers surrounded by large sterile flowers.

May 3, 2026
Doublefile Viburnum (Viburnum plicatum)

Photographed April 24.

An ornamental bush from Japan that has escaped and made itself at home in our area. These plants were blooming in the Kane Woods Nature Area, where the showy drifts of white flowers punctuate the woods in the spring. There are two kinds of flowers: the little ones in the middle that actually do the work, and the sterile flowers around the edge of the corymb that act as the plant’s advertising agency, bringing in potential pollinators.

Once in a while, a plant bears all sterile flowers, which form a white ball that gives that form of the bush the name Japanese Snowball.

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April 24, 2026
Highbush Cranberry (Viburnum trilobum)

Not really a cranberry, but it has berries that can be a good substitute for cranberries. The leaves are much like the leaves of Red Maple (Acer rubrum), and the flowers come in dense cymes. Most of the flowers are tiny, but the outer flowers in each cyme, which are sterile, are immensely overgrown, making the whole cyme much more showy and a much more attractive target for pollinators. This bush was growing at the edge of the woods in Harmarville, where it was blooming in late May.

This species is often considered as variety americanum of the Eurasian species V. opulus, and so Gray classifies it:

VIBÚRNUM [Tourn.] L. ARROW-WOOD, LAURESTINUS. Calyx 5-toothed. Corolla spreading, deeply 5-lobed. Stamens 5. Stigmas 1-3. Fruit a 1-celled 1-seeded drupe, with soft pulp and a thin-crustaceous (flattened or tumid) stone. —Shrubs, with simple leaves, and white (rarely pink) flowers in flat compound cymes. Petioles sometimes bearing little appendages which are evidently stipules. Leaf-buds naked, or with a pair of scales. (The classical Latin name, of unknown meaning.)

§ 2. ÓPULUS [T onrn.] DC. Winter-buds scaly; leaves palmately veined and lobed; drupe bright red, acid, globose; stone very flat, orbicular, not sulcate.

V. Ópulus L., var. americànum (Mill.) Ait. (CRANBERRY-TREE, HIGH-BUSH CRANBERRY, PIMBINA.) Nearly smooth, upright, 1-4 m. high; leaves 3-5-ribbed, strongly 3-lobed, broadly wedge-shaped or truncate at base, the spreading lobes pointed, mostly toothed on the sides, entire in the sinuses; petioles bearing 2 glands at the apex; cyme broad, the marginal flowers neutral, with greatly enlarged flat corollas; stamens elongate. (V. americanum Mill.) — In woods and along streams, Nfd. and e. Que. to B. C, s. to N. J., Pa., Mich., Wisc., and n. e. Ia. June, July. (E. Asia.) — The acid fruit of this and the next is a substitute for cranberries. The well-known Snow-ball Tree, or Guelder Rose, is a cultivated state of the typical Old World form, with the whole cyme turned into showy sterile flowers.

June 10, 2011
Southern Arrowwood (Viburnum dentatum)

One of several Viburnum species found in our area; this one is not recorded in the literature as occurring in Allegheny County, though it is known to occur in neighboring Butler County. (Shafer’s Preliminary List of the Vascular Flora of Allegheny County, Pennsylvania, lists it as a possible resident, on the grounds that it occurs in neighboring areas.) Nevertheless, these bushes seem to be Viburnum dentatum, and they did grow in Allegheny County, near the edge of the woods on the grounds of Harmarville Rehabilitation Hospital. (We’d be delighted to be corrected in this identification if it’s wrong.) The one above was blooming in early June; the one below in late May. The similar Viburnum recognitum, which may also grow in our area, is sometimes classified as a variety of Viburnum dentatum.

Gray describes the genus and the species, which he places in the Euviburnum or Viburnum-proper section of the genus:

VIBÚRNUM [Tourn.] L. ARROW-WOOD, LAURESTINUS. Calyx 5-toothed. Corolla spreading, deeply 5-lobed. Stamens 5. Stigmas 1-3. Fruit a 1-celled 1-seeded drupe, with soft pulp and a thin-crustaceous (flattened or tumid) stone. —Shrubs, with simple leaves, and white (rarely pink) flowers in flat compound cymes. Petioles sometimes bearing little appendages which are evidently stipules. Leaf-buds naked, or with a pair of scales. (The classical Latin name, of unknown meaning.)

§ 3. EUVIBÚRNUM Koehne (restricted). Winter-buds scaly; leaves pinnately veined (except in V. acerifolium), the veins straightish and terminating in coarse teeth; cymes never radiant, peduncled; drupes blue to black; stone usually grooved.

Leaves cordate or subcordate at base, coarsely toothed, prominently pinnate-veined.

Stone very deeply sulcate ventrally; leaves rather slender-petioled.

V. dentàtum L. (ARROW-WOOD.) Smooth, 1-4.5 m. high, with ash-colored bark; leaves broadly ovate, glabrous, or with hairy tufts in the axils beneath, very numerously sharp-toothed and strongly veined, 5-8 cm. long; fruit globose-ovoid, 6 mm. long; cross-section of stone between kidney- and horseshoe-shaped. — Wet places, N. B. to n. Ga., w. to w. N. Y. and s. Ont. June, July.

June 7, 2011