It is not easy to sort out the taxonomy of this humble weed; we are going by the listing in the USDA PLANTS database, but the species name is also spelled caespitosum (Classical Latin rather than Medieval Latin), and a whole group of related Polygonum species is often separated into the genus Persicaria, in which case this becomes Persicaria posumbu. It is one of several similar smartweeds that frequently pop up in urban areas. This one is distinguished by its dense spike of tiny pink flowers and pointed leaves without markings; the similar Lady’s Thumb (Polygonum persicaria or Persicaria maculosa) has a dark thumbprint mark on each leaf. Look also for tufts of hairs at the stem joints. These plants were growing by a fence in Beechview, where they were blooming in the middle of July.
Gray describes the genus Polygonum and the section Persicaria. He does not describe this species, because it seems to be a twentieth-century introduction; but the description we have given above should distinguish it from the other members of its genus that grow around here.
POLYGONUM [Tourn ] L. KNOTWEED. Calyx 4-6 (mostly 6)-parted; the divisions often petal-like, all erect in fruit, withering or persistent. Stamens 3-9. Styles or stigmas 2 or 3; achene accordingly lenticular or 3-angular. Embryo placed in a groove on the outside of the albumen and curved halfway around it; the radicle and usually the cotyledons slender. Pedicels jointed. — Ours all herbaceous, with fibrous roots (except in P. viviparum), flowering through late summer and early autumn. (Name composed of poly-, many, and gony, knee, from the numerous joints.)
PERSICARIA [Tourn.] L. Flowers in dense spikes, with small scarious bracts; leaves not jointed on the petiole; sheaths cylindrical, truncate, entire, naked or ciliate-fringed or margined; calyx colored, 6-parted, oppressed to the fruit; stamens 4-8; filaments filiform; cotyledons accumbent.
Teasels grow everywhere along roadsides; they can be annoyingly invasive, but—like thistles—extraordinarily beautiful if you take the time to admire the form and structure. They are also much beloved by bees and butterflies, who, if they could only talk, might wax rhapsodic in their praises of the human pioneers who first brought this plant to North America. Our photographs here demonstrate the enthusiasm with which the insect world greets the Teasel blooming season. The name Teasel and the Latin name fullonum (“of fullers”) both come from the use to which they were put: fullers used the dried flower heads to tease the fibers on fabrics. Some people still use them for carding wool today.
The spread of this species has been rapid: Gray calls it “rather rare,” which it certainly is not in our area. This was one of a patch blooming in the middle of July at the edge of a parking lot in Beechview. Gray describes the genus and the species (which he calls D. sylvestris):
DÍPSACUS [Tourn.] L. TEASEL. Involucre many-leaved, longer than the chaffy leafy-tipped bracts among the densely capitate flowers; each flower with a 4-leaved calyx-like involucel investing the ovary and fruit (achene). Calyx-tube adherent to the ovary, the limb cup-shaped, without a pappus. Corolla nearly regular, 4-cleft. Stamens 4, inserted on the corolla. Style slender.—Stout and coarse biennials, hairy or prickly, with large ovoid-ellipsoid heads. (Name from dipsen, to thirst, probably because the united cup-shaped bases of the leaves in some species hold water.)
D. sylvestris Huds. (WILD Т.) Prickly; leaves lance-oblong, toothed and often prickly on the margin; leaves of the involucre slender, ascending, longer than the head; bracts (chaff) tapering into a long flexible awn with a straight point. —Roadsides, rather rare. (Nat. from Eu.)
This attractive bush is a garden favorite, but it seeds itself and can become invasive. It likes a wet location, especially along the banks of a stream. This bush was growing along the Squaw Run in Fox Chapel, where it was finishing up its blooming in the middle of July.
In the horticultural trade, “Spiraea” is often spelled “Spirea.”
Gray describes the genus and the species, which in his time had not gained much of a foothold in North America:
SPIRAEA [Tourn.] L. Calyx 5-cleft, short, persistent. Petals 5, obovate, equal, Imbricated in the bud. Stamens 10-50. Pods (follicles) 5-8, not inflated, few-several-seeded. Seeds linear, with a thin or loose coat and no albumen. — Shrubs, with simple leaves, and white or rose-colored flowers in corymbs or panicles. (The Greek name, from speiran, to twist, from the twisting of the pods in some of the original species.)
Flowers In compound corymbs.
Calyx -tube top-shaped, pubescent.
S. japónica L. f. Stems 1 m. or more high; leaves 7-9 cm. long, glaucous beneath; petals pink to deep rose-color. — Frequent in cultivation, and occasionally escaping, s. Ct. (Graves) and e. Pa. (Introd. from Asia.)