Persicaria bicornis (Raf.) Nieuwl.

Pink Smartweed


CC = 5
CW = -3
MOC = 12

© SRTurner

Family - Polygonaceae

Habit - Taprooted annual forb.

Stem - Ascending to erect, to 1.8 m, sometimes from a spreading base, glabrous or appressed to spreading-hairy, most of the hairs usually gland-tipped. Ocreae persistent, usually tearing with age, 6-20 mm long, somewhat inflated toward the base, tan to white and papery throughout, the margin entire or nearly so, lacking bristles or occasionally with short, slender, extensions of the veins to 0.8 mm long, the surface glabrous or minutely roughened-hairy toward the base, not gland-dotted.

Persicaria_bicornis_stem.jpg Stem.

© SRTurner

Persicaria_bicornis_ocrea.jpg Ocrea.

© SRTurner

Leaves - Alternate, with petioles 0.1-2.2 cm long. Leaf blades 2-13 cm long, 1.0-2.5 cm wide, lanceolate, angled at the base, angled to a sharply pointed tip, the surfaces glabrous or appressed-hairy along the main veins, the undersurface with impressed glands, the upper surface sometimes with a reddish or purplish, chevron- to triangular-shaped area.

Persicaria_bicornis_leaf1.jpg Leaf adaxial.

© SRTurner

Persicaria_bicornis_leaf2.jpg Leaf abaxial.

© SRTurner

Inflorescences - Terminal and axillary from the upper nodes, usually several to numerous per main stem, 0.8-6 cm long, 10-18 mm wide, erect or ascending, dense, uninterrupted, the stalk 0.8-7.0 cm long, glabrous or hairy, the hairs usually gland-tipped. Ocreolae overlapping, the margins entire or with bristles 0.1-0.8 mm long, the surface usually glabrous, not gland-dotted.

Flowers - Plants heterostylous, with flowers of some plants with the stamens exserted and those of others with the styles exserted. Flowers sometimes functionally pistillate or staminate, 2-11 per fascicle. Perianth light pink, bell-shaped, not gland-dotted, the tepals 5, 4-6 mm long, fused below the midpoint, the nerves relatively prominent but not anchor-shaped. Stamens 6-8, exserted or not, the anthers pink or red. Styles 2-branched from near the base, not or only slightly exserted, not persistent.

Persicaria_bicornis_peduncle.jpg Peduncles.

© SRTurner

Persicaria_bicornis_inflorescence1.jpg Heterostyly: exserted stamens.

© SRTurner

Persicaria_bicornis_inflorescence2.jpg Heterostyly: exserted styles.

© SRTurner

Fruits - 2-3 mm long, 2.0-2.8 mm wide, not or only slightly exserted, discoid or appearing slightly 3-angled, beakless, the faces flat to slightly convex, 1 of the faces usually with a central hump, the surface smooth, blackish brown to black, shiny.

Flowering - July - October.

Habitat - Swamps, pond margins, wet areas.

Origin - Native to the U.S.

Lookalikes - Persicaria pensylvanica, P. maculosa, P. glabra.

Other info. - This species of smartweed is thought to be relatively uncommon in Missouri. Its known range in the U.S. includes a few centrally located states, extending as far southwest as Arizona.

In Missouri Persicaria bicornis may be undercollected, as it strongly resembles the more common P. pensylvanica. Furthermore, reliable differentiation of these two species can only be made at the population level, since the primary distinguishing character is whether the flowers are heterostylous. The heterostyly exhibited by P. bicornis means that flowers of some plants have stamens which are longer than the styles, and in flowers of other plants the reverse is true. This feature cannot be evaluated with a single specimen; a population must be examined, and furthermore a significant proportion of the flowers within that population need to be at anthesis (open and functional). A second difference is in the achenes, which in P. bicornis are humped on one side, though this feature can be subtle. Finally, the inflorescence stalks in P. bicornis are usually hairy, whereas those of P. pensylvanica are more distinctly glandular and less visibly hairy. See the page for P. pensylvanica for an image of the peduncle in this species. However, more study is required to establish the reliability of this character.

Photographs taken at Otter Slough Conservation Area, Stoddard County, MO, 9-10-2019 (SRTurner).