Waterpepper
Persicaria hydropiper
Knotweed family (Polygonaceae)

Description: This introduced or adventive plant is a summer annual about 1-2' tall, branching occasionally. It has a tendency to sprawl and lean on adjacent plants for support. The hairless stems are green or red (often the latter). The alternate leaves are up to 3" long ad " across. They are lanceolate, smooth along the margins, and usually hairless. Their upper surface is often olive-green. Each leaf has a short petiole; the base of this petiole has a membranous sheath (an ocrea) that wraps around the stem. This sheath has a few bristly hairs along its upper rim. The sheaths of adjacent leaves on each stem are about 1-2" apart.


The upper stems terminate in spike-like racemes of flowers that are about 1–3" long. These racemes are quite slender and their flowers are sparsely distributed (or interrupted); they often nod at their tips or droop sideways, rather than remaining erect like the racemes of other smartweeds. In addition to these terminal racemes, very short axillary racemes often develop from the sheaths of the upper leaves. Each greenish white flower is about 1/8" long, consisting of 5 sepals, 2-3 styles, 4-6 stamens, and no petals. Each outer sepal is light green toward its base, becoming white toward its upper edge; rarely are the outer sepals pink. The green portion of each outer sepal is pitted with pale yellow glands; these tiny glands usually turn brown when the outer sepals become dried out (a hand-lens with 10x magnification may be required to see them). The flowers are shy to open and usually bud-like in shape. The blooming period occurs from mid-summer to early fall and lasts about 1-2 months. There is no noticeable floral scent. Each flower is replaced by a seed that is 3-angled, dull-looking, and black or dark brown. The root system branches frequently and is rather shallow and fibrous. This plant spreads by reseeding itself and often forms colonies.

Cultivation: The preference is full or partial sun, wet to moist conditions, and a mucky soil that contains abundant organic material. In disturbed wetlands, this plant can spread aggressively. It has few problems with pests and disease, except for Popilio japonica (Japanese Beetle). This insect is very fond of the foliage of smartweeds.

Range & Habitat: Waterpepper is a common plant that occurs in most areas of Illinois. According to most authorities, it is native to Eurasia, but not North America; others believe this species has a circumpolar distribution and is native to both Eurasia and North America. Habitats include soggy openings in woodlands, logged-over woodlands in floodplains, soggy alluvial meadows, seeps, edges of rivers and ponds, roadside ditches, and poorly drained areas of fields. Disturbed wetlands are preferred, although this plant also occurs in higher quality habitats.

Faunal Associations: The flowers of this smartweed attract few insect visitors. Charles Robertson observed three species visiting its flowers for nectar: Halictus rubicunda (a Halictid bee), Exorista mella (a Tachinid fly), and Polistes fuscata (Dark Paper Wasp). Smartweed is eaten by the caterpillars of several Copper butterflies, Strymon melinus (Gray Hairstreak), and various moth species (see Butterfly & Moth Table). The seeds of smartweeds, including those of Waterpepper, are a very attractive source of food to waterfowl and granivorous songbirds (see Bird Table). Because of the biting peppery foliage, Waterpepper is rarely eaten by mammalian herbivores; livestock reportedly avoid it. The foliage of this species contains an impressive array of acidic substances, including formic acid. Formic acid occurs in the stinging hairs of Nettles and is injected by ants when they bite potential prey or intruders.

Photographic Location: At the edge of a small river in Meadowbrook Park, Urbana, Illinois.

Comments: This is one of the many weedy smartweeds in Illinois, which can be difficult to identify. Waterpepper can be distinguished from other smartweeds primarily by its racemes of flowers, which have the following key characteristics: 1) Its racemes are slender with sparsely distributed flowers, 2) its racemes are usually nodding or pendulous, rather than erect, 3) there are usually very short axillary racemes from the sheaths of the upper leaves, 4) its flowers are usually greenish white (rarely pink), and 4) the outer sepals of its flowers have tiny glandular pits that are pale yellow or brown. Other smartweeds lack one or more of these characteristics. 

A native specices, Persicaria punctata (Dotted Smartweed) has a similar appearance to Waterpepper, but its racemes of flowers are more erect. Another difference between these two species is the distance between the bases of the leaves on the stems: the leaf bases of Waterpepper are about 1-2" apart, while the leaf bases of Dotted Smartweed are about 2-3" apart. There is another species, Persicaria hydropiperoides (Mild Waterpepper) that is occasionally encountered. The foliage of this species has a mild taste, while the foliage of Water Pepper has a strong taste that is very peppery and produces a burning sensation. The sepals of Mild Waterpepper lack the fine glandular pits of Waterpepper. While the seeds of Waterpepper have a dull dark appearance, the seeds of many other smartweeds are dark, but shiny. Both the scientific name Polygonum hydropiper and common name 'Marshpepper Smartweed' refer to Waterpepper.

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