Knotweed family (Polygonaceae)
Description: This native plant is an annual or short-lived perennial that is 1-2½' tall, branching occasionally and rather erect in habit. The round stems are hairless and somewhat broader where the ocreae occur. The alternate leaves are up to 6" long and ¾" across. They are lanceolate-ovate or narrowly ovate, tapering to short petioles. These leaves are usually hairless, except for a few hairs along the lower midrib in some cases, and they have smooth margins. At the base of each leaf, is a sheath (ocrea) that wraps around the stem. This sheath is membraneous and hairless, except for a few long bristles along its upper edge. With age, it falls away from the stem.
The upper stems terminate in more or less erect spike-like racemes about 2-6" in length. Each raceme has small flowers that are sparsely distributed along its length. Each flower is about 1/8" long, white or greenish white, and its outer surface (consisting of sepals) has glandular dots that are either pale- or dark-colored (if pale-colored, a hand lens may be necessary in order to see them). The 5 sepals of the flower are more or less tightly folded against each other, while the short style is divided at its base into 2 or 3 segments. The blooming period occurs from mid-summer to early fall, and lasts about 1-2 months. There is no noticeble floral scent. Each flower is replaced by an achene that is shiny, dark brown to black, three-angled, and rather oblong. This plant often forms colonies of varying size in wet areas.
Cultivation: The preference is full or partial sun, moist to wet conditions, in mucky soil that is high in organic matter. This plant tolerates shallow standing water.
Range & Habitat: Water Smartweed is a common plant that occurs in almost every county in Illinois (see Distribution Map). Habitats include moist openings in floodplain forests, swamps, seeps, borders of ponds and small streams, and drainage ditches. This plant is often found along the edge of water that is stagnant or slow-moving, and appears to favor degraded wetlands.
Faunal Associations: The nectar of the flowers attracts short-tongued Halictid bees, various kinds of wasps, Syrphid flies and other kinds of flies, and the occasional beetle, including ladybird beetles. The Halictid bees often collect pollen as well. The caterpillars of the butterflies Lycaena hyllus (Bronze Copper) and Lycaena helloides (Purple Copper) feed on the foliage, as do the caterpillars of several moth species and the adults of Gastrophysa polygoni (Leaf Beetle sp.). See the Moth Table for a list of moth species that feed on smartweeds. The nutritious seeds of wetland smartweeds are popular with many species of ducks, seed-eating rails, and various songbirds (see Bird Table for a list of species), which may help to distribute the seeds. The plants and seeds of wetland smartweeds are a minor source of food to muskrats.
Photographic Location: A sunny opening in Busey Woods at Urbana, Illinois, in a poorly drained area. There were sedges and Mimulus alatus (Winged Monkeyflower) growing nearby. The ochrea in the lower photograph is starting to fall off the stem.
Comments: This is one of the more common smartweeds in wetland areas. It also occurs in flood-prone areas of woodlands. Water Smartweed is average-sized and not particularly showy, but the seeds are valuable to wildlife in wetlands. To identify a Smartweed species correctly, it is necessary to examine the racemes and ochreae (the leaf-sheaths that wrap around the stem). Water Smartweed produces erect racemes with sparsely distributed flowers, whereas some other Smartweed species produce racemes that droop (e.g., Persicaria lapthifolium), or that are densely packed with flowers (e.g., Persicaria pennsylvanica). The ochreae of Water Smartweed are hairless, except for a few long bristles at the top. Other smartweed species may have ochreae that are hairy across the surface, and they often have very short or no bristles at the top.