Knotweed family (Polygonaceae)
Description: This annual plant is about ½–2' tall and more or less erect, although individual plants may sprawl. The stems are usually light green, round, and glabrous or slightly pubescent. The alternate leaves are up to 6" long and 1" across, although usually smaller. They are lanceolate or linear-lanceolate, hairless, smooth along the margins, and sometimes slightly ciliate. Each leaf has a short petiole or it is nearly sessile; there is a membranous sheath (ochrea) that wraps around the stem at the base of each petiole. This sheath may have a few longitudinal veins and there are deciduous bristles along its upper rim that may exceed 1 mm. in length. The upper surface of a leaf often has a black smudge that is oval or triangular-shaped; this smudge may be dark and conspicuous or faint and barely perceptible.
Each upper stem terminates in 1 or 2 spike-like racemes of flowers; there are often shorter racemes that develop from the axils of the upper leaves on peduncles. Each raceme is about ½–1½" long, more or less erect, and oblongoid in shape from the crowded whorls of small flowers. The sepals of the flowers may be pink, red, greenish white, or purple, even on the same raceme; usually pink flowers are the most common. Each flower is about 1/8" long and shy to open; it consists of 5 sepals, 6 stamens (usually), a style that is divided into 2-3 parts toward the middle, and no petals. The sepals are not glandular-punctate, and the stamens are not exerted beyond the sepals. The blooming period can occur from late spring to early fall; a colony of plants will typically bloom for 1-2 months during the summer. There is no noticeable floral scent. Each flower is replaced by a seed that ovoid, flattened or slightly 3-angled, black, and shiny. The shallow root system doesn't produce rhizomes. This plant often forms colonies, particularly in disturbed wetland areas, or it may occur in drier areas as scattered plants. Reproduction is by seed only.
Cultivation: The preference is full sun or partial sun, wet to mesic conditions, and fertile soil with organic matter. However, this adaptable plant will also grow in light shade and poor soil containing clay, gravel, or sand. After the seeds germinate, it develops quickly and may bloom while only a few inches tall.
Range & Habitat: The non-native Lady's Thumb is a common plant that has been observed in most counties of Illinois; it probably occurs in every county of the state. Habitats include marshy areas, edges of streams and drainage canals, mudflats, roadside ditches, moist weedy meadows, vacant lots, fallow fields and edges of cultivated fields, edges of yards and gardens, moist areas along railroads, and waste areas. This species prefers disturbed areas, but can invade higher quality wetlands to a limited extent. Lady' Thumb is native to Europe.
Faunal Associations: The nectar of the flowers attracts Halictid bees, wasps, and Syrphid flies primarily. Less comon visitors include small butterflies and bumblebees. Halictid bees also collect pollen occasionally. The foliage is eaten by the caterpillars of some Copper butterflies and several species of moths, while the flowers and fruit are eaten by the caterpillars of the Gray Hairstreak butterfly (see Butterfly & Moth Table). Japanese beetles are also quite fond of the foliage of this and other smartweeds. Flea beetles that feed on the foliage or roots include Chaetocnema concinna (Brassy Flea Beetle), Systena frontalis (Red-Headed Flea Beetle), Disonycha pensylvanica, and Disonycha conjugata. Mammalian herbivores rarely feed on the foliage of Lady's Thumb because the foliage is pungent, peppery, and slightly bitter. However, White-Tailed Deer may chomp off the tops of young plants upon occasion. The seeds of smartweeds are very popular with waterfowl and granivorous songbirds. The seeds of Lady's Thumb, in particular, are eaten by birds in both upland and wetland habitats (see Bird Table). The ecological value of this little plant is rather high, notwithstanding its weedy nature.
Photographic Location: Disturbed open ground at the Arboretum of the University of Illinois in Urbana, Illinois.
Comments: Lady's Thumb is probably the most common and adaptable Persicaria sp. (smartweed) in Illinois. It occurs in both wetland habitats and surprisingly dry habitats. I have even found small plants of this species flowering underneath the shade of lawn trees. It is fairly easy to distinguish Lady's Thumb from other Persicaria spp. (smartweeds) because its leaves often have black smudges on their upper surfaces. Sometimes these black smudges aren't produced, however. The spike-like racemes of Lady's Thumb are somewhat distinctive: they are rather stout, erect, and densely crowded with small flowers; these flowers tend to have conspicuously different colors on the same raceme. Other smartweeds often have racemes that are more slender, nodding, and less densely crowded with flowers. For these latter species, the flowers of individual plants are more likely to be homogenously colored. Another scientific name for Lady's Thumb is Polygonum persicaria.