Knotweed family (Polygonaceae)
Description: This perennial plant is 3-5' tall and unbranched or sparingly so. The central stem is light green to reddish green, hairless, and terete. The alternate leaves are up to 12" long and 2½" across, becoming smaller as they ascend the stem. The lowest leaves have petioles about as long as the blade, while the middle to upper leaves have short petioles. At the base of each petiole, there is a papery sheath that wraps around the stem. Each leaf is oblong-lanceolate, smooth or slightly undulate along the margins, and hairless; the margins are not crisped. The central stem terminates in 1-4 whorled racemes of greenish flowers; each raceme is up to 12" long. Each flower consists of 6 tepals, 6 stamens, and a tripartite style; it is about 1/8" across. The tepals are green, while the anthers of the stamens are white or pale yellow. Each flower droops downward from a slender pedicel that is up to 2/3" (16 mm.) long. The blooming period occurs during the summer and lasts about 2 weeks. Pollination is by the agency of the wind. Each flower is replaced by a 3-valved fruit up to 1/6" (4 mm.) long when it is fully mature. Each valve is a modified inner sepal; it is deltoid-ovate, flattened, and smooth along the margins. Each valve encloses a lanceolate tubercle that is located along the upper-half of its central vein. Each tubercle contains a single achene (or grain); there are 3 achenes per fruit. Each achene is reddish brown and shiny. The 3-valved fruits persist into the fall, at which time they detach from the mother plant; these detached fruits can be blown about by the wind or float on water, distributing the achenes. The root system consists of a taproot.
Cultivation: The preference is light shade to full sun, wet conditions, and mucky soil. This plant can tolerate standing water better than most Rumex spp. (Docks). It also tolerates shade better than most Docks.
Range & Habitat: The native Swamp Dock is occasional to locally common in most areas of Illinois; it is least common in SE Illinois (see Distribution Map). Habitats include edges of vernal pools and streams in woodlands, swamps, soggy islands in partially shaded areas, marshes, and roadside ditches.
Faunal Associations: The caterpillars of Copper butterflies and several species of moths feed on Rumex spp. (Docks). See the Butterfly & Moth Table for a listing of these species. The rather large seeds of Docks are eaten to a minor extent by various upland gamebirds and granivorous songbirds. These species include the Ruffed Grouse, Bobwhite, Woodcock, Red-Winged Blackbird, Bobolink, Song Sparrow, Swamp Sparrow, and other sparrows. Among mammalian herbivores, White-Tailed Deer feed on Swamp Dock readily. Some local populations of this plant may require fenced protection in swampy woodlands where White-Tailed Deer are abundant. Livestock eat the foliage of Docks, but the nitrates and oxalates in the leaves are mildly poisonous if they are eaten in quantity.
Photographic Location: At the edge of a vernal pool in Busey Woods of Urbana, Illinois. The flowers in the upper photograph are starting to form fruits.
Comments: This tall slender plant is often found in shady wetlands. While the flowers are not showy, it is relatively conspicuous in such wetlands because of its size. Distinguishing the different Rumex spp. (Docks) that occur in Illinois is relatively difficult. However, Swamp Dock can be identified by the long drooping pedicels of the flowers/fruits; these pedicels are 2-5 times longer than their attached fruits. Other Docks have pedicels that are no longer than twice the length of their fruits. Other key features of Swamp Dock include: 1) the margins of its mature leaves are flat rather than crisped, 2) each fruit has 3 tubercles/achenes, and 3) the margins of its valves are smooth (they lack teeth). Another native species, Rumex altissimus (Pale Dock), has a somewhat similar appearance to Swamp Dock, but it has shorter pedicels and each of its fruits has a single tubercle/achene.