Knotweed family (Polygonaceae)
Description: This adventive perennial plant consists of a rosette of basal leaves, from which occasional flowering stalks are produced. The rosette of basal leaves typically spans about 4-6" across, while the flowering stalks are about ¾–1½' tall and more or less erect. A full-sized basal leaf is about 3" long and 1" across (including the petiole, which is about as long as the leaf blade). It is hastate in shape (i.e., arrowhead-shaped, but with spreading basal lobes), hairless, and smooth along the margins. The leaf is usually broadest above the middle, while the small basal lobes are often rounded, rather than pointed. The slender flowering stalks are angular or ridged, terminating in a panicle with spike-like racemes of tiny flowers. The few leaves that occur on these stalks are alternate, lanceolate or linear, sessile, and greatly reduced in size.
Because Sheep Sorrel is a dioecious species, the flowers of a plant are either all-male or all-female. Each flower is about 1/12" across and consists of 6 sepals and no petals. A male flower has 6 stamens, while a female flower has a pistil with a white tripartite style. Each division of the style is itself divided into long narrow lobes and has a frilly appearance. The sepals are initially green, but become red or reddish brown while the flowers are in bloom and the achenes ripen. The 3 inner sepals do not develop membranous wings as they mature, unlike other Sorrel species. The achene of a female flower is reddish to yellowish brown and 3-angled, tapering to a point at both ends. Its surface is more granular than shiny. The surrounding sepals are about the same length as the achene, but they do not fully enclosed it. The root system consists of a taproot that is shallow and slender, and long rhizomes that snake out in all directions. This plant often forms vegetative colonies.
Cultivation: Sheep Sorrel is usually found in full or partial sunlight, mesic to dry conditions, and a poor acid soil that is either sandy or gravelly (although not limestone). It can also flourish in rich loam or clay-loam soil at highly disturbed sites. This plant is quite aggressive and difficult to eradicate.
Range & Habitat: Sheep Sorrel is a common plant that occurs in every county of Illinois (see Distribution Map). Habitats include sand prairies, chert and granite glades, abandoned fields, areas along railroads, gardens and lawns, gravel beds along buildings and around shrubs, and dry sunny waste areas. This species has the capacity to invade disturbed areas of some natural habitats. It is adventive from Europe.
Faunal Associations: The flowers are wind-pollinated and attract few insect visitors. The leaves are a preferred source of food for the caterpillars of Lycaena phlaeas americana (American Copper), which has been interpreted as an indication that this butterfly is an introduction from Europe. Upland gamebirds eat both the seeds and foliage, while many songbirds, especially sparrows, eat the seeds (see Bird Table). Various small mammals occasionally eat the seeds or foliage, including the Cottontail Rabbit, Prairie Vole, and White-Footed Mouse.
Photographic Location: The plants were growing in or near a gravel bed along the apartment building of the webmaster in Urbana, Illinois. The lower photograph depicts female flowers at the bud stage or in bloom.
Comments: Other common names for this species are Red Sorrel, Field Sorrel, and Common Sorrel. Sometimes the leaves are added in small amounts to salads and other culinary dishes to provide a sour taste. However, they contain significant amounts of oxalic acid, which can be toxic to the kidneys if a sufficiently large quantity of leaves is eaten. Sheep Sorrel closely resembles in appearance its native counterpart, Rumex hastulatus (Wild Sorrel). Wild Sorrel occurs in only two counties in SW Illinois and is state-listed as endangered. It differs from Sheep Sorrel primarily by its inner sepals – they become winged and heart-shaped with maturity, fully enclosing each achene. The root system of Wild Sorrel consists of a taproot; it may form clumps of plants through short rootstocks, but doesn't create new plants from long spreading rhizomes. Another species that is rarely encountered in the wild is Rumex acetosa (Garden Sorrel). This introduced species is a bigger plant than Sheep Sorrel. Garden Sorrel has sagittate leaves (with basal lobes that are downward pointing, rather than spreading), and its cauline leaves clasp the flowering stalks.