Madder family (Rubiaceae)
Description: This herbaceous plant is a summer annual that has stems about ½–1' long. These stems are usually erect or ascending, although sometimes they sprawl across the ground. The stems are unbranched or sparingly branched, light green to red, bluntly 4-angled (at least above), and short-pubescent (var. teres) to hairy (var. setifera). Pairs of opposite leaves occur along the entire length of each stem. These leaves are up to 1½" long and ¼" across; they are linear, linear-lanceolate, or linear-oblong in shape, entire (toothless) and involute (rolled downward) along their margins, and sessile with prominent central veins. The upper leaf surface is medium green and appressed short-pubescent to glabrous, while the lower leaf surface is slightly more pale and short-pubescent primarily along the central vein. The leaf bases along each stem are joined with merged stipules that form shallow cup-like structures. The exterior of these stipules is green to whitish green and sparsely short-pubescent to hairy. Along the upper rim of each pair of merged stipules, there are long erect bristles up to ½" long; these bristles are light green, white, or red.
Either solitary or small clusters of 2-3 flowers are produced from the axils of the middle to upper leaves. Each flower is up to ¼" long, consisting of a tubular-funnelform corolla with 4 spreading lobes, 4 green sepals that are lanceolate in shape, 4 stamens with pale yellow to white anthers, and an inferior ovary with a single white style. The corolla is lilac, pink, or white; its exterior is often finely short-hairy. The sepals are sparsely short-pubescent to hairy; they are shorter than the corolla. The style has a knobby (capitate) tip. Both the stamens and style are included or only slightly exserted from the corolla. The blooming period occurs from mid-summer to early autumn, lasting about 1-2 months. There is no noticeable floral scent. Afterwards, the flowers are replaced by dry fruits (schizocarps) that are obovoid in shape with remnants of the persistent sepals at their apices. Immature fruits are green, while mature fruits are brown. These fruits are sparsely short-pubescent (var. teres) to hairy (var. setifera); their fine hairs are straight and ascending. Eventually, these fruits divide into 2 nutlets each. The nutlets are about 3 mm. (1/8") long, half-obovoid in shape, brown, and more or less covered with persistent fine hairs. The root system consists of a slender taproot with secondary feeder roots. This plant often forms colonies by reseeding itself.
Cultivation: The preference is full sun, dry conditions, and poor soil containing an abundance of sand, gravel, or compacted clay. This plant will also tolerate partial sun and moister conditions with fertile soil, in which case it will be displaced by taller plants eventually. Drought tolerance is quite good because of the long taproot.
Range & Habitat: Rough Buttonweed occurs occasionally in the southern half of Illinois, where it is native, and a few counties in the northern half, where it is probably adventive. Illinois lies along the northern range limit of this species. Habitats include upland prairies where there is sparse vegetation, hill prairies, sand prairies, rocky glades, gravel bars along rivers, pathways with compacted soil, gravelly areas along roadsides, gravelly areas along railroads, and barren waste ground. Areas with a history of disturbance are preferred. Rough Buttonweed is regarded as a common weed in the southern states, but it is less ubiquitous in Illinois.
Faunal Associations: Limited information is available about this plant's relationships to various fauna. The nectar and pollen of the flowers probably attract small bees and flower flies (Syrphidae). It has been reported by Tietz (1972) that caterpillars of the Tersa Sphinx (Xylophanes tersa) feed on Rough Buttonweed. This moth has a southern distribution, but it is a strong flyer that migrates to the northern states during the summer. Another insect that feeds on this plant is a flea beetle, Strabala rufa (Clark et al., 2004). This flea beetle has a brownish orange to red carapace. The Greater Prairie Chicken eats the seeds, and possibly other gamebirds feed on them as well.
Photographic Location: The photographs were taken of plants growing on a little-used path with compacted soil. This path was located near an abandoned railroad in Champaign County, Illinois. Rather large colonies of Rough Buttonweed occurred in this area, primarily in sunny areas where Partridge Pea (Chamaecrista fasciculata) and Flowering Spurge (Euphorbia corollata) also occurred. The photographed plants are the less hairy variety of Rough Buttonweed, Diodia teres teres.
Comments: Rough Buttonweed (Diodia teres) is not particularly showy and it is easily overlooked. This plant resembles some of the other buttonweeds (Diodia spp.), but it has more narrow leaves and it prefers to grow in drier, sunnier places. Compared to Smooth Buttonweed (Diodia virginica), Rough Buttonweed has shorter flowers, undivided styles, and corollas that are often lilac or pink. Smooth Buttonweed has white flowers with slender divided styles. Other common names of Diodia teres are 'Poorjoe' and 'Poverty Weed.' These are primarily southern names that refer to this plant's preference for poor soil. There are two varieties of Rough Buttonweed: the typical variety (var. teres) and a more hairy variety (var. setifera). This latter variety occurs in southern Illinois.