Diodia teres teres
Madder family (Rubiaceae)
Description: This native annual plant is ½1' and unbranched, except near the base. The green or reddish stems become bluntly 4-angled toward the top, and have a tendency to sprawl. Depending on the variety, the stems are slightly pubescent to quite hairy. Opposite leaves occur at intervals along the stems, and are sessile. They are up to 1½" long and ¼" across, and narrowly lanceolate to linear in shape. There is a prominent central vein, while the slightly ciliate margins are quite smooth. One or two axillary flowers occur above the upper leaves near the stems. These flowers are light purple, lavender, or nearly white. Each tubular flower is about 1/6" long, and has 4 spreading lobes, 4 white stamens, and an undivided stigma that is white and knobby at the end. The green calyx divides into 5 triangular lobes that are much shorter than the corolla. The blooming period occurs from mid-summer to early fall, and lasts about 1-2 months. There is no noticeable floral scent. Each flower is replaced by a rounded 2-celled fruit about 1/8" long that is broader at the top than the bottom. This fruit looks like a swollen button and is green and shiny, eventually turning brown. The hairiness of the fruit depends on the variety of the species. Each cell contains a single seed. Seed distribution is rather limited, unless animals facilitate their dispersion in some manner. A distinctive characteristic of this plant is the presence of rather long bristles near the flowers and fruit at the base of the leaves. The root system consists of a slender white taproot with secondary feed roots. This plant often forms colonies by re-seeding 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. It doesn't appear to have many problems with disease, and is quite drought tolerant.
Range & Habitat: Rough Buttonweed occurs occasionally in the southern half of Illinois, and a few counties in the northern half, where it is probably adventive. Habitats include dry upland areas of prairies where there is sparse vegetation, hill prairies, sand prairies, glades, gravelly borders of lakes, pathways with compacted soil, and gravelly areas along roadsides and railroads (including the railroad ballast). This plant is more likely to occur in disturbed areas, and tends to be more common in the southern counties of the state. It is regarded as a common weed in the southern states, but 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. It has been reported by Tietz (1972) that the caterpillars of Xylophanes tersa (Tersa Sphinx) feed on this plant. This moth also has a southern distribution, but migrates to the northern states during the summer. The Greater Prairie Chicken eats the fruit capsules, and possibly other gamebirds 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 Chamaecrista fasciculata (Partridge Pea) and Euphorbia corollata (Flowering Spurge) also occurred.
Comments: This little plant is not particularly showy and is easily overlooked. It resembles some of the other buttonweeds, but it has more narrow leaves and prefers to grow in drier, sunnier places. Rough Buttonweed differs from Diodia virginica (Smooth Buttonweed) by having shorter flowers that are often tinged with purple. It also has an undivided style that is knobby (i.e., capitate), while Smooth Buttonweed has a slender divided style. Other common names for Rough Buttonweed are 'Poorjoe' and 'Poverty Weed.' These are primarily southern names, which reflect this plant's preference for poor soil. The other variety of Rough Buttonweed is Diodia teres setifera. This latter variety is a hairier plant that occurs only in southern Illinois.