Diodia teres Walter

Rough Buttonweed

© SRTurner

Family - Rubiaceae

Habit - Annual forb, sometimes woody at base.

Stem - Ascending to erect, to 35 cm, 4-angled, minutely roughened or sparsely to moderately pubescent with short, straight to wavy hairs of various lengths.

	Stem and node.
© SRTurner

Leaves - Opposite, simple, sessile, entire. Stipules 1-2 mm long, spanning node and fused to leaf bases on either side, each side bearing 5-11 bristles on the margins, variously 1-10 mm long. Leaf blades 1.0-3.5 cm long, 2-5 mm wide, narrowly lanceolate to narrowly elliptic or nearly linear, angled to somewhat rounded at the base, angled to a sharply pointed tip, the margins minutely pubescent with stiff, spinescent hairs, frequently becoming revolute (especially in dry microsites), the surfaces glabrous or nearly so, the venation with the midvein and usually 2 or 3 pairs of pinnate, secondary veins visible.

	Leaf adaxial.
© SRTurner

	Leaf abaxial.
© SRTurner

Inflorescences - Axillary flowers, usually solitary, produced at most of the nodes.

	Inflorescence.
© SRTurner

Flowers - Calyces deeply 4-lobed, the lobes 1-2 mm long, triangular, often unequal. Corollas funnelform, 6-7 mm long, pink to less commonly pale purple, externally glabrous or hairy, internally glabrous, the tube 4-5 mm long, the lobes 2.0-2.5 mm long, lanceolate. Stamens 4, inserted near the top of the corolla tube. Ovary fully inferior, 2-locular, the ovules 1 per locule. Styles unbranched, with a 2-lobed, capitate stigma.

	Flowers.
© SRTurner

	Corollas.
© SRTurner

Fruits - Schizocarps 3.0-3.5 mm long, 3.0-3.5 mm wide, broadly obovoid, smoothly rounded to broadly angled on the sides, splitting into 2 mericarps.

	Fruits.
© SRTurner

	Fruits.
© SRTurner

Flowering - June - October.

Habitat - Glades, upland prairies, sand prairies, bluffs, streambanks, pastures, old fields, railroads, roadsides, open disturbed areas, often in sandy or acidic soils.

Origin - Native to the U.S.

Lookalikes - Sherardia arvensis.

Other info. - This little species is common across most of Missouri except for the northwestern corner of the state. Missouri is near the northwestern corner of the plant's main range, which occupies the southeastern quadrant of the continental U.S. It is found, less commonly, in southwestern regions of the U.S. and continues into Central and South America.

The plant is capable of flourishing in some relatively inhospitable habitats, and can form large populations. It is easily recognized by its opposite leaves and small pink flowers at the leaf nodes. It somewhat resembles Sherardia arvensis, but that species has considerably smaller flowers and whorls of leaves at nodes, and lacks the stipular bristles found in Diodia teres.

Photographs taken at Hawn State Park, Ste. Genevieve County, MO, 9-1-2008, Canaan Conservation Area, Gasconade County, MO, 8-13-2013, and at Sand Prairie Conservation Area, Scott County, MO, 8-15-2021 (SRTurner).