Arenaria serpyllifolia L.

Thyme-Leaved Sandwort

© DETenaglia

Family - Caryophyllaceae

Habit - Taprooted annual forb.

Stems - Sprawling to erect, to 45 cm, usually branched, moderately pubescent with minute, downward-curved hairs, sometimes also stipitate-glandular.

	Stem and node.
© SRTurner

Leaves - Opposite, fused basally into a sheath, usually sessile. Leaf blades 2-7 mm long, elliptic to broadly ovate, tapered at the base, acute, entire, glabrous.

	Leaves.
© DETenaglia

Inflorescence - Axillary single flowers or terminal loose cymes. Pedicels elongating in fruit to 1 cm, retrorsely pubescent. Bracts paired, leaflike.

	Inflorescences.
© SRTurner

Flowers - Sepals 5, 2-3 mm long, becoming elongated to 4 mm long at fruiting, free or basally fused, lanceolate to narrowly ovate, sharply pointed, the margins thin and whitened. Petals 5, free, usually shorter than the sepals, 0.6-2.7 mm long, oblong, not tapered to a stalklike base, entire at the tip, white. Stamens 10, the filaments white, glabrous, distinct. Pistil with 1 locule, sessile. Styles 3, white, glabrous, each with the stigmatic area along the inner surface. Ovary superior, green, glabrous, 1.5 mm long, 1 mm in diameter, unilocular. Placentation free-central.

	Flower.
© DETenaglia

	Flower.
© SRTurner

	Calyx.
© DETenaglia

Fruits - Capsules, 3.0-3.5 mm long, dehiscing apically by 6 ascending to recurved teeth. Seeds 10-15, 0.3-0.6 mm wide, kidney-shaped, the surface tuberculate, black.

	Fruit.
© DETenaglia

Flowering - April - August.

Habitat - Glades, fields, bluffs, roadsides, railroads, disturbed areas.

Origin - Native to Europe.

Lookalikes - Chickweeds (Cerastium spp., Stellaria spp.).

Other info. - This tiny plant is common throughout most of Missouri and large portions of the continental U.S. It is small and inconspicuous, and often overlooked, although large populations can be showy in their own way on warm, sunny spring days. This species can be identified in the field by its small, opposite, ovate leaves and its small white petals, which are smaller than the sepals.

Missour plants can be subdivided into two varieties: var. serpyllifolia, which has more conical fruits and larger seeds, and var. tenuior, which has more cylindrical fruits and smaller seeds. The distinction is subtle and only possible if mature fruits are present (or by counting chromosomes).

Photographs taken in Gainesville, FL., 2-11-03, and at Providence Prairie, Lawrence County, MO., 6-18-05 (DETenaglia); also along the Katy Trail west of Augusta, St. Charles County, MO, 5-12-2017, and near Labadie, Franklin County, MO, 4-24-2020 (SRTurner).