Flower:

2 to 6 flowers on slender stalks arising from leaf axils all along the stem. Flowers are about 1/8 inch (4 to 5 mm) across with 5 rounded petal-like sepals, white on the inner surface and green on the outer; 3 or 4 white to yellowish tipped stamens surround the green ovary in the center.

Leaves and stems:

Leaves are whorled in groups of 3 to 8, linear-elliptic to spoon-shaped, ¼ to 1½ inches (.5 to 4 cm) long, up to ~½ inch (1 to 15 mm) wide, toothless, hairless, rounded or pointed at the tip, stalkless or nearly so. Stems are hairless, branching from the leaf nodes, swollen to varying degrees at the node. Plants are typically sprawling, spreading in all directions from the base and can form a mat up to 16 inches (40 cm) wide, though a colony of plants can form much larger patches

Fruit:

Fruit is an egg-shaped capsule 2.5 to 4 mm long, slightly longer than the sepals, containing up to 35 dark reddish-brown seeds.

Notes:

The origin of Carpetweed is debatable but a common theory is it is not native and migrated north from South America. I have most often encountered it in sandy soils, but it is apparently not very particular about its habitat and grows most anywhere in dry to moist, disturbed soil. It regularly takes hold in lawns and gardens. The whorled leaves resemble one of the Bestraws (Galium spp.), which all have flowers with 3 or 4 petals and their leaves tend to be a consistent size and shape, where Carpetweed flowers are 5-petaled and leaves can be rather variable on any given plant.