Purple ammannia

This plant is a summer annual that often branches at the base, otherwise it is usually unbranched. The erect or sprawling stems are about ½–1' long. They are light green, glabrous, and either  round or angular in circumference. Pairs of opposite leaves occur at intervals along these stems. They are up to 2½" long, ¼" across, linear or linear-lanceolate, smooth along the margins, and glabrous. Each leaf clasps the stem at the base, where it often has a pair of small shallow lobes (i.e., it is auriculate). Along the upper surface of each leaf, there is a conspicuous central vein that runs along its length. From 1-7 flowers are produced in tight clusters in the upper axil of each leaf. These flowers are sessile, or nearly so. Usually, fewer flowers are produced per axil in the upper leaves (about 1-3) than in the lower leaves (about 3-7). Each flower is about ¼" across, consisting of 4 rounded petals that are pink or purple, a tubular calyx that is divided into 4 segments that form ridges at their edges, 4 or 8 stamens with yellow anthers, and a stout central style. The calyx is initially green or purple, but it later becomes red when the seeds begin to ripen. In the middle of each calyx segment, there is a secondary ridge along its length; these 4 secondary ridges are slightly less pronounced than the 4 primary ridges between the segments of the calyx. The blooming period occurs from mid-summer to early fall and lasts about 2-3 months. Individual flowers are short-lived. An ovoid seed capsule develops within the surrounding calyx that causes it to swell in diameter. This capsule contains numerous tiny seeds that are shiny and yellow. These seeds are small enough to be blown about by the wind, and they probably float on water. The root system consists of a shallow tuft of roots. This plant spreads by reseeding itself.