Common agalinis is an annual in the broomrape family (Orobanchaceae) native to the eastern United States and southeastern Canada. In Pennsylvania, it grows in wet meadows, bogs, fens, marsh edges, lakeshores, and serpentine barrens, typically reaching 2–4 feet in height with a branched, open habit about 1 foot across. 

This species blooms from mid July through September, producing tubular, pink to purple flowers that arise from the leaf axils along the branching stems. Each flower measures ¾–1 inch long and about 1 inch across at the open end. The corolla has five spreading lobes, the upper two smaller than the lower three, and all are finely hairy.

The throat of the flower is spotted purple with yellow lines, and the interior contains four hairy, white‑tipped stamens and a slender white style that is about the length of, or slightly longer than, the floral tube.

The calyx is hairless, with five pointed lobes that are one‑third to one‑half as long as the tube.

The leaves are opposite along the stem and linear to linear‑lanceolate in shape, measuring 1–3 inches long with a prominent midrib and smooth margins. The upper surface can feel slightly rough and may show a purplish tinge, especially in strong sun or dry periods.

This plant is hemiparasitic, forming root connections (haustoria) with nearby grasses and other herbaceous (rarely woody) plants to supplement its water and nutrient intake while still photosynthesizing on its own. Agalinis purpurea is pollinated primarily by native bees, with occasional visits from butterflies such as the Common Buckeye, contributing to the ecological diversity of open wetlands.

Common agalinis spreads by seed, producing a small round capsule that develops from the flower’s ovary. As the capsule dries, it splits open and releases numerous tiny seeds, which fall to the ground and may be moved short distances by wind or water.