Agalinis tenuifolia, commonly known as slenderleaf false foxglove, is a branching annual in the Orobanchaceae family, native to eastern and central North America. It typically grows in dry, open habitats such as forest edges, thin‑canopied woodlands, wooded slopes, granitic outcrops, meadows, and roadsides with coarse, well‑drained soils. Plants often reach 1 to 2 feet in height, occasionally taller in ideal conditions.

Slenderleaf false foxglove is hemiparasitic, drawing water and nutrients from the roots of nearby grasses and herbaceous plants. It is pollinated by bees, syrphid flies, and butterflies such as the Common Buckeye. The species is named for its slender leaves; tenuifolia means “narrow-leaved.” Leaves are opposite, becoming alternate or sub-alternate toward the flowering portion of the stem; simple, linear, typically 1 to 3 inches long and 1–3 mm wide, with smooth margins and a prominent midrib. In dry conditions, the foliage may turn purplish or reddish.

It blooms from August to October, with each flower lasting only a day. The blossoms are pink to purple, funnel‑shaped, and typically ½ to ⅗ inch long, with five lobes; the lower three are larger than the upper two. Flowers are borne singly in the upper leaf axils on slender, thread‑like pedicels. The interior of the corolla is marked with purple spots and pale yellow to white lines. The stamens and style are pale in color, with the stamens characteristically hairy.

A. tenuifolia reproduces by seed, forming small round capsules after the flowers wither. When the capsules dry, they split open and release numerous tiny seeds, which fall to the ground and may be further scattered by the wind.