Houstonia purpurea aka Large Summer Bluet

Houstonia purpurea is a small perennial in the Rubiaceae family. Common names include mountain bluet, purple bluet, and woodland bluet. It grows wild in the central and eastern United States. In North Carolina, it occurs in the mountains and Piedmont. Its habitats range from damp woods to rocky upland sites. It also grows near streams, lakes, and wooded floodplains. Other sites include pastures, rocky bluffs, slopes, and limestone glades.

The plant forms low clumps with several upright stems. Listed garden dimensions give heights of 0.5–0.67 feet [0.15–0.20 metres]. The broader reported range is 0.33–1 foot [0.10–0.30 metres]. Clumps spread to about 1 foot [0.30 metres] across. Growth is moderate, though the clumps spread slowly. Each plant is short-lived, but self-seeding can keep a patch going. The branching stems have hairs and four distinct angles. They may be green or purple and have no scent.

Green leaves grow in opposite pairs along the stems. They are simple, smooth, and have unbroken edges. Each leaf has three veins and no leaf stalk. Leaf shapes range from narrow and lance-shaped to oval. The leaves at the base disappear by flowering time. Broad flower clusters form at the tips of branches. Each cluster holds three or more small blooms. Flowers appear from late spring into early summer. The bloom period lasts about a month. Four petals join to make a deep tube. Their spreading tips give the flowers a star-like outline. Colors include lavender-blue, pink, purple, white, and near-white shades. The central bloom in a cluster tends to open first. Small capsules follow the flowers.

Grow mountain bluet in fertile soil that drains well. It accepts sandy, loamy, and shallow rocky soils. Soil rich in organic matter suits it well. It grows in acidic to neutral soil. Moist conditions suit it, but it also tolerates occasional dryness. Light can range from full sun to partial shade. Dappled light beneath trees is another suitable setting. Its listed USDA hardiness range is zones 3–8. New plants can come from seed or division.

Mountain bluet suits rock gardens, path edges, and pollinator gardens. It also serves as a low ground cover. Mowing is possible, but wait for seed set to preserve self-seeding. Bees, flies, butterflies, skippers, and beetles visit the flowers. The plant needs little upkeep. Slugs and snails are its listed pests.