Ageratum houstonianum aka Bluemink

Ageratum houstonianum has two common names: flossflower and Mexican ageratum. It belongs to the aster family, Asteraceae. Its native range spans Mexico and extends into Central America. It grows in pine woods and on land people cultivate. Records also place it by roads and on disturbed ground. Gardeners grow it as an annual. It cannot withstand frost. Kew records both annual and perennial forms within the species.

The plant forms low clumps with a round outline. Garden plants reach about 0.3–1.5 feet [0.1–0.46 metres] tall. Their spread can reach 1.5 feet [0.46 metres]. Useful Tropical Plants describes taller plants. These reach 3.3 feet [1 metre]. Stems may branch and lean along the ground before rising. Their colour ranges from green to shades of red. The green leaves are simple, with hairs on both sides. Some have an egg shape; others have a triangle shape. Their bases have a heart shape. Small, round teeth line their edges.

Small flower heads form groups at the stem tips. Each head is about a quarter-inch wide. The flowers form tubes. They lack the outer ray flowers that daisies have. Their fine parts give the heads a soft, fluffy look. Blue is the main flower colour of the species. Species accounts also report pink, white, and lavender flowers. Bloom starts in late spring or early summer. It lasts until frost. Butterflies visit the flowers and pollinate them. The dry fruit is a cypsela, a type of achene.

Grow flossflower in sun, with rich soil that drains well. It also grows in average garden soil and light shade. Shade in the afternoon helps where summers are hot. Keep the soil moist through the growing season. The plant can cope with some drought. But dry soil can make it wilt fast. Sources list USDA zones 2–11 for use as an annual. This does not mean it survives winter across those zones. Sow seeds indoors. Then plant outside after the last frost. Plants suit beds and border edges. They also suit rock gardens and pots.

Deer and rabbits seldom damage this plant. Aphids and whiteflies can attack its leaves and flowers. Poor air flow can let powdery mildew develop. Do not water from above where mildew causes trouble. Poor drainage can lead to root rot. Useful Tropical Plants reports garden escapes in many places. Sources report no known edible uses or hazards. They record the use of plant juice on wounds. This use is external and comes from tradition. It does not prove that the juice is safe or works.