Culture
Annual. Easily grown in average, moist, well-drained soils in full sun. Prefers rich soils with good drainage and consistent moisture throughout the growing season. Plants tend to wilt quickly if soils are allowed to dry out. Tolerates light shade, and prefers some light afternoon shade in hot summer climates. In climates where powdery mildew is a problem, avoid overhead watering and site in areas with good air circulation. Ageratums are easily grown from seed. Seed may be started indoors in late winter and transplanted outdoors after threat of frost has passed. Seed may also be sown directly in the garden after last frost date, however flowering season will be shorter (late summer to frost). Seeds are tiny and difficult to work with, particularly for direct outdoor plantings. For gardeners who do not wish to use seed, live plants are available from many nurseries in cell packs/six packs and flats. Space taller varieties 12” apart and shorter varieties 6” apart. Spent flowers will fall to the ground as they fade to be replaced by new bloom throughout the growing season. Prompt deadheading of the faded flowers is not necessary, but keeps a planting tidy and tends to promote a better uninterrupted bloom. Plants may not bloom to frost in some hot summer climates, but usually do so in cooler ones. First autumn frost will kill most if not all plants, at which point they may be removed from the garden.
‘Blue Horizon’ is an F1 hybrid that may be grown from seed.
Noteworthy Characteristics
Ageratum houstonianum, commonly called floss flower, is a frost-tender annual of the aster family that provides continuous bloom for beds, gardens or containers from late spring to frost. Features fluffy flowers (basically asters with ray flowers absent) in flattened to slightly rounded clusters atop compact, mounded plants generally ranging from 4” to 18” tall. Some taller varieties (e.g., the triploid ‘Blue Horizon’) are more erect and grow to as much as 30” tall. Species flowers are medium blue, however many cultivars are available in commerce in various shades of blue, pink, red, white and bicolor. Leaves are typically rounded, cordate-at-the-base, hairy, slightly quilted and soft green. Taller varieties make good cut flowers.
Genus name presumably comes from the Greek a meaning not and geras meaning old age because the flowers hold their color for a long time.
Specific epithet honors William Houston (1695-1733) who collected species plants in Mexico in the early 1700s.
Common name of floss flower is in reference to the thread-like appearance of the fluffy blooms.
'Blue Horizon' features fluffy lavender to purplish-blue flowers.
Problems
No serious insect or disease problems. Watch for aphids and whiteflies. Powdery mildew is an occasional problem, particularly in hot and humid climates where soils are kept on the dry side and air circulation is poor. Root rot may occur in poorly drained soils. Taller plants usually benefit from some support.
Uses
Shorter varieties are excellent as bedding plants, edgers (along paths, walkways and border fronts) and container plants (hanging baskets, window boxes, urns or other containers). Also effective in rock gardens. Taller varieties are appropriate for mid-borders or cutting gardens.