General Culture: Click for monthly care information.
Annual. Easily grown in average, moist, well-drained soils in full sun. Prefers rich, fertile soils with good drainage and consistent moisture throughout the growing season. Plants tend to wilt quickly if soils are allowed to dry out. Prefers some light afternoon shade in hot summer climates such as the St. Louis area. Flowering suffers in too much shade. In climates where powdery mildew is a problem, avoid overhead watering and site in areas with good air circulation. ‘Blue Horizon’ is an F1 hybrid that may be grown from seed. Start seed indoors in late winter (6-8 weeks before last spring frost date) and transplant outdoors after threat of frost has passed. Seed may also be sown directly in the garden after last spring 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 plants about 9-12” 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.
Noteworthy Characteristics:
'Blue Horizon', commonly called floss flower, is a frost-tender annual of the aster family that provides continuous bloom for borders, drifts or gardens from late spring to frost. It features fluffy lavender to purplish-blue flowers (basically asters with ray flowers absent) in flattened to slightly rounded clusters that bloom atop strong upright stems typically rising to 18-30” tall. Leaves are typically rounded, cordate-at-the-base, hairy, slightly quilted and soft green. Ageratum comes from Greek words meaning “non-aging” in reference to the long life of the flowers. Specific epithet honors William Houston (1695-1733) who collected species plants in Mexico in the early 1700s. The common name of floss flower is in reference to the thread-like appearance of the fluffy blooms.
Problems: Click for detailed list of pests and 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. Southern blight and botrytis may occur.
Uses:
Borders and cutting gardens.
© Missouri Botanical
Garden, 2001-2009
More photos:
(3 ratings) --- 
