Ageratum houstonianum (flossflower, bluemink, blueweed, pussy foot, Mexican paintbrush) is a cool-season annual plant often grown as bedding in gardens. This plant is native to Central America, in Guatemala and Belize and adjacent parts of Mexico, but has become an invasive weed in other areas. It has also been naturalized in large parts of the tropics and in the southern United States. Their habitat is pastures, moist forest clearings and bushes up to altitudes of 1,000 meters.
Today, it is widely used as an ornamental plant for summer borders and balcony boxes, and as cut flowers. The species is cultivated once a year, having numerous varieties whose crowns may be dark blue, purple, pink and white. Preferring cool soils and exposure in full sun, high varieties reach stature heights up to 60 centimeters (24 in).
This herbaceous plant grows to 0.3–1 m high, with ovate to triangular leaves 2–7 cm long, and blue flowers (sometimes white, pink, or purple). The flower heads are borne in dense corymbs. The ray flowers are threadlike, leading to the common name. The flowering period is from May to November in the Northern Hemisphere.
Varieties include: A. houstonianum var. angustatum, A. houstonianum f. isochroum, A. houstonianum f. luteum, A. houstonianum var. muticescens, A. houstonianum f. niveum, A. houstonianum f. normale, A. houstonianum var. typicum, A. houstonianum f. versicolor. The cultivars 'Blue Danube' and 'Blue Horizon' have gained the Royal Horticultural Society's Award of Garden Merit.
Ageratum has evolved an ingenious method of protecting itself from insects; it produces a methoprene-like compound which interferes with the normal function of the corpus allatum, the organ responsible for secreting juvenile hormone during insect growth and development. This chemical triggers the next molting cycle to prematurely develop adult structures, and can render most insects sterile if ingested in large enough quantities. A. houstonianum IS toxic to grazing animals, causing liver lesions, as it contains pyrrolizidine alkaloids.
Ageratum houstonianum is prone to becoming a rampant environmental weed when grown outside of its natural range. It has become an invasive weed in the United States, Australia, Europe, Africa, China, Japan, New Zealand, and the Philippines.
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