Ageratum houstonianum

Ageratum houstonianum, commonly known as flossflower, bluemink, goatweed, blueweed, pussy foot or Mexican paintbrush, is a cool-season annual plant often grown as bedding in gardens.

Description
This herbaceous annual or dwarf shrub grows to 0.3 – high, with ovate to triangular leaves 2 – long, and blue flowerheads (sometimes white, pink, or purple). The flower heads are borne in dense corymbs. The ray flowers are threadlike and fluff-haired, leading to the common name. The narrow lanceolate bracts are pointed, denticulate only at the top and glandular hairy. The flowering period is from May to November in the northern hemisphere. The plant attracts butterflies.

Range
The 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 was also naturalized in large parts of the tropics and in the southern United States. Its habitat is pastures, moist forest clearings and bushes up to altitudes of 1,000 m.

Today, it is widely used as an ornamental plant for summer borders and balcony boxes, high varieties also 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 cm.

Chemistry
Ageratum has evolved a unique 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.

Toxicity
Ageratum houstonianum is toxic to grazing animals, causing liver lesions. It contains pyrrolizidine alkaloids.

Weed risk
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.

Varieties

 * Ageratum houstonianum var. angustatum B.L. Rob.
 * Ageratum houstonianum f. isochroum
 * Ageratum houstonianum f. luteum
 * Ageratum houstonianum var. muticescens
 * Ageratum houstonianum f. niveum
 * Ageratum houstonianum f. normale
 * Ageratum houstonianum var. typicum
 * Ageratum houstonianum f. versicolor

The cultivars 'Blue Danube' and 'Blue Horizon' have gained the Royal Horticultural Society's Award of Garden Merit.