Mexican ageratum

Ageratum houstonianum is a soft-stemmed annual herb in the family Asteraceae, growing 0.3 to 1 m tall, native to the highlands of Guatemala, Belize and adjacent southern Mexico, where it occurs in pastures and moist forest clearings up to about 1,000 m elevation. Stems are pubescent and bear opposite, ovate to triangular leaves 2 to 7 cm long with crenate margins. The plant is grown worldwide as bedding ornamental for its dense terminal corymbs of fluffy blue (sometimes white, pink or purple) flowerheads, the disc florets fringed with long projecting style branches that give the inflorescence its distinctive powderpuff texture; ray florets are absent. Flowering extends from late spring through autumn. The species naturalises readily and has become an invasive weed in many warm regions. All parts contain pyrrolizidine alkaloids and a precocene-type methoprene-like compound; the plant is hepatotoxic to grazing livestock and should not be consumed.