The ordinary Ageratum is a perennial, herbaceous plant or a dwarf, or shrub. The plant grows to 0.3–1 m high, with ovate to triangular leaves 2–7 cm 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 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. Their habitat is pastures, moist forest clearings and bushes up to altitudes of 1000 meters.
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 centimetres.
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. 
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.
- A. houstonianum var. angustatum B.L. Rob.
- 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
|Wikimedia Commons has media related to Ageratum houstonianum.|
- "Ageratum houstonianum Mill.". The Global Compositae Checklist (GCC) – via The Plant List.
- Eckehart J. Jäger, Friedrich Ebel, Peter Hanelt, Gerd K. Müller (ed.): Rothmaler - excursion flora of Germany. Volume 5: Herbaceous ornamental and useful plants . Spektrum Akademischer Verlag, Berlin Heidelberg 2008, ISBN 978-3-8274-0918-8 , p. 553 .
- New South Wales Flora Online, Ageratum houstonianum
- Nesom, Guy L. (2006). "Ageratum houstonianum". In Flora of North America Editorial Committee (ed.). Flora of North America North of Mexico (FNA). 21. New York and Oxford – via eFloras.org, Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.
- Species profile
- I. Kiss; et al. (September 1988), "Biological activity of precocene analogues on Locusta migratoria", Cellular and Molecular Life Sciences, 44 (9): 790–792, doi:10.1007/BF01959168
- Acamovic, T., Stewart, C.S., Pennycott, T.W.,"Poisonous Plants and Related Toxins", 2004
- Noa, M., Sanchez, L.M., Durand, R., "Ageratum houstonianum toxicosis in Zebu cattle", Veterinary and human toxicology, 2004, vol.46, no4, pp.193-195.
- Wiedenfeld H, Andrade-Cetto A., "Pyrrolizidine alkaloids from Ageratum houstonianum Mill.", Phytochemistry, 2001 Aug, pp1269-71 
- Global Compendium of Weeds, Ageratum houstonianum (Asteraceae)
- JSTOR Plant Science
- RHS Plant Selector Ageratum houstonianum 'Blue Danube' AGM / RHS Gardening
- RHS Plant Selector Ageratum houstonianum 'Blue Horizon' AGM / RHS Gardening