Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Asterales
Family: Asteraceae
Tribe: Eupatorieae
Genus: Ageratum
Species: A. houstonianum
Binomial name: Ageratum houstonianum
Synonym: Ageratum mexicanum
Common names:  Bluemink, Blue billy goat weed, Flossflower, Blueweed Pussy foot, Mexican paintbrush, Goatweed.

Ageratum houstonianum is a short-lived (annual or biennial) herbaceous plant native of Central American and Mexico but has become an invasive weed in other countries. It is often grown as bedding in gardens. It is found mainly in the North Island but it is reported in the Christchurch area. Its habitats are wastelands, roadsides, pastures, lake margins, river terraces and open sunny spots along forest edges.
This herbaceous plant which grows to 0.3–1 m high has softly hairy stems and leaves. The leaves are ovate to triangular leaves (2–7 cm long) toothed and are oppositely arranged at the base of the stems, but they are often alternately arranged at the top of the stems.
The flower-heads (capitula) are arranged in dense clusters at the tips of the branches (terminal corymbs) and do not have any obvious 'petals' (ray florets). Each flower-head (5-8 mm across) has numerous tiny tubular flowers (tubular florets) that are surrounded by two or three rows of greenish-coloured, hairy bract bracts (an involucre). The florets (2-3 mm long) range from pale lavender to blue, pink or purplish in colour and each has two elongated projections (style branches). Flowering can occur all year round.
This species reproduces by seed. The tiny, black or brown, light, seeds (>1.8 mm long) are often dispersed by wind or water. Also, they readily become attached to animals, clothing and vehicles and may also be spread in contaminated agricultural produce.
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. This chemical triggers the next moulting cycle to prematurely develop adult structures and can render most insects sterile if ingested in large enough quantities. Grazing animals tend not to eat it as it is toxic if they ingest it. However, the flowers are a great nectar source for butterflies and bees.
Many garden centres offer hybrid selections which tend to be less weedy than their wild counterparts.

The sap was used to treat cuts and wounds and the crushed leaves were rubbed on the exposed skin as an early mosquito and biting insect repellent. 

Ageratum houstonianum Floss flower .JPG

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Close-up showing the numerous tiny flowers in each flower-head 
Ageratum houstonianum Floss flower -002.JPG

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Thanks to Wikipedia for text and information: