Ageratina (Asteroideae- Eupatorieae)

Chromolaena odorata (L.) R.M.King & H.Rob. (Images by J.M.Garg (Id by Akramul Hoque & Validation by E S Santhosh Kumar), Balkar Singh (Id by Gurcharan Singh), Surajit Koley, Dinesh Valke & Neil Soares (id by Dinesh Valke) (Inserted by J.M.Garg) (For more images & complete details, click on the links))

Differences:
Ageratina: Phyllaries persistent, 8–30 in 2(–3) series, 0- or 2-nerved, lanceolate to linear, ± equal (herbaceous). Florets 10–60; corollas white or lavender, throats obconic to campanulate (lengths 1.5–2 times diams.); styles: bases sometimes enlarged, glabrous, branches linear, seldom distally dilated.
Chromolaena: Phyllaries usually readily falling (at least in fruit), 18–65+ in 4–6+ series, 3–5-nerved, ovate to oblong or lanceolate, unequal (papery or herbaceous), outer shorter. Florets [6–]15–40[–75]; corollas white or purple to blue, lavender, or reddish, throats cylindric (lengths 3–4 times diams.); styles: bases not enlarged, glabrous, branches linear to linear-clavate.

– …, if you want a shortcut:
Ageratina adenophora ALWAYS occurs in higher altitudes, whereas, Chromolaena odorata is usually found in plains (may be with some exceptions). A.a. has purplish stems and petioles, while C.o. is generally green throughout.
A. adenophora can also be distinguished by its leaves which as triangular (deltoid) in outline with a more or less truncate (straight) base (except subcuneate at petiole).  
Ageratina genus in India is represented by four wild or naturalized species (A.adenophora, A.ligustrina, A.riparia and A.trapezoidea).
One of the most common species is A.adenophora (Spreng.) R.M.King & H.Rob.