3 (2) Peduncles minutely puberulent and sparsely to densely pilose, eglandular; phyllaries oblong-lanceolate, abruptly tapering to subulate tips 0.5–1 mm, glabrous or sparsely pilose, margins often ciliate, abaxial faces eglandular   3 Ageratum conyzoides
+ Peduncles mixed pilose, stipitate-glandular, and viscid-puberulent; phyllaries narrowly lanceolate, gradually tapering to indurate-subulate tips 0.8–2 mm, margins not ciliate or inconspicuously ciliate, abaxial faces stipitate-glandular and sparsely to densely pilose   4 Ageratum houstonianum
1 Leaves basally cordate or truncate; phyllaries narrowly lanceolate, apex long acuminate, margin entire, stipitate glandular, with long hairs.   1 A. houstonianum
+ Leaves basally obtusely or broadly cuneate; phyllaries broad, oblong or lanceolate-oblong, acute, glabrous or with long, non-viscid hairs, eglandular.   2 A. conyzoides


Comparison of Ageratum conyzoides L. & Ageratum houstonianum Mill. as below:


Ageratum L. genus is represented in India by two species- A. conyzoides and A.houstonianum.
These closely related pantropica weed species can be differentiated as follows:
Involucral bracts linear, sharply acute-acuminate, sparsely hairy; Corolla of the florets 1.5mm long, shorter than awned pappus scale…. A.conyzoides L.
Involucral bracts linear setaceous, densely hairy; Corolla of the florets 2.5mm long, as long as or longer than awned pappus …. A.haustonianum Mill.

It (Ageratum houstonianum) has larger and robust plants and flowerheads with more florets than A.conyzoides.  

In A conyzoides plants and phyllaries are not that densely hairy, and heads are smaller and more spreading 


Thicker cordate leaves and longer hairs especially on involucre bracts are clearly seen in Ageratum houstonianum.  

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