I've recently discovered that the plant going around as Rotala verticillaris is not what it appeared to be and in fact, isn't even in the same family as Rotala (Lythraceae), which has been confirmed by a botanist specializing in those genera.
Ghazanfar had some growing emersed in a tub on his front porch that had some buds forming as the weather was getting cold. I put them into a small emersed setup I have going and they flowered, as you can see here:
Compare it to the inflorescence of Pogostemon yatabeanus:
And that of P. stellatus:
I'm pretty sure I know the species but I need (as the ones pictured have faded) to be sure of a few more things. Everything I've checked so far fits. It's interesting that the real R. verticillaris looks very much like it, apart from its sessile (right up against the stem), solitary flowers.
I have been informed that this species is now being sold as a Pogostemon in Japan.
If you look closely at the flowers of the Pogostemon, you'll notice that the filaments of the stamens have trichomes (hairs). No species in the entire Lythraceae family has that feature (that was what I meant to say above). Additionally, the flowers of real R. verticillaris are quite small and are right up against the stem at the nodes, totally unlike the flowers shown previously.
I noticed too, that the Japanese site Grassy Aqua lists it as "Rotala cf. verticillaris" (cf meaning similar to) and gives the place of collection as in the state of Maharashtra, in Western peninsular India (P. erectum is endemic to this area), where R. verticillaris does not grow (it does grow in the Southeastern side of the peninsula and in Sri Lanka). Not conclusive by itself, but certainly interesting.
In this case, I was fortunate to have not only a good description (Aquatic and Wetland Plants of India by C.D.K. Cook), but also well-preserved actual specimens to which I could compare my flowering material. Very neat stuff.
Wow! That's quite a site to see a species dominate like that. All the wetlands here are a hodge-podge of different plants. The only ones with a 'majority species' are those getting overtaken by AlligatorWeed.