Asystasia variabilis

An indigenous common herb found in waste lands, forest edges, along borders of paddy fields etc. in wet lowlands and hill country.  An indigenous herb common under shade along streams and footpaths in wet lowlands to mid country secondary forests. Flowering from August to March.*

* Text and species name of the initial post was changed according to the Jep de Vlas's comment below and correct identification given in the subsequently published his second book on flora of Sri lanka.