Stachytarpheta urticifolia, Blue Rat-tail, Verbenaceae, is a herb from tropical America naturalised in Sri Lanka. Flowers open only a few at a time on the long spike.
It is a nectar source for butterflies (van der Poorten & van der Poorten, 2016, The Butterfly Fauna of Sri Lanka).
I have observed an Acherontia lachesis (Bee Robber) moth larva devouring the leaves of this plant.