This is a plant that we bought from Hua Hng. It is related to the balsam plant and originates from China as a common weed in their padi fields.
It simply loves growing in waterlogged sites and the hollow stems require a lot of water for it to be turgid and erect. Without sufficient water, it would flop over.
The lanceolate leaves are serrated with a red edge whilst the clusters of three beautiful pink flowers are absolutely free flowering when the plant is planted under full sun conditions.