I am a member of Postcrossing, and I received my very first postcard from Vietnam. The caption on back is in several languages. The English is: "Reaping of water caltrops".
I did not know what a water caltrop was, so I had to look it up. It's obviously a water plant, with a stem that can be 12-15 feet long with roots anchored in the mud, and produces a fruit shaped roughly like a head of a bull. Inside this fruit is a large seed which can be roasted & eaten. It's cultivated in parts of Asia and Africa. However, in places like the Eastern US and Australia, it's apparently considered a weed - and a nasty one at that.
It's an interesting card, my first from Vietnam.