Water chestnut, Trapa natans

We will have to see how well these will do in our climate. While they should be hardy enough to get through the winter, Ken Fern (Plants for a Future) reports he hasn't had much success keeping these or getting them to flower. 

The seed is edible raw, cooked or dried and ground into a powder. A sweet floury and agreeable flavour, similar to sweet chestnuts (Castanea spp) .

This is a floating plant.