Mirabilis jalapa, commonly called Four O'Clock plant.

It has tuberous roots that can be stored over winter, similar to Dahlias. The seeds are extremely poisonous, but it self seeds easily. Also known as Marvel of Peru. Maybe they don't have clocks in Peru!!

From what I can find out about the climate of the Charente area, you could probably leave the tubers in the ground--they have been known to grow to over 4 kilograms in weight.