Answer: when it is a "Lucky bamboo". Dracena Sanderiana commonly called lucky bamboo is a small shrubby plant that grows as an understorey plant in rainforests, it is a native of Cameroon in tropical West Africa where it grows to 1.5 metres and is not related to bamboo. Although it grows better in soil it is often sold with its roots in water as an aquatic plant. While it can live for months like this it will eventually rot unless sprouts are allowed to grow above the surface.
WARNING: "Lucky bamboo" (Dracena Sanderiana) is toxic to pets.