Sounds like you've got a problem with your soil.
Buy another one and keep it in it's tin. You can always surround the tin with wood or stone, or drop it in a large pot to disguise it. It'll also help to stop the tin conducting heat from direct sunlight and cooking the roots.
In Kaya we can only grow the purple ones. The others are killed by the winter frosts. In the spring we wait for the first new shoots to appear and then cut back all the dead wood to the fresh growth.