If you want potatoes that will store well, wait til the tops of the vines have died before you start harvesting. If you want early potatoes, gently dig down and take only what you need. Reset your plant and the potato should continue to grow. This is from my own experience. Hope it helps. I love growing potatoes. I have not try growing blue potatoes. Do you know what they taste like? Textures? Very curious. Please share what you find out later after you harvest.