You can start to harvest potatoes when the plant dies off (the leafs are brown and falling off. I have harvestes mine. Mark sure you harvest before a lot of fall rains if the ground is too wet for long periods of time this will cause the potatoes to rot. Some will show sights of this as they will have little whit spots on them. Be sure to wash and let dry outside in the shade. The sun with cause them to turn green. Unwashed potatoes dry out and will not last as long. A tip for next years potatoes during the winter when potatoes start to grow eyes set them asides and plant them in the spring