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But it can take 70-90 or more for potatoes to mature. This depends on the variety. The potatoes can carefully be harvested at 7-8 weeks for a early harvest of small potatoes. For full grown potatoes you would harvest in the fall when the leaves turn brown. But it can take 70-90 or more for potatoes to mature.
When the potatoes are ready for harvest, the plant tops yellow and die back. Dig up a few tubers and examine their skin. Immature potatoes have thin skin that bruises easily while mature potatoes have firmer skin. If you can rub off the skin easily with your thumb, then delay the harvest until the skin is set.