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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.
In this case, harvest them about a week after the potato plant flowers first appear. New potatoes only produce a couple of handfuls of potatoes per plant, so dig up the whole plant. Source(s): http://www.garden.org/foodguide/browse/veggie/potatoes_harvesting/574.