Also whats the best way to store them and how long should they last?
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I wouldn't wash them before storing -- if they're very dirty, brush off the clods with a brush, but keep them totally dry.
They'll keep the longest in a cool, dry, dark environment. Imagine a root cellar!
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DEFINITELY don't wash them before storing. They need to be completely dry. That soft dirt is also protection from the "elements."
…and away from your onions.
Do not wash the potatoes. They provide a protective coating. This is not just for potatoes but is a good rule in general for storing vegetables.
Unwashed, cool, dark, dry in their sacks----but also humid as to the surrounding air, as in a basement. That way, they will keep until March, when they will sprout, even in the dark.
Besides not washing and storing in cool dry, place, also prudent to remove any potatoes that have green areas. When potatoes get a sunburn, they turn green, a marker that toxic alkaloids have accumulated on the surface. I either discard these potatoes or use immediately peeling deeply to remove all the bad stuff.