Earthing up does all sorts of jobs. It makes the plants form more tubers (stolons eh? - ya learn something every day!), it stops the spuds going green in the sun (as Chris said), it kills young weed seedlings (as would any kind of soil movement) but most importantly at this time of year it protects the young leaves from frost. Potatoes will sulk very badly if they get frosted, and may refuse to give you any kind of crop at all (though some varieties are tougher than others).

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Resistance is fertile