The reason? Cooking aboveground vegetables simply requires softening the cell walls to make them more palatable and digestible. Because most green vegetables are small with thin cell walls, that process doesn’t take very long. So all you need to do is boil water, add the vegetables, and cook until they are just tender.
Root vegetables, on the other hand, contain a great deal of starch, and that starch needs to be dissolved before most can be eaten.
Starting potatoes off in cold water creates more even cooking. Throwing cold potatoes into boiling water gelatinizes the starches at the surface of the potato too fast, leaving you with a mushy exterior that falls apart and dissolves into the cooking water before the center cooks through. By starting in cold water, the temperature in the potato rises more gently.