Bad ginger root begins to display mold, a deep green, white or gray, often at cut edges if you have previously trimmed the root. Not quite as obviously bad is mushy ginger root, in which the aromatic root has dried out and lost some of its flavor and much of its texture. In a pinch, you can trim off the bad part of the ginger and use the rest -- but only if the root is still firm. Ideally, replace the ginger root with fresh.
Tip Ginger root with moist blue flesh may be fresh, high-quality blue Hawaiian ginger, especially if the blue appears as a circle inside the flesh.
You can keep ginger root fresh for weeks with correct storage techniques. Wrap the ginger root in a dry paper towel and keep it in a zip-top bag in the refrigerator but not in the crisper. Or slice it and keep it in sake, vodka or cooking sherry.
Alternatively, freeze it and microplane off slices as needed, or plant the root in a pot with loose soil and grow your own fresh ginger plant. You can snap off pieces of root as needed.