Place the roots in a container of water, cover the greens with plastic, and refrigerate for up to one week. If purchased without the roots, cover cilantro with a damp cloth, refrigerate in a perforated plastic bag, and use within four to five days. Cilantro loses its flavour quickly and develops a harsh, unpleasant taste, so use it as quickly as possible after purchase. Store whole coriander seeds and ground coriander in tightly closed containers in a dark, cool cupboard. The whole seeds keep for a year or more. Once ground, coriander loses its flavour more quickly. Discard ground coriander when it becomes mild and flat-tasting.