Pots, made of terracotta or sometimes plastic, and window boxes have been the most common containers. Terracotta containers are usual for long-term use in back yards. The reason is that they are porous, and water evaporates through the pores. This keeps the contents aerated. Plastic pots are usually for short-term use, and often sold complete with flowers and earth. Window boxes are shallow and open, and may be used for above-ground windows. 