Ruellia dipteracanthus is a slightly disreputable member of the family Acanthaceae. It is a commonly seen, tough, low-growing species (height 20 cm) from Mexico, with funnel-shaped, purple flowers in the warmer months. It copes with sun or shade, and forms a good groundcover in difficult areas, layering to form a wide mound. It is one of those plants often given to young couples when they are starting a garden, on the premise that it can't be killed, even by complete novices. Mine was certainly obtained that way! Just make sure you keep an eye on it. I have seen it clipped to form a very effective garden border (as illustrated above). It will grow in sun or shade and is quite drought tolerant. Trim it back hard in late winter and give it some fertiliser at this time.