The rhizome mustn't be covered indeed, but exposed to as much sunshine as possible. Overwatering and underwatering have the same symptoms, but different causes so care needs to be taken which of the two it is:
* Overwatering displaces air and causes roots to rot which in turn make it unable for the plant to take up water: it wilts and goes yellow/brown beginning at the tips.
* Underwatering also causes wilting and yellowing/browning at the tips, but here the plant can't take up enough water because there is none; the rate of transpiration is greater than the available soil moisture. Ways of easily decreasing transpiration: right amount of soil moisture, shading and protection from wind
