Om Vigneshwaraya Namaha
There was a demon by name Analasura(Anala means fire). He became very powerful after getting boon from Lord Brahma. He had a boon that he could emit fire from his mouth and eyes, so he started to trouble all the Devas.
When the Devas were not able to bear the Asura’s atrocities, approached Lord Ganesha, the obstacle remover and begged him to help.
Lord Ganesha then took a giant form and swallowed the demon as a whole. Since the demon was made of fire, swallowing him caused severe heat in Ganesha’s tummy and entire body.
All the Gods tried cooling Ganesha with the Chandrakanthi, moonlight or soothing glow of the moon and with lotus flowers, but Lord Ganesha didn’t get any respite from the burning sensation in His entire body.
Finally, all the sages taking the advice of Lord Dhanwantari, the divine physician of Gods, placed 21 blades of Durva (Bermuda) grass on his head, which cured the body heat and Lord Ganesha felt relieved from the burning sensation.
Lord Ganesha then declared that anyone who worships him with Durva grass will receive his blessings, so from that day onwards, Ganesha is worshipped with all green leaves and especially Durva grass.