Why we offer grass (durva) to Lord Ganesha?At one time a ruthless demon named Anlasur created havoc around the world. Even the gods in heaven were not spared. He would swallow saints, sages and innocent people alive. There were terror all around. The chief of gods, Indra, tried to control Anlasur several times by engaging him in battle. However, there was no respite. Anlasur was beyond Indra’s control.
Panic stricken by Anlasur’s terror, all the gods went to Lord Shankar for help. He revealed that only Sri Ganesha could help them. Sri Ganesh was pot-bellied and could swallow Anlasur. There was enough space in his belly to accommodate the demon.
The gods offered prayers and pleased Sri Ganesh, who agreed to help them. He chased Anlasur, caught and swallowed him whole. But this created a great burning sensation in Sri Ganesh’s stomach. Many remedies were tried. None worked. Sri Ganesh was in great agony. When sage Kashyap heard of Sri Ganesh’s plight, he went to Mt Kailash, collected 21 stems of of durva grass, and offered them. The moment Sri Ganesh ate this grass the burning vanished.
Since that time devotees are offering durva to Lord Ganesh.