Darbha grass is known to be the purifying object. In Kumbha poojas the grass is used to purify different objects of worship by sprinkling holy waters using the grass. Brahmins put this grass always in their houses and use for any purificatory ritual they perform.
Kusha or Darbha grass is used in Hindu rituals for purificatory process. Kusha grass is meant to have Lord Vishnu’s potency. It is believed that this grass has immense purifying properties.
The sanctity of dharba, also known as kusha (or, kusa) grass, is as old as the Indian gods. Puranas tell how Vishnu assumed the form of the Cosmic Tortoise (Skt. kurma) whose shell served to support Mandara, the mountain that served as a dasher in the Churning of the Sea of Milk. As the mountain rotated, several hairs were rubbed from the tortoise's back. With time, they washed ashore and became Kusha.
Later, when the amrita was obtained and distributed among the gods, some drops fell on the grass which further sanctified it imbuing it with healing properties.
Therefore, in the tradition hair-cutting of Vaishnava toddlers, the hair is touched with kusha before it is cut. It was used as a ritual seat as far back as the Vedas, and the Bhagavad Gita (ch. 6) stipulates that, covered with a skin and a cloth, it is the appropriate seat for meditation. Therefore, it was one of the first offerings made to the Buddha.
Kusha, whose name signifies sharp in the sense of acute, is the root for the Sanskrit word for "expert," kosala. That is because the edges of the long leaves that grow in pairs along the tall stems are very sharp, so like the sword, it is used as a symbol for discernment or "discriminating wisdom." It grows beside brackish (salty) water such as found at the mouths of rivers and is a kind of tussock grass; that is, it grows in clumps.
When it is dry, kusa straw is called durva or dharbai. However, some say these are two different species: Kusha is Poa cynosuroides and Durva, Agrostis linearis. In the Mythology, to remove the slavery of his mother Vinata from Kadruva, the King of Birds, Garutman (Garuda) went to Amara Loka and brought flagon contains Ambrosia (Amruta Kalasah) and kept on the Kusah (Sacrificial Grass) and shown to cousins ( step mother’s , Kadruva’s sons) and asked to liberate his mother’s (Vinata’s) slavery from their mother.
Because of placing the Amruta Kalasah ( pot containg Ambrosia) the Sacrificial Grass (Darba) became sacred. For this reason, since then Kusah (Darba) has become headmost and worthy for all the rituals of Godly Sacrificial Fires (Yagnas and Homas etc.,). In the Mythology the mighty Demon Vrutrasura was killed by Indra Deva. Then that Asura fell on the gound and became as Kusah (Darbh). Hence Kushah is treated as sacred in all the Hindu Rituals.