Ganesha is the one who removes all obstacles, but for Ganapati Pooja Tulasi leaves are not taken. Durva grass is offered. Here is the reason –
Tulsi is very sacred plant without which Vishnu Pooja is incomplete. Tulsi was once a girl, a great devotee of Mahavishnu. Once when Tulsi came across Ganesha she expressed her wish to marry him. Ganesha declined saying he will follow Brahmacharya always so cannot think of think of marriage.
Tulasi was annoyed by this reply that she cursed Ganesh saying he will undergo forceful marriage. Hearing this Ganesha cursed Tulsi that she will be married to an Asura.
Tulasi after realizing her mistake asked for forgiveness. Pleased with her Ganesha told that she will marry an Asura in 1st birth & later will be very dear to Vishnu but will not be taken for Ganesh Pooja. It was because of this, Tulsi leaves are not taken for Ganesha Pooja.
Tulasi is not used for Ganesha but only on Vinayaka Chaturthi days along with 21 patras we use Tulasi.
