Yahya related to me from Malik from Nafi that Abdullah ibn Umar would never wash his head while he was in ihram except if he had to do ghusl because of a wet dream.

Malik said, "I have heard the people of knowledge say that there is no harm in someone who is in ihram rubbing his head with certain kinds of plants after he has stoned the Jamrat al-Aqaba but before he has shaved his head, because once he has finished stoning the Jamrat al-Aqaba it is halal for him to kill lice, to shave his head, to clean himself of body hair, and to wear normal clothes."