What is required when doing ghusl in the case of janaabah is to make the water reach all of the hair, even if it is long. At-Tirmidhi (106) narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Under each hair is janaabah, so wash the hair and clean the skin.” And it was narrated from ‘Ali (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Whoever leaves impurity the size of a hair and does not wash it, such and such will be done to him in the Fire because of it.”
Narrated by Abu Dawood, 217. These two hadeeths were classed as da‘eef (weak) by Shaykh al-Albaani (may Allah have mercy on him).
But the report in which these words are attributed to ‘Ali (may Allah be pleased with him) is saheeh.
The scholars of the Standing Committee were asked (5/323):
Are the woman who has long hair that is not braided and the woman whose hair is braided regarded as the same in the case of ghusl for janaabah, or is it essential to wash the hair completely in either case?
They replied: The one who is in a state of janaabah and the one whose period has ended have to make water reach every part of the body and hair with the intention of purifying themselves, whether the hair is long or short, and whether it is braided or not braided. End quote.
Shaykh ‘Abd al-‘Azeez ibn ‘Abdullah ibn Baaz; Shaykh ‘Abd ar-Razzaaq ‘Afeefi; Shaykh ‘Abdullah ibn Ghadyaan; Shaykh ‘Abdullah ibn Qa ‘ood
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: It is not permissible for her to wash the outer part of the hair only; rather it is essential that the water reach the roots of the hair and the skin of the head (scalp). But if the hair is braided, she does not have to undo it; rather she has to make the water reach every single hair by placing the braid under the water spout and squeezing it so that the water reaches all of the hair.
End quote from Majmoo‘ al-Fataawa, 11/163.