I did interviews with elderly Kikuyu men and women to get primary data so not all info can be cited by referencing a book.
However, according my informant, Mzee Waweru Michael (deceased), Mutamaiyo (Olea chrisofila) an olive tree, was used for the same purpose as Mukuyu (Ficus sycamora) or Mugumo (Ficus natalensis) where the former two were unavailable - sacred trees for religious ceremonies.
Leakey L.S.B. (1973) states that this olive tree was the female in that sacred tree family.
Leakey goes on to say that “Ngai mweru” was regarded as the female deity and was addressed first by women...
By inference, Ngai Mweru (White God) was represented by the Mutamaiyo (Olive tree).
I hope that answers your question.
Leakey, L.S.B., 1977, The Southern Kikuyu before 1903, Vol I,II & III, Academic Press, London.