He then decided to keep growing it for as long as he could handle it, cut it off and raise money for Sativa's cause while he was at it.
Mr Moore said he would shave all of his hair off a week after Sativa's anniversary on October 8.
Donations made to Mr Moore's Givealittle page would go to Starship Hospital and Make a Wish foundation.
It would be weird to feel the wind on the back of his neck again, he said.
Sativa's mum, Sheree Roose, said William and Sativa were very close.
"It was pretty hard for the whole family. As a parent your grief surrounds you every day. You just learn to cope with it better. It's not so raw any more but coming up to her anniversary it gets quite hard and difficult for me.
"The fact that somebody else, without me mentioning it, is trying to help her legacy live on is really awesome. When William set the page up, I cried. It was so lovely."
Each year on Sativa's anniversary, Ms Roose would blow up purple balloons and light Chinese lanterns for her little girl, something she would do again this year to keep her memory alive.
Sativa Eagle's funeral was held at Greerton Bible Church. Photo/File
What is leukaemia?
Leukaemia is the name given to a group of cancers that develop in the bone marrow. Under normal conditions the bone marrow contains a small number of healthy immature blood cells, sometimes called blast cells. These immature blood cells mature and develop into red cells, white cells and platelets, which are eventually released into the bloodstream.
The 16-year-old Tauranga runner lowered his own national U17 and U18 1500m records at the Sound Running Sunset Tour in Los Angeles.
Video / Athletics NZ