It was a magical fantasy type of novel, where a girl living with I believe it was her grandmother goes through to a magical realm from a tree in their garden (I believe this tree was on the cover and had something to do with the title).
I remember a part where a rainbow was also a portal to the fantasy realm. And there was a girl who tried to catch a fish with nets made of her hair to catch a wish fish to wish for the protagonist's father or grandfather to visit her again. The protagonist accidentally caught the magic fish and got to make a wish.
It was so beautifully written but I read it more than 9 years ago at the very least.