Q.Red Japanese Acia Tree How Big Will
Hi, I need to know if I have to put a retaining wall up or not… In the photo you’ll see my fence on the left. Then there’s a slight gap then another fence behind it.

Certified GKH Gardening Expert
I can't tell if you are referring to an Acacia tree or a Japanese Maple. either way, it will depend on the cultivar. Both Genus have trees that exceed 30 feet in height, while some stay under 4 feet. Some selections will exceed your space, and some will remain small enough to be planted with no help, or structure.
It will all depend on the cultivar that you choose, which should match the dimensions that you are limited to.