Tried a 4-inch mulch ring instead of the usual 2-foot circle on a young oak in my yard, and the growth difference after one season was wild. | Arborists - Guest Forum
My neighbor did a six inch ring around her maple and it looked so silly, like a tiny life preserver. Oliver_adams is right, that's basically a mulch bracelet for a tree. I switched to a proper three foot circle last year and the roots look way happier now.