Pull all fabric, soil, mulch and all plants away from that trunk! This is the easiest way to kill a tree or even other woody perennials. Burying the trunk no matter what your drainage mitigation might be will kill that tree.
Roots belong underground. Where the roots and trunk begin and end is a very important place; bark has to be out of the ground and never compromised with mulch, rocks, fabric of any kind. That bit of constant moisture will encourage the decomposers and some other bacteria to start decomposing the bark and right beneath the bark is the life line of the tree; the cambium. The vascular system.
This vascular system is sometimes as thin as a piece of paper. The Phloem and Xylem of the tree one takes water and chemistry upwards to the photosynthetic factories and one takes carbohydrates to be stored in the root system.
'Girdling' a tree is too easy; take a sharp knife or saw and cut into the bark into the vascular system right below and you have killed the tree. It will happen slowly but surely. Burying the bark of the trunk allowing moisture next to the bark to decompose the bark and the cambium just beneath does the same thing as a sharp knife. Allows bacteria to kill the vascular system.
Plant your flowers in pots with potting soil. Set them around the base if you want, make groups of flowering potted plants on your decks or patios.