Old root problem? Don’t try to kill your mower, plant it up. Try this “River series ” of Tiarella. Alow the Tiarella to flow along side that old root and brighten up planting path under that tree.
I used to be made by mom to take a grass shears and cut grass around tree roots, mail boxes and lamp posts in the yard. This makes more sence. Plant perennials; there is a vast array of perennials for any situation.
Create gardens around trees, not mulch compost sights.
There is nothing good about this mulch job. Dig back to ground level and plant a Tiarella. The Tiarella will have dense foliage enough to keep weeds down and provide cool root area for the tree in summer.