Hey there Cyber,
You need to excavate more soil from the base of that tree. Get in there and get dirty. We want to see you post another picture showing a noticeable buttress flare. This is paramount for long term health. Strangle the base of a tree and you shorten the life span exponentially.
Lose the annuals. They get fertilized with miracle grow, right. This is terrible for the tree. The flowers and that tree share the exact same space in the soil. You had to disrupt the trees fibrous roots to even get the flowers in there. Think holistically.
Keep up the efforts and you can enjoy many more years of fall color from this tree.