Do Not Girdle Your Trees
You see tree girdles (like the one in the photo) all the time. Girdling a tree results in the eventual strangulation of a tree. This tree owner saw an easy way to protect a crepe myrtle from the lawnmower and the weed eater but did not realize the tree would be suffering a slow death from this protection. Seems it really needs protection from the tree owner.
It is just not a good practice to cover a tree's trunk base with plastic or metal for protection from mechanical yard tools - especially on a permanent basis. Instead, think about using a good mulch that will keep the tree's base weed free and worry free. In combination with a small amount of annual herbicide, the mulch you apply will conserve moisture as well as prevent weed competition.