Moral of the story? Don't do what I did.
Make home-made ice cream every week during the summertime. Pour the saltwater onto the tree - every week.
Came to post this. Remedy is what we use on mesquite stumps and has been 100% effective.Skip the roundup and basal spray with diesel and Remedy (20% Remedy). Must wait a few months before removing tree. Or cut flush to ground and spray the fresh cut with same. I've heard just diesel works too but have always mixed Remedy/diesel when spraying trees.