To be honest, it's good that Water Pennywort isn't doing well. Many years ago, Bob Lusk and I talked over the Pennywort at my hatchery. He stated that if you have another type around it, leave it there or the Pennywort will grow tremendously and hard to manage. I left Water Primrose around it and it worked.
My guess is that something is eating it, and I have no clue what it would be.
If you want to treat it, then
Texas A&M AgriLife Extension is the way to go.
