Is it common for roots to be growing off the stems above the substrate? I have pulled my ludwigia out and trimmend the roots above the substrate 3 weeks ago and sure enough they are back at full strength. Is this just part of the prunning process?
If you don't like it, yes it's part of the pruning process. This is completely normal for Ludwegia, especially if they are thriving.
This is one of the reasons why I like to keep them short. When short and bushy, the 'air' roots are harder to detect.