Not really, i mean i dont know personly how hardy Ludwigia is but i'm pretty sure they are hardly. You can just cut the tip top with half an inch of stem and replant it and it should grow, you can cut branches and it would grow, the roots dont really effect it chances because well it will probably grow no matter what(it just way get started a tiny tad faster). I started out with 1 plant and now i have over 20 i trim the tops when they get to tall and replant them, i do the same to some branches. I had this really hardy plant that one time i was trimming and i accidently cut a leaf in half, i'm not kidding you a root grew out of the half of that leaf and started a new plant, i was amazed but that was a realy hardly plant, haha. Just trim away and replant and you will get yourself a few new plants, like i said trim anywhere, the bigger the stem left on the trimming the faster it will get going. And sometimes the orginal may not grow from the top portion but it will branch out anyway. Use very sharp scissors it will help the orignal recover faster and allow it to continue from where the trimming was, lously scizzzors leaves scars and the original plant might not grow where it was trimmed.