What I have done, as soon as a ditch lily bloomed, I cut it back to the ground and brushed it with grass and weed killer, being careful not to get any on my good plants. I use a concentrate killer, and mix a little strong. You have to stay on it, no matter what you do. It has been 3 years since I saw any in that bed, and 1 bloomed just a few days ago. Gone now, but I would not be surprised to see another. I found that easier than digging, but I have bad knees.
