Answer: I am somewhat surprised to hear that a Ligularia is being invasive. Usually it is somewhat difficult to grow it well. Here is a photo to see if this is really the plant.
I am wondering if it might be a Lysimachia (notoriously invasive) instead?
Or possibly a look-alike Petasites, also known as a terribly invasive plant especially in damp soil.
All three of the above plants thrive in moist rich soil. If you are fertilizing or watering the area you could consider stopping while you try to eradicate the problem plant. To slow the spread, first prevent the plants from setting seed. Then begin by digging them out as best you can. Lysimachia will regrow from any shred of root left in the ground. Then use an herbicide such as glyphosate, be sure to read and carefully follow the label instructions. When plants have invasive tendencies and strong root reserves, it can take several applications over a period of time to achieve full control. Since this is intermingled with other desirable plants you may find the wipe-on application method is better in terms of protecting the plants you want to keep. You should also try to mulch the area to prevent seedlings from emerging.
If the roots have worked their way throughout the area you may have to clear it and then start over once you are certain the invader is controlled. If it is this invasive I would suggest you not even try to keep a small patch of it as it will always be a problem.
Your local county extension may also have some control suggestions and may be able to help you identify it for sure as well. I'm really sorry you are having trouble.
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