Answer: This plant (Physostegia) can be difficult to control in certain locations such as a mixed perennial bed because it will spread aggressively in a spot where it is happy. You can keep trying to dig it out; and/or you can try using an herbicide containing glyphosate. I would apply it by wiping it on rather than spraying as this makes it easier to avoid contacting your desirable plants with the herbicide. Be sure to read and carefully follow all of the label instructions.
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