Answer: I'm so sorry you are having this problem. This plant is a notoriously invasive spreader in the garden under conditions it prefers -- especially in rich, damp soil. You will have to be ruthless in grubbing out every trace of root you can find (six inches is probably not deep enough yet, keep going until you think you have truly found them all) then be vigilant next summer to remove any remnants that may sprout. If in doubt, trace the root back toward the butterfly bush and see, but if it looks like the others it is probably the loosestrife. Try to make a clear perimeter around the loosestrife you want to keep, then shovel prune it regularly to keep it in bounds. You need to create a clear line between it and your sedum to prevent it ruining the sedum planting, then from that clear area continue to work back into the loosestrife until you have it controlled to where you want it. If it has gone into the sedum, your best bet is to lift the sedum and bare root it (or else start new sedum plants from tip cuttings next spring) to replant elsewhere in the garden until you have the loosestrife under control. You may also want to consider using an herbicide containing glyphosate on it next summer; it will not be effective until the plant is in active growth for the season and is non-selective so do not allow it to contact your desirable plants. And, certainly, do not water or fertilize it. Best of luck!
Q&A Library Searching Tips