Answer: These are vert persistent plants, and one method of rejuvenating old overgrown ones is to cut them to the ground to encourage vigorous new growth from the roots. So the ones you cut to the ground will be trying to regrow using all their root reserves and may even be stronger than the ones you simply trimmed back. The most effective methods for getting rid of them are to dig them out completely, alternatively spraying them with a herbicide containing glyphosate can work -- be sure to read and follow the instructions -- although it may take more than one application. Note that the herbicide must be absorbed through foliage, so they will have to regrow somewhat before you can spray them effectively. You will need to be persistent to do the job completely.
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