Answer: Based on your description I think you might be seeing Japanese knotweed (Polygonum cuspidatum) a truly invasive and difficult to control weed. The stalks on this plant are definitely reminiscent of bamboo. Here is a photo and some control information.
If it was a flower garden before, you might have inherited Polygonum orientale, Kiss me over the garden gate, and old fashioned but also very aggressive plant -- it is related to the above. Here is a photo and description. You may need to cut and paste the complete url into your browser to make it work correctly.
If neither of these seem right, you might want to consult with your local county extension and/or experienced professionally trained nurseryman to see if they can identify it for you.
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