If it is shrub proportion but dies to the ground each year, and really is a giant herbaceous perennial (in other words, not woody, but with greenish stems with reddish bads around them), then I think it’s giant knotweed, Polygonum cuspidatum. Some people mistake its stems for a bamboo. Here is a link to this aggressive creature’s profile, with some links off it so maybe you can see if it looks like your guy: http://www.ecy.wa.gov/Programs/wq/plants/weeds/aqua015.html
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Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.