Japanese knotweed, Polygonum cuspidatum (syn. Fallopia japonica), also sometimes revered to as Mexican bamboo. Why, I have no idea as it's not from Mexico and its not bamboo.......or any other grass-like species.

Usually considered one of the most pernicious invasive weeds around. I am surprised they seem to have this one under semi-control. Not even close to possible in my location!!

Here is a link that might be useful: Japanese knotweed

Is someone growing that on purpose? Its bad, not as bad as running bamboo but still not a good thing to see. It escapes into the wild and does major havoc on streams and waterways.

Altho' it doesn't look like it, if it's Japanese knotweed, hope you never see it in your garden--it's an invasive species. If you Google your state + invasives, chances are it's listed. It's a plague here in the northeast. I tried burning it, whacking it and a few other ideas before giving up.

Good luck.

Here is a link that might be useful: Japanese knotweed

The leaves and flowers look like it but I wonder it they stripped the lower parts of the stems to resemble bamboo? That part looks different.

Is someone growing that on purpose? Its bad,

This particular instance of this plant is not invasive as the homeowner did say that it has been in this contained clump for many years. Having said that I will let the owner know of the potential unconstrained spread of this plant if successive winters are too moderate.

not as bad as running bamboo but still not a good thing to see.

It is rare that bamboo ever becomes invasive in a zone which includes a real winter as it will die back completely with the associated cold of say...a zone 4. In fact it may well be difficult to keep it alive..

Oh, no no! I didn't mean to say that bamboo would be a problem growing there, just bamboo in the more general sense because you had compared it to that.

Good luck identifying the plant.

This post was edited by GreatPlains1 on Sun, Sep 8, 13 at 23:12

Japanese knotweed is a big invasive plant here in NH and VT! It is now known to spread by seeds; persistent and extremely difficult to get rid of. From one small patch north along the brook when I bought my property 13 years ago, it now covers the brook banks for miles to the Connecticut river and is now found in large inland areas.

I was curious, clumping means no knotweed. I googled a little bit, the names are confusing but there seems to be a selection of P. Cuspidatum with no runners.
Well lets see, what others may add, fascinating. Bye, Lin

Here is a link that might be useful: Garden Polygonum

More related discussion here:

Here is a link that might be useful: Polygonum Cuspidatum 'Crimson Beauty'

Persicaria 'Crimson Beauty' looks a lot like Japanese knotweed, but it remains in a clump, never sends out runners and never drops fertile seed. I've had one for over 10 years--in sandy soil where everything that can run or seed does. And it's coming into its glorious cherry red bloom right now. A great, great, plant.