This is not a bamboo. It is mistakenly called Mexican bamboo but is not. It is actually a noxious weed... Here is a posting on it from another inquiry....Mzkrista
Posted by gardengal48 PNW zone 8 (My Page) on Tue, Jun 13, 06 at 23:02

I'm suprised no one has posted a bit more emphatically about this plant. It is on the noxious weed list of nearly every state in the nation as well as many foreign countries. It is extremely adaptable to a variety of conditions and has naturalized rampantly in some ares. In my state, erradication, if growing on your property, is a requirement.
There are supposedly non-invasive cultivars of Polygonum cuspidatum, but I wouldn't like to test that theory:-) Most members of this family - that includes persicaria, fallopia, tobira as well as polygonum - have very aggressive growth habits, although few that I know of that are quite so incredibly rampant and robust as the Japanese knotweed.

Your reference to the bamboo forum is not all that unusual - another common name for this plant is Mexican bamboo. The hollow, jointed canes do resemble those of bamboo and many bamboos can spread just as aggressively as the knotweed. And they are not any harder to remove from unwanted locatons :-)

In short, get rid of this plant or encourage your mother or her friend to do so. It's a menace!

Here is a link that might be useful: http://www.ecy.wa.gov/programs/wq/plants/weeds/aqua015.html

MZkrista,
I think I will gladly keep my specimen until I find something wrong with this plant. I decided to try this mexican bamboo after reading an article from Fine Gardening, Sept/Oct 2001, pg 40.
"'Mexican bamboo is related to Japanese knotweed, a plant that's become a troublesome weed in many areas of the country. But don't be afraid of 'Crimson Beauty'- this awesome cultivar doesn't run and hasn't generated so much as a single seedling after five years in my garden. And it's no fluke; I've seen this gem at the New York Botanical Garden. where one of the gardeners told me it hadn't displayed even a hint of claim-jumping. There, the plant stood 5 or so feet because the gardeners cut it back in late spring. In my own garden, spared from shears, 'Crimson Beauty' soars to about 12' in heightand is nearly as wide. It thrives in full sun in regular soil and appears to have a cast-iron constitution. I've yet to note any significant pest or diseases, but Japnaese beetles do chew a few of its leaves gagged." (This article was written by Steve Silk from the issue I mentioned above.)

Yes, cut it to the ground before new growth starts.

I believe the P cuspidatum that I have growing is 'Crimson Beauty' and despite the reputation of the species as a dreadful pest, this particular cultivar is not a problem. I rescued a start from what was once my great-grandmother's garden and in seventy years or more it had only spread across an area maybe ten feet in diameter.

To answer your question, yes, do cut down last year's growth. The stems are rather woody and may take quite some time to rot down on their own. I usually wait until early spring, because I like the form against the winter landscape.

Highlander,
Thank you for confirming my research! And for the cut-back information. By the way...the photo in the Fine Gardening magazine of the Crimson Beauty in the fall was so awesome, that I had to have that plant!

'Crimson Beauty' is indeed a very impressive plant. Best of luck with it.

I bought this plant as "Fallopia" 'Crimson Beauty'. We grew it at the NY Botanical Garden where I volunteer in the perennial garden. It never self-sowed there. But I'm sure we would cut it down once it's flowers were gone.
In my garden, where I let it languish into seeds, I've had quite a few seedlings. They can be hard to pull out once they've become established.
I have other Fallopia that have never run or self-sowed.