Well, no one's tried to answer this, so let me see what I can do. There are many plants known -- formally or informally -- as "Knotweed." Most (maybe all?) are in the Polygonaceae family. You're probably familiar with Common knotweed, Polygonum arenastrum [don't know if that Latin name is up-to-date: many plants formerly in the Polygonum genus are now classed as Persicarias], which can be flat as a pancake and is commonly found in sidewalk cracks and what are called "waste places." It's one tough plant, a real survivor, and was common in the Bay Area when I was growing up. (We have that or something similar here; I think I mostly see it on the side of the road near my mailbox.) http://www.ipm.ucdavis.edu/PMG/WEEDS/common_knotweed.html And you may know Japanese knotweed, Fallopia japonica, a shrub which is trying to take over most parts of the country, including here. http://nyis.info/index.php?action=invasive_detail&id=43 Many of the Persicarias may be referred to as "knotweed," just as they're often referred to generally as "smartweeds" or "lady's thumbs." There are also garden cultivars, such as the older Kiss-me-over-the-garden-gate and all sorts of modern varieties, which I gather are more common in Europe than in the U.S. Here are some examples: http://www.finegardening.com/bistort-persicaria-amplexicaulis https://www.rhs.org.uk/Plants/67494/Persicaria-affinis-Superba/Details http://www.paghat.com/persicaria.html http://www.bethchatto.co.uk/o-r/persicaria-polygonum/ So what is the flower in your photo? To me, it looks like it might be something classified as Persicaria, or some relative of the Persicarias -- but I can't say what it is.

Hi - we managed to get rid of knotweed completely - will tell you how I did it. I'm here because of a BBC article and got me remembering and thinking. I don't understand why everyone says this is so difficult to get rid of it seemed really easy to me anyway. 4 years ago we bought a flat - it had knotweed noted on the survey, it was pretty big area of huge tall weeds, but we haggled the price and bought for cash. Told we wouldn't get a mortgage because of the knotweed. Neighbours told us it was a nightmare they had battled for years and told us all the stories of council officials coming out to look at it and trying to apply for grant funding etc etc to get it dug out with JCB's and burned. Anyway, we decided ourselves to get an estimate from a professional company - priced in the £000's. I did some reading and research about the plant and controlling it. Went to b and q and bought a large quantity of the strongest glyphosate I could find, it said it could treat like 500m/sq. this patch was maybe 20 metres by 5 metres - pretty big for me anyway. It was supposed to be pretty diluted but I only used about 10% water maybe. I also bought a £15 manual spray pump and some gloves and some sponges. I went with some ladders on a Saturday morning in August, it was about 15 degrees C, cloud cover and dry. i coated every single leaf I could with the stuff - several times during the day and again after it dried off or was absorbed. I then found all the thickest stems and bunches and cut them down about halfway, and poured the stuff into the stems - filled right to the top. Then topped them off later that evening - and used the last of the strong spray solution I had made for another final spray coating. Over the next few weeks they starting visibly dying. The winter came and went and by July the following year everything was completely dead. I took plenty of before, during and after photos which I still have. That was 4 years ago I did that and not one single sign of it has returned. It cost me like £80 of materials and maybe 12 hours of my time. I knew nothing whatsoever about Japanese knotweed or plants and gardening and still don't. I was later told that national rail and the local council had actually sent 'a man' every season for 'several years' previous to our buying the place to 'spray the plants' to try and get rid of them. With little success apparently. What I find totally and utterly bizarre is that someone like me, completely clueless about horticulture and plants, was able to completely kill - permanently - a pretty substantial bank of this stuff in like 1 day, with stuff bought from the local hardware shop - and yet the Internet is full of stories and news reports of people spending fortunes and despairing in heartache over it. Was I lucky somehow? Genuinely I've always been curious about this and would welcome input from those more experienced.

It is not knotweed, this is pokeweed (Phytolacca americana).

So now you know how it spreads,seed & rhizome,especially the latter. My methods,never let it flower.Always cut close to ground level,just below a node,and spray/pour glyphosate mix into the cavity.Do that with every cane. Once done,you can mulch,heavily,though that will be in the way when the devil re-sprouts.....not if,but when. If you dig up roots,you MUST find every tiny offcut,lest a new plant form.You have to sequester the roots away from water,sunlight and any organic matter,they remain viable far too long.I used polythene barrels. Digging roots is nightmarish,but breaking the underground connexions speeds depletion of plant reserves.You can spray root ends in the ground if you can't remove at once,though the herbicide is claimed for foliar use.I am NOT saying that flooding root infested soil with herbicide will help! If you are a composter,the leaves(no seeds) and stalks,NO ROOTS,make good feedstuff. Hope this helps,and good luck.