Not sure where you live but here in UK we are supposed to report to the local council officers when we find this plant (knotweed) as it will spread along a whole road even through concrete in 2 years, you cant dig out all the roots as they break when you do and this means you leave little bits in the soil, I think your local officer will advise you what to use, IF it is knotweed. IF it is the dreaded plant, then your neighbours waont be happy IF it gets into their area. Go as quickly as you can to the offices in town and take a picture close up with you, make sure you photgraph the stems, root area and the leaf.
Best of luck. WeeNel.
Looks to me like if it is Japanese Knotweed, you could cook it and eat it, then possibly live longer to be frustrated by it that much longer! In all seriousness, maybe you could make marmalade from it and actually turn a profit? You know what they say about life giving you lemons...
Ohhh Noo... Better get some rest, trying to get rid of this is going to work you like a dog, good luck!
Hint: This weed laughs at roundup, and makes roundup look like a wimpy wuss, so use PURE Brush Killer.
Whatever you do, IF you cut this plnt down, have a good roaring fire going because the smallest bit of this plant will grow roots and a huge plant again, I would ask your local gov officer to help you out because IF you have it, be assured it is elsewhere in the neighbourhood, the plant spreads like you see when a camera is speeded up, it sends out underground suchers and wherever these pop up as they grow, a new colony is formed.
Seriously, if I were you, I would ask for profesional halp from the appropriate officer who knows the plant, knows how it grows and most important how to either contain it or get rid.
It's one of those plants that plant hunters Years ago, seen and liked, imported it and propogated it for there huge estate gardens and before we knew it, this plant had migrated to river banks, gardens, edge of woodlands and now it has colinated areas that will not be used for growing anything in for years as every year there are more new shoots found growing, so every part of this plant has to be burned and make sure there are no unburnt bits left, it is almost un-distructable.
Best of luck. WeeNel.
Cut it short, throw down hardy grass seeds, or better yet, roll out some grass over the cut stuff. Mow/cut the grass often when ever you see them poke out, and eventually it will push back to the areas you don't mow or weed whack. Those things freak the hell out of me. They grow so fast over night!