First of all absolutely DO NOT let this plant grow tall and flower and have time to send its rhizomes to the center of the earth. With new shoots use Ortho Total Kill Lawn Weed Killer. It kills knotweed and many other weeds but does not kill your lawn. I've used it for the same purpose and it really works great. It actually lists knotweed in the list of weeds it kills. Wait until new shoots grow to about 3 or so inches tall with some leaf growth and then spray. The next day you will already see the knotweeed browning and curling over in its sure death. Also, be careful Ortho Total Kill comes in two varieties one that is safe for lawns and one that isnt. Look at Home Depot for the green bottle, not the blue one, if you do not want to kill your lawn. Also I would suggest buying the 32 oz. concentrate and making your own mix. Mix a tiny bit heavier to really feel the satisfaction of knocking this plant back to Japan from where it first came. Bye bye knotweed.

Allelopathy is not the main issue with Japanese knotweed. This is a plant which can grow 6 to 8 feet tall, root down twice that depth and spread to cover acres of allowed. It is the number 1 worst possible invasive species in th UK. I would stop even trying to grow vegetables until the JK has been totally eradicated. If necessary dig up the entire garden! I had it in my woods and it took 3 years to remove by a professional company applying industrial herbicides 3x per year. Unfortunately if it's coming from the neighbour's you are pretty much scuppered unless they control theirs too. This stuff is so awful that it affects property prices over here and there are criminal charges brought against those transporting or dumping it.

floral_uk z.8/9 SW UK 2 days ago Have you fixed the price? Over here JK could be grounds for a reduction. Could be? Could be? Based on my reading, that is the understatement of the year. https://slate.com/technology/2019/05/japanese-knotweed-invasive-plants.html

wonderful ... except that you already broke it off .. yikes.. be a bit more patient .. its only the second day of spring .. in the future... let us know in every post.. where you are.. big city name .... e.g. i think of knotweed as a warm soil plant .. you dont seem to notice it till later in spring .. ergo.. i wouldn expect to see it in my MI on mid march ... welcome to the forums.. come back often ... ken ps: many of us recommend not doing much the first summer in a new garden ... just observe and plan .... lets see what he prior owners let youl.. good or bad ... killing knotweed would be approved ... lol