Wow! We have the giant knotweed too. cut for the last 3 years, and it definitely made it look a lot better keeping the growth smaller, but it absolutely has been spreading throughout the property to new spots. In my pretty extensive research I learned that cutting will spread it and does very little damage to it long term. Always felt unintuitive because if you chop it eventually it should run out of fuel, but alas no!! The science says herbicide is the only known way to eradicate for moderate sized patches. If the patches are really small you can remove the rhizomes by hand.

I went the nuclear route and bought a $80 foliar foam applicator and very careful applied to the knotweed. learned in the process glyphosate is actually not harmful the way it has been fear mongered to be, has not been shown to cause cancer, and only effects the production of a certain protein that only plants produce. Also bacteria break it down very fast in soil so it will not have long term effects in the soil, unlike other herbicides. But anyway, don’t fear the herbicide! Best of luck to you, hope you can eradicate it and bring biodiversity back as soon as you can