Random Garden Tip:
Marigolds are the MVP of your garden. Aromatic and gorgeous, these plants likewise keep away hazardous bugs. But their greatest selling point is their effective roots. Marigold roots prevent nematodes that can fatally damage your garden and generally cleanse the soil. You go, Marigolds.
So then by August or September after flowering, the plant will be 3 feet high, and then you could spray treat it with glyphosate.
If it is in a sensitive site, you could either not cut it and spray it when it’s at the 6-foot height, which is generally what I recommend because you get the maximum intake of herbicide to really begin the dieback of the root system, which is necessary in order to maintain control. Or if it’s a very small infestation, you could actually inject the stems with glyphosate concentrate. Only five milliliters per stem will kill each stem.
I’ve been at sites here across Connecticut where there’s maybe 15 or 25 stalks of knotweed in a very small infestation. And after injection in the second year, there’s next to no knotweed left.