How can I eradicate knotweed?
It's an old invasion in my small, otherwise cultivated, South Philadelphia back yard, and as much as I dig or spray with systemic poison, it comes back. Advice?
Answer
Polygonum cuspidatum is the tall invasive weed commonly called knotweed. There is also a prostrate knotweed.
The non-chemical treatment calls for cutting the weed back at least three times during the growing season and mulching heavily. It will take several years to kill it.
Chemical treatment can irradicate it in a single season, but it often takes multiple years to fully accomplish killing it. The chemical of choice is a Glyphosate commonly marketed as Roundup. It is best applied in late summer or fall. Roundup is systemic and will kill the roots. Follow label directions. It is difficult to irradicate because is has a massive root structure.