Interesting that you are mentioning the knotweed right now. Susan, who works w/me in the garden, and I just were talking about it this past week. My stand of one species (a very large towering thing) was so well-behaved for years and years (like 10), and now is suddenly out of control. We are planning to cut away its entire perimeter and leave just a small bit where I originally planted it (and we are going to bag up all the remains for the garbage collector, as I am now suspicious of the "non-invasive" promise the plant came with). I suspect this moist year made it a little too happy. Is yours a tall kind with big bamboo-like stems? Flowers reddish in a week or two or three from now? Whichever it is, you can cut it back anytime you like (nothing will kill it, I suspect), including right now. But I think we have to both face the fact that if we don’t dig soon, we are going to have an all-persicaria-all-the-time bed on our hands. Susan and I are going to do that in another week or so I think. And probably every week hereafter for a lifetime.