Looks rather deceptively like Japanese knotweed but it would be helpful to see the plant growing in situ.

I think you really narrowed it down for me. I think it is some kind of knotweed. I'm going through all the species now and I still haven't found one with the bicolor leaves, but I will continue to look. I will also try to get and post a picture of the plants growing where I took the sample.

Those ocreas looks convincingly like persicaria/polygonum, do you see
flowers?, consider polygonum pensylvanicum. though many are similar

strange my post will be visible for other readers several hours from exact moment I posted it, something wrong with my account

Yes that is strange, because I just got the email notification of your post at 9:22 am EST, but your post says it was posted 9 hours ago. Hope that info will help you figure out what the issue is.

Thanks so much for your info - I did not see any flowers but I'll look again. I looked up polygonum pensylvanicum but the leaves on that are too narrow. However---based on your post, I google-imaged both persicaria and polygonum and got some images I hadn't gotten before. Some of which look exactly like my plant, except for the bicolor leaves. I did find one pic with leaves that are slightly bicolor. I will post the pix I found shortly. Thanks again