What is this odd looking plant?

I found a couple of these in my flower bed, which is still under construction. I have deliberately neglected weeding for a couple of weeks because I chucked some seeds down. Now I have no idea whether this should come out or not. The seeds I randomly chucked in were Purple Loosestrife, Pot Marigold, Cornflower and some big poppy things that grew last year. I don't think this is any of those. Any ideas?

 

It is a weed-can't remember it's name, I am plagued with the stuff in my garden.

 

Maybe Persicaria maculosa or the common name being Redshank

 

Is it one to rip out or one for the Round-Up bottle? I.e. does it put down deep and complex roots like bindweed that will produce two or three new plants for every one I pull out, or is it more of a Rosebay Willowherb type affair, where one pull will lift the lot out?

 

It's a member of the Knotweed family but it's a only a summer annual weed so like with Rosebay Willowherb as long as you rip it out before it sets seed you should be fine =]

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Thanks all. I'll rip it out tomorrow. I'm glad I don't have to risk the Round Up on it, as it is in close proximity to some of my actual chosen plants, although the Round-Up is still employed, as I still keep finding the odd bindweed coming up.

It's interesting about the seeds staying viable for 45 years. I've never seen this weed in my garden before, nor in the neighbour's gardens. At the back end of last year I dug the soil really deep to get to the Leylandii roots. I bet the seeds were lurking deep down in the soil for years waiting for me to dig deep enough to bring them back up to the surface.