The surest way to kill it would be to dig it up. Failing that, buy a bottle of concentrated Round-up (or a generic version containing glyphosate), poison oak/brush killer (the active ingredient of the one I use is triclopyr), or a combination thereof. In my area, Lowes and Home Depot carry these; I've never looked at real garden centers or WM; agricultural supply places may also have them (but the only one I'm familiar with sells only its own brand). It must be the concentrate (usually 41%). Regular strength won't do. [This comes in large bottles, and isn't cheap. So if the price puts you off, talk to all the people you know who might be battling poison oak, or who have large yards or live in more rural areas. They can probably let you have a quarter-cup, which will last you forever.] To apply the concentrate, you can 1) put it in a cheap squeeze container (reused mustard dispenser, or the $1 version you can buy in a grocery's kitchen-supply aisle) or 2) apply it with a small paintbrush which you will never use for anything else. Then, on a nice warm day (at least 65*, and 70* would be better), cut off some of the branches with green leaves and immediately cover the surface of the cut branches (including the edges of the cut bark) with the concentrate. You don't want it to go into the soil: just on the plant. Depending on the plant's susceptibility, you may see it die within a week or so. If new growth appears, cut off another branch or two and repeat.

Can I just say Thank you!!!!! This conversation had me on a google search that lead to this: http://www.oardc.ohio-state.edu/weedguide/singlerecord.asp?id=1050 It looks like this is PERENNIAL SOWTHISTLE Other Names: corn sow thistle, creeping sow thistle, dindle, field milk thistle, field sow thistle, gutweed, hare's colewort, hare's lettuce, hare's palace, milk thistle, swine thistle, swinies, tree sow thistle. I probably never noticed it flower thinking it was a neighboring dandelion. If anyone else runs it this nightmare hopefully they can catch it quicker than I did. Thanks again!!!! These 2 suggestions on what it was gave me the right search string. Now off to find a way to destroy it! :)

I think that is one of the myriad members of the dandelion family. I have something similar that gets small yellow flowers that has been identified as a dandelion. Dig out the long tap root it will have and compost the leaves.

Not a doubt in my mind with the supporting photographic evidence that it is knotweed. I would have come to the exact same conclusion just based on your description - the rapid spring growth; the hollow, bamboo-like stems and even the coloring are all classic attributes of this pernicious weed.