The picture makes the weed look kinda spindly, but it's not. It's stem is a good nickel size on the tall ones.

OK...I finally discovered what it is while searching on Google. It is called Japanese Knotweed. Anyone familiar with it?

Yep, Japanese Knotweed. A lot of info/discussions on the Net about it - JK is a real problem.

It spreads by runner, cutting, and seed. Don't let cuttings lay on the ground...they will root. Cut off any flowering parts and bag them and put them in the trash. It's hard to get the runners as they go many feet and can come up through asphalt and house foundations. If small plants come up in your lawn, keep mowing them. Cut larger ones down and apply round-up to the stump with a paint brush or drizzle some into the hollow stem. This will be trans-located to the roots so give it a little time to work(10 days is the norm I think). Repeat all of the above as needed...this will be a long-term battle. Contact your state/county noxious weed control board to see if they can help you with more info.

The only way to kill Japanese Knotweed is with Roundup. Lots and lots of Roundup. Foliar spray early in the year as needed, then cut-and-paint the stem from July on when the plant is sucking nutrients to the ground.

You WILL succeed if you are diligent, but think in terms of years not months. JK will be your new hobby. After five years, I have reduced my invasion by about 95%.

I generally spray and/or cut and paint the stalk of each plant about 4 times per year. In my experience, it takes about five to ten applications of Roundup to completely kill established plants.

The Nature Conservancy has had more rapid success with using a needle to inject full-strength Roundup directly into the base of the stalk.

When spraying, use normal concentrations. When painting, use full strength concentrate, or at most cut 50/50 with water (from the full strength concentrate).

Don't compost it, don't leave any parts of the plant in contact with soil

Good luck! And sorry; this thing sucks some of the joy out of gardening because it takes so much time to kill that other things get neglected.

You can always put some round up in a balloon (Not filling like a water balloon, just a little, enough to fill the balloon without stretching it), cut the stem of the weed and put the balloon opening over the cut stem, It will stay on the stem and not go allover the yard. Works well. High constant dose!

I thought that's what it was when I saw the pic...I have it in my back yard behind my garage. I hate it!! The stems are hollow and if the plant is big enough, I cut it about 2-3" from the ground and use a large medicine syringe to put Round-Up down inside the stalk. That's what the guy at the local garden center told me to do. Works.