Japanese Knotweed - Invasive Species

So...the saga of the backyard septic installation continues.  We were happy to see our grass growing in the new backyard fill, even though there was also a prevalence of weeds.  There was one plant that I had never seen before, which I hoped was some sort of nice bush I could use in my landscaping.  It sprouts with a reddish tinge, grows quickly, and has broad oval pointed leaves.

 Unfortunately, as a visiting friend warned us, this plant is Japanese knotweed, which is highly invasive.  Apparently there was a landslide where various contractors were getting fill dirt from, but was from a location highly infested with knotweed - a highly aggressive invasive species.  So, my options are now to use poisonous chemicals, cover with heavy black plastic for a number of YEARS, or cover with 5 FEET of new clean soil.  Pulling up from the roots can actually spread it worse, since root systems can be 45 feet long, and only a small root fragment can restart a colony.  Another option is to cut it down up to 6 times a season for 5 years or more, which is sometimes used for large stands of it, but it can regrow to full size within a month or two.

 I asked the Kodiak Water and Soil Conservation District to come and look at it, and they suggested they could target spray it with something, and only kill a little grass since the knotweed are not yet well established.  It might or might not fully address the problem.  What a pain!