PlantFiles: Giant Knotweed
Fallopia sachalinensis

Look like a very massive version of Fallopia japonica - this will behave itself for 4 to 5 years - after that it will start becoming more aggressive, sending rhizomes deep in the ground going horizontally about one foot deep, 5 to 10 feet away from the parent plant. Pulling the suckers didn't help as it detach easily from the rhizome and quickly grows a new shoot from the 1/2 to one inch thick rhizome. Even trying to compost the rhizome and it will try to spread.

In zone 4 the seeds won't germinate for me. That's a good thing. I don't understand why but a few species which normally seed themselves in warmer zones have seeds that are more vulnerable to winter cold than the parent plant that the seeds form.

Grows quickly vertically, but spreads very slowly compared to Japanese, Bohemian, or Himalayan knotweed.

When I say "slowly", I mean that the average person would be alarmed by the speed of the spread, but for the invasive plant lover, it's not that big of a deal.

This plant will eventually get large fantastic looking leaves about the size of a keyboard, with stalks about to 18' high.

It DOES have perfect flowers though, so unwanted seedlings might be a problem, thus being said, if you are the lazy type, you probably don't want to plant this in your yard for obvious reasons.

Extremely invasive, kills everything else, almost impossible to eradicate