Originally Posted by beccabunga

It is a Fallopia [also known as Polygonum]. If it stop growing at the height in your photograph and produces red pokers, it is likely to be Polygonum amplexicaulis, which is a thug but relatively decorative and can be dealt with.

If it continues to grow to about 6 feet and has white flowers you have the dreaded Japanese knotweed.

If the former, you can dig it out relatively easily. If the latter, you have a problem.