That looks like Japanese knotweed, a highly invasive and difficult-to-eradicate plant. Read about it here: https://extension.psu.edu/japanese-knotweed. 

I've had success with digging out as much as I could, then systematically removing any new growth, sometimes on a daily basis, to starve the roots. It still comes up in one place after 10 years of this, but it's 99% reduced.

6”? JK would be 6” .... at least. I’m thinking some other Polygonaceae such as Fallopia convolvulus or Buckwheat.

Hmm, good point. And I hope for the OP's sake that you're correct!

Fallopia scandens. The leaf base doesn't match Reynoutria japonica.

It could be F. convolvulus also.

We will need to see it with flowers and seeds to make a species ID.

Your pessimistic views are obstructing our work here Ken. We can do without your doom prophesies. It's Name That Plant, not Kill That Weed.

What Ken said.

I had Japanese Knotwood on one side of my yard for years. I didn’t know what it was for a long time until I took a piece to a knowledgeable landscaper. They identified it as JK and told me it was very invasive and powerful enough to crack house foundations. I always thought it was pretty, dark red with pretty pink flowers. When I found out about it’s invasivness, I started digging it out……that was virtually impossible. So deep and tough are the roots, it could bend the spade or shovel. Little by little, year after year I dug more out. Eventually it died back to one small area and then just dies off. All this to say, it did not look like that plant above. Perhaps there are several varieties, I don’t know.

debra

Check out: countrylife.co.uk

I remember pink flowers, but it was a long time ago.

debra

OP’s plant is not Japanese Knotweed, of any variety.

This forum is not the possession of a single individual that thinks they know more than anyone else and assumes they can monitor what opinions others may share. ken's opinions are held and shared by many others. Weed ID is important but not necessarily critical and rarely ever requires going much further than genus. I rarely bother ID'ing any weed in my garden.....if I didn't plant it, it doesn't belong and out it comes. Especially anything in the Polygonaceae!!

I grew JK for several years, and it never spread? I dug it out because it's not native. Any potential vollunteers of Fallopia can easily be dealt with simply by pulling or scuffling when they are newly emerged. There is an abundance of online info about the aggressive and invasive nature of plants simply by googling the species name. In my other plant group comments must pertain to the identification of the plant. You can say an ID including the word invasive at the beginning or end like 'invasive Reynoutria japonica ' or 'Reynoutria japonica, invasive.' but you cannot elaborate, and comments about whether the identified species is edible or toxic aren't allowed. The OP is expected to google the name for themselves. I can definitely see why they have those rules! Of course discussion is allowed here, that's why some gw members will post here, and also on the Facebook ID group, so they can have a better chance of getting the correct ID (from 1 group or the other), and still be able to chit chat. Fallopia scandens has attractive flowers, and many people grow it purposely for that reason.

"Any potential vollunteers of Fallopia can easily be dealt with simply by pulling or scuffling when they are newly emerged."

Well, that is patently not true!! Invasive knotweed identification and control.

Because many of us first came here when it was Gardenweb, or even before that, I personally tend to regard this as a gardening forum, rather than a pure botany forum. Therefore, imo, in addition to an accurate id, which enables further research, I believe comments about a plant’s habits, uses and attributes are perfectly fine, whether one agrees with them or not. This ’chit chat’ can provide valuable information for the poster. You can always argue amiably against an opinion if you disagree.

Btw, I had to bring in professionals using chemicals unavailable to the general public to get the JK out of my woodland. It took three applications per year for three years. Volunteers were definitely not ’easily dealt with’ and are still appearing years later. However, The OP doesn’t have JK.

Hi, I just wanted to pop in and say that as a novice gardener who has much to learn about identifying plants (especially invasives as I live next to a woodland), I really appreciate comments about the habits and origins of plants. That info is as valuable to me as the identity of unfamiliar plants. I'd also add that I agree that once a poster has been provided with an ID, doing further research on their own is an important next step.

And, while I'm here, I'd like to thank all of the people on this forum who take the time to share their knowledge of plants with those of us who are not as knowledgeable. I hope that one day, I will have enough gardening knowledge and experience to be able to contribute more here than simply asking questions. :)