I just bought a house on Cape Cod this winter and have a mystery plant coming up that I'd love to identify. I've attached a photo. It's obviously a perennial. The plants are appearing in multiple places around the garden, in both sunny and very shady places. The first one to pop up appeared a couple of weeks ago (mid-April) and it is already about 2 feet tall. The soil here is mostly sand, by the way.

I'd really appreciate help in identifying it!

Compare to:
Virginia Knotweed (Persicaria virginiana)

Purpleinopp, thank you very much for answering my question. I thank you because even though your identification was incorrect, it was close. It led me along the right path and enabled me to find the correct answer. Very unfortunately, the plant is Japanese Knotweed, an invasive species that is evidently very hard to get rid of due to its thick web of roots - rhizomes. A speck of the root that breaks off is likely to start another plant. Now I have to deal with getting rid of this. I believe this came into my yard because if work that was done to install a new septic leach field. This required that an excavation company come in and dredge out 3/4 of my small yard, and because there was no place to put the soil, they took it offsite and then when the work is done they brought it back. It seems likely that some pieces of the root got mixed into my soil.

Yes, Japanese Knotweed (Reynoutria japonica) is apparently an invasive, plant in many areas.

https://www.massaudubon.org/le...

Happy to suggest but so sorry to hear about what happened.

I learned a lot once I realized what the plant was. I confirmed my identification by taking a piece of it to a local garden center. For anyone reading this, a couple of the key things I found out are that the plant's root system can go down several feet and spreads horizontally. It's very tough and can grown in sun or shade and the poorest soil. It can get into cracks in foundations of buildings and ruin the foundation. It can easily destroy a driveway. In the UK, people who have this stuff on their property can be denied a mortgage and are likely to have trouble selling the property. Here are a few ways to get rid of it: 1) dig it out (but very thoroughly because tiny pieces of the rhizome will regenerate the plant, and can lay dormant for 20 years and then spring up again. 2) cover it tightly with black plastic and let the plastic cook it in the sun until it dies. 3) Inject the stalks with pesticide. 4) spray the plant with pesticide. Both 3 and 4 are not one-time operations, and may take several years. After consulting with the garden center, I decided on a combination of 3 and 4.

We have a herd of local goats that are commissioned to clear properties of this plant:
http://www.cbc.ca/news/canada/...
They also go after the roots.

I forgot about the goats - I read that people rent out their goats for this purpose!

I also forgot another viable solution, to bring in a particular insect that eats the plant. I forget it's name. Evidently researchers have not found any problems to other species by going the insect route.