Looks like a weed that invades my gardens. I seem to have to pull a lot of them.

Compare with Lythrum salicaria.

Thanks everyone, I pulled it out.

It doesn’t look like Lythrum salicaria based on my experience with that plant. The way the leaves join the stem doesn’t look quite right, and I have not ever seen the puckered look on purple loosestrife leaves.

I don’t have a different suggestion, however.