Don't have a clue, this came up in some roses we got from California. I thought it was a weed and just never got around to pulling it out. It's pretty. Does anyone know what it is? It's got lots of lateral branches and is about 3 feet tall.
willow herb family.
dont let it seed!! 1 year seed = 7 years weed
glad to know that, it is pretty, but I'll not let it seed.
Rosebay Willowherb - Epilobium angustifolium.
The roots are rhizomatous even pulling it out will not get rid of this weed forever, it even withstands a certain amount of fire damage and as Mark says it will seed everywhere.
If left to it's own devices it will quickly take over a large area in a very short amount of years.