This is a rapidly growing plant; likely a gift from a passing bird, as it was not there last year. It's about 4 ft tall now. The first photo is the entire plant, second shows the new leaves growing in the top, and the third is a close up of one of the lower branches showing the older leaves.

The stem is much like the tree dahlia, D. imperialis but the leaves are different. They do grow quickly!

Look at Dahlia tenuicaulis ..

http://www.pacifichorticulture...

Dahlia excelsa .. I really must try one of these! I lost my D. imperialis over a very harsh winter, it never managed to open flower buds before frost got it.

http://www.slow-life.co.uk/tag...

You can click on the next pic of a tree dahlia to see the stem closer.

http://www.slow-life.co.uk/wp-...

I can't really see it growing that quickly from seed though. Did you happen to plant any tubers you forgot about?

Dahlia tubers do not survive our winters, so it's not a stray tuber, and I know for a fact that I did not plant anything in that spot. It's in a fairly shaded area, too, facing west on the north corner of the house.

I have a Dahlia specie which is very hardy, I grew a packet of mixed species seed years ago and most survived until we had an extremely cold winter 2010/11 with 7 weeks constantly below freezing down to -17C at least. The survivor has cut leaves with pink nodding flowers, it looks like D. australis. I have Bishop of Llandaff which survived too, and several self crosses between the species and Bishop's Children which I also grew from seed but the original BC have all but disappeared. The crosses set themselves around, some even grew from dropped seed after the freezing winter!

Dahlia species like some shade, they droop in hot weather but it doesn't seem likely that you have a dahlia. Notice how the two leaflets on one side nearest the stem in your last shot have an extra leaflet, almost like it's part of the main leaflet with that side reduced?

So, what has stems like a tree dahlia?

Maybe Fraxinus latifolia? Or another ash growing in the PNW?

http://science.halleyhosting.c...

http://www.wnps.org/landscapin...

It's close, but the leaves on this plant all have definite serrating where F. latifolia barely has any. The leaf bases are different too, notice how the leaves on this plant are mostly lop sided at the base.

https://davisla3.files.wordpre...

Elms have lop sided leaves but the venation is different and leaves are alternate.

What about?

Elderberry has a hollow stem, so yes it could be!

Sambucus nigra ssp. Canadensis is close ...

http://newfs.s3.amazonaws.com/...

http://www.florafinder.com/Spe...

Sambucus racemosa var. racemosa is even closer!

http://www.florafinder.com/Spe...

http://science.halleyhosting.c...

http://pnwplants.wsu.edu/Plant...

http://luirig.altervista.org/f...

And this photo from the database looks very like the photo I posted

Wonder if I can successfully move it, because it can't stay where it is. Kind of a nice looking plant, too.

The leaf bases on Sambucus nigra are narrow but yes, it looks very similar.

You could try to move it once the leaves have dropped, I wouldn't try it now.

If it blooms this year, that should make it easier to identify.
I'm surprised it has gotten so big in just this one season. Any one know anything about the root system? Tap root or shallow roots?

They make spreading roots ..

https://www.google.co.uk/searc...

And the stems...trunks are pithy. They can be quite a chore to saw through. I was thinking the plant reminded me of something. We just cut down an 8 foot tall Sambucus nigra ...absolutely a DUH moment.

http://www.rnr.lsu.edu/plantid...

There are native elderberries in this area, too, so there's a very good chance it is indeed an elderberry. (Thanks, birdies. They gave me a golden currant two or three years ago, too. )
Thanks for the help, all!