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?
