Posted: 23/09/2013 at 18:08
I agree it's concerning that it's not flowered yet if it's to be an annual. Even my most tortured annuals have managed something. I don't like to think I may have to keep it another year or so before I can know what it is though! Next year I shall not let weeds get my interest!
The stem is indeed very solid and bumpy lower down, which I assume is where roots would come out were they nearer the ground.
I thought it came from the bird seed, as this plant is near to a bird table and all around the base of the table there are what appear to be seedlings of it. I hoe them down frequently, but unfortunately this giant fellow got itself under a cloche and was quite large before I found it, so I don't know for sure it's the same, though it seems likely enough.
The leaves look right for the Himalayan balsam, so I shall have a look through other Impatiens and see if there's another giant that might fit the bill. If only it were Impatiens niamniamensis!