This arrived in a pot on my condo balcony last
year, in which a clematis had been planted the previous year.
It is now well over 6-ft high and quite bushy.
I would love to have it identified
Thank you for posting your question to the Ask a Master Gardener website. Since your photo was first sent in (via the TBG) quite a few of us have had a look at it, but no one is completely positive what it is. I presume the photo is from fairly early in the spring. I haven’t found an exact match to the visible characteristics, but the little catkins suggest that you have a tree or bush, rather than a flower or vine — and indeed you say that it has become bushy now. The leaves could be those of a young willow tree. (Certain willows are the source of pussy willow catkins early in the spring.) Are there any trees close to the balcony that might have sent over a seed? There are other possibilities, but none of them are likely to be something you particularly want to grow on your condo balcony. So unless something unexpected happens soon — like it blooms! — I would remove it before it gets even larger. If you really like it as a privacy screen, for instance, then you can keep it pruned down to a shape you like and a manageable size. If you want to tell us a bit more about what it has done, and send in a recent picture or two, one of the whole plant, and another large close-up of the stem with leaves, we can do some more checking.