the underside of the leave. it has a silky grey underside.

This post was edited by penumbra59 on Mon, Jul 22, 13 at 17:06

And the branch itself

It looks like goat willow. Did it come by itself? Salix caprea is very common in damp areas and is a frequent self seeder.

Here is a link that might be useful: Salix caprea

This post was edited by florauk on Mon, Jul 22, 13 at 17:10

Yes it started to grow in between some seams in the concrete of my balcony.

I'll check it out, thank yo already for the advice!

It sure looks like goat willow. They make nice trees, even if they might be a little trashy sometimes. Important for bees and bumblebees because they flower very early in spring when there isn't much else in bloom