It grows between paving stones in The Netherlands. Under the paving stones is
sand, all the way down.

The tallest hollyhock I ever saw (14 feet and covered in blooms) was
also growing "between" paving slabs. On lifting the slabs (I had to in
the end), I found the roots travelling at least that far in all
directions without any contact with soil. The plant seemed to be
thriving on top of a sand/hardcore base, but I've never understood how.
Seems some plants don't need any nutrition at all.