Many "woodland" and "wayside" plants can be found growing in the most obscure of places.
Given half a chance, and with enough moisture and nutrients they will grow pretty much anywhere.
Mortar joints in brick and stone walls - small holes and crevices in tree trunks - even old discarded pots and pans etc. etc.
You name it - if the seed can germinate - it will!
Obviously the nature and location of the growing medium may well have an effect on whether the plant can attain full healthy growth.
It might be worth mentioning that one of the main reasons that the flower / plant / tree forums were created here on UK Fungi was with a view to trying to get assistance with identifying plants /trees (the associated organism(s)) when some unidentified fungus has been found growing on or nearby the plant in question.
No doubt we will all try to help ID plants/trees wherever we can, but our knowledge of the subject might be a bit lacking - I know mine is.
We had a few people with superb botanical knowledge on the now defunct WAB forums - and it would be fantastic if one or two of them were to discover and look in now and again here on UK Fungi.
Regards,
Mike.