Seeing it's been 10 years since Vince posted, hoping all's well with them.
In the UK, what we might also call 'grassroots gardening' is being tolerated by some local authorities in some circumstances. I'm a big fan.
For example, Maxilla Gardens under the London Westway in the UK, supported by Grow2Know Community Interest Company:
"[I]n 2020, Tayshan Hayden-Smith [who lost friends in the infamous Grenfell Tower Disaster of 14 June 2017], along with BBC presenter and horticulturalist Danny Clarke and local agriculturalist Ali Yellop, formed Grow2Know, a not-for-profit based in North Kensington, aiming to make horticulture more inclusive by inspiring and educating young gardeners across disused spaces in London [UK].
"The organisation has since transformed green spaces in the local area – with Tayshan founding the Grenfell Garden of Peace too."
https://geographical.co.uk/news/how-grenfell-guerrilla-gardening-helped-rebuilt-community-resilience