Want more garden than your small urban allotment? Try what I did. Take a walk around your neighbourhood and look for the weediest or bleakest church/synagogue yard. Approach the pastor or rabbi and offer to take over yard maintenance chores!
Some congregations might even have a budget and pay you for your work, or at least, expenses. The church I work at is very poor, so I bear the expense of perennials, shrubs etc. However, they are classified as donations to the church, so I submit all receipts and am issued a statement by the parish for claiming it on my income tax.
I've been at it for two years now and it's been great.