Legal? – To plant trees/flowers in goverment areas

I doubt its legal, but you could probably get permission.

A better way to go about it is to plant native species and drought-resistant wildflowers in areas of your town that are neglected or abandoned. This is called guerilla gardening and its one of the fastest growing urban renewal movements. This way, your spreading nature through your city into the places that need it the most, and your not encroaching on land that is already set up as park space.
Guerrilla gardening is wholesome mischief. It breaks the law but improves public property. Because its wholesomeness is clearly apparent, some cities have started to institutionalize guerrilla gardening, with programs that invite local residents to “adopt” and plant traffic circles, boulevards, and other pieces of public propert.
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