Lynn is right. However I would advocate planting food everywhere, no exception. People who eat organic, should be able to save enough seed from their organic produce to overgrow their entire city if they plant every seed they get. And that's the way it should be, you shouldn't be able to walk 20 feet without bumping in to another edible plant. There are food shortages in the world but we're not using every inch of soil to grow it? wtf?
Also boss, if you go and volunteer on some local farms, I guarantee the farmers will stack you with your favorite produce (whether it's veggies, fruit. meat or something else!), plus you'll learn a ton, meet great new people, get exercise, and really feel fulfilled. Plus you get to create, you get to have a real impact on other people's lives. The work you do will go to help sustain their health and it will literally become a part of them. That shouldn't be taken too lightly! I can send you the WWOOF list for florida in email if you want. If you're not familiar, it is a list of organic farms in your state that you can either work or volunteer at. It costs money to buy the membership from their site, so if you're on a budget I'd be happy to share it.
That's something I want to do when I come down this fall, is go visit some local miami farms.
However, as a point of practicality, like Lynn said, if you choose areas that are already pretty scrubby or places that aren't well groomed, your efforts are likely to remain instead of getting weeded the next time the landscaper or gardener comes through. Still, you can also ask any friends who eat organic to save you their seeds. Think of how many seeds you get out of one pepper, one strawberry, one tomato, one melon, sooooo many! And it's east to scratch up some dirt, drop a mix of seeds and cover it back over, next time it rains, your seeds will come to life and do what they do best! With that much seed, you should be able to plant even in tended areas, some plants will always get overlooked. And yes, I'd stick to organic seed only, planting GMO's is not good, and even planting over bred conventional seed is not good. Stick with organic for effectiveness, nutritiousness, taste, and resilience.