Ever hear of community gardens? They have that in my city, Detroit. Gardens at which everyone eats, for free, in other words. Homeless people can feed themselves that way too. A couple of things though, especially for homeless people. You shouldn't use "natural" fertilizers when you do this, because of the risk of certain diseases. And you should have a faucet nearby in any event, so they can wash the dirt off before they eat it.
Uh, unless the homeless like raw food or have access to a kitchen, a garden won't help much. Food banks can be a better option, especially if they can provide precooked food. Also the homeless are often forced to move on, making it hard to maintain a garden.