It’s a simple question with a complex answer. When I was younger the 1000 acres behind the house and the 2000 across the road answered that question. Today it is where ever I can. Indeed, a woman at the local health food store that I’ve been trying to get interested in foraging recently said “there’s no place to forage.” She is wrong, but more right every year.
To be utterly frank, I forage where ever I find the plants and (that “and” is very important) and I think the soil and water are wholesome. This means yards, parks, fields, vacant lots, cemeteries, bike trails, the back of businesses, residential neighborhoods, abandoned citrus groves, working citrus groves and local woods. This is also a tad iffy since this state says you can’t take plants from public land and everything that is not private is public. However, that usually applies to plants they don’t want removed whereas I am usually taking plants they want to get rid of. No one loves weeds but me and thee. I also forage in lakes, ponds, rivers, the inner coastal waterway and seaside.
The truth of the matter is I spy the edible plant first and then I ponder where it is. If it is a pindo palm on a lawn I ask the homeowner if I can have the fruit and if he or she puts down pesticides. If it is a body of water I examine the quality of the water. A spring and a ditch are quite different. If it is a field it depends where. The less witnesses the better.
It might be more instructive to say where I don’t forage. I try to avoid any land or water that has parking lot run off. I don’t forage down hill from the interstate or a heavily traveled road. Uphill I might is it were far enough away. Avoid anyplace near a garage. And the absolutely worst place to forage is railroad lines, even old ones. Railroads are among the most toxic property in the United States, and among the longest contaminated. Folks have died from chemicals put down around the tracks to hold the weeds down. What you can do is collect seeds there and plant them to get a clean generation.
And whenever a police officer stops me I just say “I’m looking at the flora and fauna.” That usually creates a blank stare to which I add “I know something about edible plants and I’m looking for something to eat.” By then they usually decide I am not any threat to anyone except myself and they leave me alone.