Perhaps you're looking for a truly terrible place to garden. The awful place I'm speaking of is what used to be called a "tree pit." Now urban foresters prefer the more civilized term—"tree bed." These are the small spaces between sidewalk and curb that very often contain a dead tree with yellowing weeds around it.
There are a few good things about being a tree pit, or tree bed, gardener. It is a place to garden if you have no garden. A tree in your bed creates shade. Even without a tree, a bed of plants and flowers does a bit to mitigate city heat. Your cultivated bed absorbs storm water, keeping it from running off bare, hard soil and overwhelming your community's sewer system. You slightly increase your property values, and you cheer up many neighbors and passers-by. It's the ultimate gardening mitzvah.
If you're in a ho-hum suburb, you make your street less boring. If you're in a messy, careless city, you create a space that is beautiful and cared for. (Check with the local authorities; you can't put up a stockade fence or barbed wire.)
As with many good deeds, the initial process is not easy. You have to improve the soil, which is almost certainly full of weeds and trash and worse. The earth in the bed is probably compacted, hard for tree and plant roots to penetrate. (A nice comparison comes from an article by James Urban in the April issue of Arborist News. Think of stepping on a bag of popcorn and squashing the fluffy part of the kernels, Urban suggests. City soil, much stepped-on and sun-baked, has lost its air spaces.)
As you get started, wear cotton gloves you can throw away on top of disposable surgical gloves. Have a big garbage bag handy for rocks and litter, along with small plastic bags for plucking out various revolting things. Use a small fork to loosen the soil gently. When the bad stuff is bagged, fold in good stuff—compost or aged cow manure, not fertilizer.
If your tree bed has a tree, be very careful working around its roots. People are often surprised to hear that tree roots don't grow straight down. Most roots grow out in a shape like the base of a goblet. Because the oxygen and water and nutritious organic matter are in the soil's upper layer, few roots go deeper than 3 feet.
Be careful that the soil level doesn't rise above the tree's flare, the place where the roots meet the trunk. A common and disastrous practice is to build a cement or wooden enclosure around the tree bed and fill it with soil, covering up the bottom few feet of the tree trunk. This may look very sweet with pink impatiens planted in it, but stifled under soil, the trunk's bark will rot, and the tree will die.
The next stage in the process may well be difficult emotionally. The plants that you purchase and place out in the cold, hard world will be peed on, mostly by dogs. People, as well as dogs, may step on them. Drivers learning to parallel park may run a wheel over them. (I have seen this.) Delivery vans are likely to rip branches off any little tree you plant.
The positive way to think about this is that not only are you learning about plants, you're getting a course in urban anthropology and human psychology.
So what innocent, trusting, blameless green things should you put out there in harm's way?
There are, if you're in the East or Midwest, already too many London plane trees, Norway maples, and Callery pears. Here is an opportunity to increase your town's tree diversity.