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.
Consider the Halesia, the Syringa reticulata, or the Tilia tomentosa, three underused and beautiful trees. Learn more about them
Annuals, those plants that last only a summer, are great for tree pits because you can refresh the space every year. A few suggestions for sunny plots are Portulaca, Tagetes patula, and Osteospermum. In the shade, Semperflorens begonias, Coleus, and Torenia work well. Find out more about these annual flowers
Some argue that ground cover and flowering annuals compete with tree roots for water. But their presence reminds you to water, and, more importantly, green plants and flowers signal that someone cares for this space. Something pretty discourages all but the most ruthless bike parkers and trash tossers.
It's also important to avoid having bare soil, which dog owners see as fair game. (Interestingly, professional dog walkers are much more careful about protecting plantings in sidewalks or parks than many dog owners.)
This is a place to use thuggish spreaders you might not want in your garden—Houttunyia and Aegopodium, also known as bishop's weed, both have dazzling variegated foliage. (If anyone ever offers you a little plant of either of these, run.) Another spreader, Liriope spicata, is less aggressive.
Stay away from ivy, often recommended; it's the ground cover favored by city rodents, who like to scuttle along under it. Click
When you've made your plant choices, tuck them in with 3 inches of mulch. As the mulch breaks down, it makes the soil less compact.
Water immediately. In fact, water for the rest of the summer. Unless it has rained more than an inch, a young tree needs 10 to 15 gallons a week, delivered slowly. City spaces lose water fast. Your plants and soil are dried out by reflected heat from sidewalks and buildings. Tall buildings create a rain shadow, which means a lot less rain falls on the plants in your little plot than would onto plants in an open meadow.
So you have mulched and watered and carefully designed a lovely little selection of appropriate flowers, and then some oaf stomps on it, or a vigilante gardener comes at night and adds a wilted poinsettia to your composition.
Accept that to garden in a city is to garden in an unpredictable and extreme environment. Speaking of extreme environments, garden-making in Greenland is said by gardeners there to require tamaviaartumik, Greenlandic for passion, ambition, and commitment.