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.