Many gardeners may not realize there is a difference between the terms "green roof" and "roof garden." While the green roof is typically lightweight with a shallow soil depth---generally accommodating only shallow rooted plants, the roof garden is a more diverse space used as an additional recreation area or a garden in an urban environment. It can even incorporate small trees in containers with deeper soil.
A number of fruit trees are especially adapted to container growing and do well on roof tops. Citrus trees in particular like the full sun available there and will provide lovely, fragrant flowers as well as fruits. Many are evergreen. The Meyer Lemon (Citris x meyeri) is esteemed by gourmets, is fairly hardy and. at 6 to 12 feet. is a perfect container tree. For extra hardiness. try the Yuzu (Citris junos). It is a rare and prized lemon, tolerant of temperatures to 10 degrees Fahrenheit.
Limes need warmth, so grow these in frost-free areas. The Key lime (Citrus aurantifolia) produces small, delicious fruits famous as the signature ingredient in key lime pies (see Resources), but if you are interested in Asian cuisine (or want something a bit more different) try the bumpy fruited Kaffir, or wild lime (Citrus hystrix). It is the only citrus whose glossy green leaves are regularly used in cooking.
For lovers of the exotic in zones 10 and 11, the Buddha's Hand (Citris medica var. sarcodactylis) is truly bizarre. It looks like a lemon octopus and has no pulp at all---only rind---but is said to make delicious candied citron. Calamondin (Citris mitis), blood oranges (Citris sinensis) and tangerines (Citris reticulata ) are other interesting varieties to grow.
With thin sweet rinds and sour flesh, kumquats are the "sweet-tarts" of the fruit world. Nagami Kumquat (Fortunella margartita) is the most commonly grown in the United States and is cold hardy. It is a close relative of citrus. Bananas (Musa species) have interesting dwarf varieties to give a tropical look to your roof top garden; or grow a winter hardy quince (Cydonia oblonga).
For early spring color and lovely heart shaped leaves, choose a redbud (Cercis canadensis). The variety "Silver Cloud" is smaller (12 feet) than other redbuds, and has pink and white variegated leaves. Another flowering tree, with very early bloom, is serviceberry (Amelanchier species also called June berry). In addition to white flowers when little else is blooming, it produces tasty berries in June. You will probably never get to taste them, however, as birds love them Of the 30-plus species, only two are not natives of North America. Downy serviceberry (A. arborea) or Saskatoon serviceberry (A. alnifolia) are best bets for containers and roof gardens.
The top place on this list has to be the Japanese lace-leaf maple (Acer palmatum). Purple/red summer foliage and unusual shape make it quite dramatic in every season, and it seldom grows above 8 feet tall. The Bay laurel (Laurus nobilis) is a striking culinary trees that does well in pots, and the common weeping laburnum (Laburnum alpinum var. Pendulum) is exceptionally ornamental, but all parts are poisonous. The dwarf chestnut (Aesculus pavia Koehnei) is another beautiful ornamental tree for small spaces.