Hardy to Zone 5, Kousa dogwood (Cornus kousa) stays small for a tree, with some varieties maxing out at just about 30 feet, making it an excellent foundation planting for a rooftop garden. In spring, the tree has small but beautiful white to pink flowers, which transition to deep green summertime foliage. Autumn offers its own benefits, with red-purple leaves and fruit (which starts to appear in August).
Harry Lauder's Walking Stick
A deciduous shrub with a funky common name -- Harry Lauder's walking stick -- Corylus avellana 'Contorta' twists itself into seemingly unfathomable shapes that are good sculptural plant accents to a rooftop garden. The heart-shape foliage on the shrub or small tree, which can reach 10 feet tall and is hardy in Zones 4-8, is interesting, too.
Note: This plant can be invasive in some areas; check local restrictions before planting it.
'Blue Star' Juniper
Also called singleseed juniper, Juniperus squamata 'Blue Star' shines with skinny needles that have a definite silvery-blue cast. Hardy in Zones 4-8, the dwarf juniper -- which puts out mounds of needles on a shrub that reaches about 3 feet tall -- is a good accent or container variety for rooftop gardens.
Dwarf Hinoki Cypress
Fanlike foliage and a yellowish-green color are two standout characteristics of golden dwarf hinoki cypress (Chamaecyparis obtusa 'Nana Lutea'). Extremely slow-growing, the evergreen will reach only 3 feet tall at its mature height. It needs a regular watering schedule but is otherwise wonderfully low-care. Hardy in Zones 5-9, the evergreen brightens corners on a rooftop garden or works well with other plants in container combinations.
Great for shadier spots on a rooftop, Astilbe 'Fanal' rewards gardeners with rich crimson color that blooms midsummer (typically July). A good perennial plant for the middle of the border, astilbe shoots up stalks of about a foot-and-a-half and is hardy in Zones 3-8. It does best in moist soil; don't allow astilbe to dry out.
'Big Daddy' Hosta
Integrating impressively sized hosta into shadier spots on a rooftop garden is a terrific way to add beautiful foliage to the space. Hosta 'Big Daddy', hardy in Zones 3-8, is just one choice; its blue-green leaves offer rich, deep color and are big enough to cut as foliage for bouquets -- perfect for multitasking rooftop gardening plants.
A groundcover that is attractive to butterflies and hummingbirds, Ajuga reptans, hardy in Zones 3-9, needs water on rooftops but rewards with steady growth. 'Silver Beauty' (pictured) is one good choice, as is 'Bronze Beauty', which has deep blue flowers and bronze-tinged foliage.
Beloved for scent, lavender (Lavandula angustifolia) is a member of the mint family and native of the Mediterranean, so it weathers the windy and dry conditions of a rooftop garden well. Its beautiful, gray-green foliage reaches heights of about a foot and a half, making it a good fit for either containers or full-fledged rooftop garden beds. Zones 5-9
Most daylilies are fairly hardy, and their consistent blooms mean that they offer rooftop gardeners a good way to add color. Try Hemerocallis 'Anna Warner', which has pinkish-lavender blooms and does well in full sun. Strappy foliage reaches about 2 feet in height, and the plant thrives in a huge Zone range -- 3-9 -- so you can enjoy it just about anywhere.
A nice fit for low-growing borders or the front of flower beds, Stachys byzantina, or lamb's-ears, has a distinctive soft, feathery feel to the silver-gray foliage. In the ground it tends to spread, so it may be best for containers on rooftop gardens. If the purplish blooms get leggy in summertime, cut them back. Zones 4-10
With fragrant blooms that hang like grapes, showy wisteria is a much-admired woody vine. Wisteria 'Blue Moon' rewards even more with a trio of bloom times in the growing season; the plant is hardy in Zones 4-9. Its rambling vines are a great accent over a rooftop garden trellis or pergola.
Note: Wisteria can be invasive in some areas; check local restrictions before planting it. Parts of this plant are also poisonous.
'Gold Flame' Honeysuckle
A cascade of 'Gold Flame' honeysuckle (Lonicera x heckrottii 'Gold Flame') is a great way to attract hummingbirds and butterflies to a rooftop garden. It begins blooming in late spring, with delicate pale pink to deep magenta blossoms that are incredibly fragrant. Hardy in Zones 5-8, honeysuckle vine reaches about 20 feet in length, making it a good fit for a trellis or above a rooftop porch structure.
Creamy white blooms decorate the vines of this self-adhering plant, which can reach an astonishing 60 feet long. However, climbing hydrangea (Hydrangea petiolaris), hardy in Zones 4-8, is easy to prune and rewards with those springtime flowers as well as a stunning autumn color transformation, when the green leaves turn to a bright yellow.
Ornamental grasses -- hardy, with few maintenance needs and year-round structure to boot -- are a good fit for rooftop gardens, and blue fescue (Festuca glauca) can be used in containers, in a border, or as an edging plant. It is hardy in Zones 4-8 and reaches just 10 inches tall.
On the other size end of the ornamental grass-size spectrum, maidengrass -- hardy in Zones 5-9 -- towers, particularly Miscanthus sinesis 'Gracillimus', which can reach heights of 7 feet. It does best in full sun and can be used in the back of a rooftop garden border. Or plant multiples in a row to block an unsightly view.
Note: Maidengrass can be invasive in some areas; check local restrictions before planting it.