Choosing rooftop plants is much like choosing plants for any garden. The plants need to be able to thrive in the growing conditions you have, and on a rooftop, that could be intense sun bouncing off neighboring buildings or it could be mostly shade from those nearby structures. Rooftop gardens also tend to have more limited space, but you may also want taller plants that can act as living screens. The following list of rooftop plants offers something for just about any lofty situation.
Kousa Dogwood
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.
'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, this low-growing juniper reaches about 3 feet tall and makes 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, this evergreen tree 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.
'Fanal' Astilbe
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 or its foliage will look crispy and tattered.
Hosta
Integrating hostas into shadier spots on a rooftop garden is a terrific way to add beautiful foliage to the space. Most types are hardy in Zones 3-8, and they come in an array of colors and sizes so you can mix and match to fit your rooftop garden design best.
Ajuga
Also called bugleweed, ajuga (Ajuga reptans) is a flowering groundcover that is attractive to butterflies and hummingbirds. This perennial needs regular watering on rooftops but its low growth makes it a good choice for windier situations. 'Silver Beauty' (shown here) is one good choice, as is 'Bronze Beauty', which has deep blue flowers and bronze-tinged foliage. Ajuga is hardy in Zones 3-9.
English Lavender
Beloved for its scent, English 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. It's hardy in Zones 5-9.
Daylily
Most daylilies are fairly hardy, and their prolific blooms offer rooftop gardeners a good way to add color to full sun spots. They come in a wide range of sizes and colors but in general, the strappy foliage reaches about 1-2 feet in height. Most types of daylilies are hardy in Zones 3-9.
Lamb's Ears
A nice fit for low-growing borders or the front of flower beds, lamb's ears (Stachys byzantina) 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 in Zones 4-10. If the purplish blooms get leggy in summertime, cut them back and new ones will often form.
Wisteria
With fragrant blooms that hang like grapes, showy wisteria is a vigorous 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.
Asian wisteria species (Wisteria sinensis and Wisteria floribunda) are invasive in some areas. Check local restrictions before planting it. Parts of this plant are also poisonous.
'Gold Flame' Honeysuckle
This variety of 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.
Climbing Hydrangea
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.
Blue Fescue
Ornamental grasses are hardy, low-maintenance, and provide year-round interest. These grasses also make excellent rooftop plants, and blue fescue (Festuca glauca) is one of the best for this use. It stays shorter, reaching just 10 inches tall, so it works well in containers and stands up well to windy conditions. It is hardy in Zones 4-8.
Maidengrass
On the other end of the ornamental grass-size spectrum, maidengrass can tower to 7 feet, particularly Miscanthus sinesis 'Gracillimus'. It does best in full sun and can be used in the back of a rooftop garden border or as a screening plant for an unsightly view. It's hardy in Zones 5-9
Maidengrass, also known as Chinese silvergrass, can be invasive in some areas so check local restrictions before planting it.