Rooftop gardening has its own set of challenges, but it’s no more difficult than traditional, in-ground gardening, it’s just different. Let’s review the growing conditions on a typical roof, and you’ll see what I mean.
Most rooftops are full sun growing situations. In cities with competing high-rises, one building may cast a shadow on a neighboring roof, but in general, rooftop gardens are full sun gardens. Growing conditions are hot and bright.
As you rise above ground level, wind gets stronger; this both helps and challenges the rooftop gardener. Good air circulation reduces the chances of fungal diseases (e.g. black spot of roses, powdery mildew on bee balm). However, wind wicks moisture away from leaf surfaces, so plants with delicate foliage (ferns, Japanese maples) may become wind burned.
When you consider the things that make rooftop gardening different from in-ground gardening, it’s clear that plant choice is important. You need tough plants that thrive in sunny, hot/cold, dry, windy conditions.
Look for plants with these physical characteristics:
Fuzzy leaves and silver foliage are adaptations that slow the evaporation of water from leaf surfaces. Consider lamb’s ears, lavenders, and sages.
Evergreen trees and shrubs are often drought tolerant. Their small leaf surfaces lose water slowly, and stiff leaves with waxy surfaces have an extra layer of protection against drying winds. Junipers, Oregon grape, and arborvitae are excellent choices.
The roots of container plants are more exposed to cold temperatures than the roots of plants in an in-ground garden. They simply don’t have as much soil insulating the root zone. When planting a container garden, choose plants for one hardiness zone colder than you would for an in-ground garden. And if your container is on a rooftop, subtract one more zone to compensate for the cooler temperatures above ground level.
Rooftops are man made structures and most were not constructed with gardens in mind. If you’re going to grow a few pots of tomatoes, you’re probably fine, but if you want to plant an extensive rooftop garden, get an engineer’s report to make sure your roof is up to the task. In all my years of rooftop gardening I’ve never had this be an issue, but let’s play it safe.
To minimize the weight of your rooftop garden, use a lightweight potting mix rather than topsoil. Potting mixes may be either soilless or soil-based. Soil based mixes will be heavier, but will retain moisture longer. Soilless mixes are lighter weight and dry out more quickly.
Containers add to the weight on the rooftop; wood, terra cotta, and metal are heavier than plastic and fiberglass. Container material also effects your watering schedule. The potting mix in porous containers (terra cotta, wood) dries out more quickly than the mix in non-porous containers (fiberglass, metal, plastic), since water can evaporate through the walls of the container. And in areas where winter temperatures regularly dip below freezing, you’ll want your containers to be winter proof. Untreated terra cotta can crack with repeated freezing and thawing, and plastic may become brittle in cold temperatures. The most durable choices for containers that overwinter outdoors are fiberglass, metal, and wood.
I’ve saved the most important consideration for last: How do you water rooftop gardens? I have two words for you: drip irrigation. It’s the most efficient way to water plants because it delivers moisture directly to the roots of the plants. It can also be automated so your garden is watered when you’re away, and it releases you from the chore of hand watering, so you’ll have more time to relax and enjoy the garden you’ve created.
A rooftop garden is a terrific way to make the most out of unused space and bring more greenery into your life. Not to mention all the great things it does for air quality, rain capture, and building temperatures. But that’s another blog post!
Ellen Zachos is a Harvard graduate and received her certification in Commercial Horticulture and Ethnobotany from the New York Botanical Garden. As the owner of Acme Plant Stuff, Ellen designs, installs, and maintains both interior and exterior gardens in the New York City area. Ellen is an instructor at the New York Botanical Garden, where she teaches for both Continuing Education and The School of Professional Horticulture. She lectures on a wide variety of topics at flower shows, nurseries, and for horticultural organizations around the world.