When the black cherry trees start budding in New York City, I know it’s time to get back into the garden. Like any good green thumb, I organize my seeds and start my transplants. I look back on last season’s mistakes, and map my growing space with the sophomoric optimism of an annual agriculturalist. I topdress my garden beds with compost to reinvigorate the tired soil. And this is where my story diverges from that of most vegetable growers.
For me, topdressing involves schlepping soil up several flights of stairs.
That is because the vegetables, herbs, and flowers I’m preparing to plant will come to life on a green roof farm three stories up in the air. I’ve been farming on the Eagle Street Rooftop Farm in Greenpoint, New York since 2009. The first commercially operated green roof turned vegetable farm in the country, Eagle Street has proven to be one of the most beautiful and challenging places I’ve ever grown plants.
As I quickly learned, the differences between rooftop gardening and ground level gardening don’t end with the stairs. Rooftop gardening requires permission, practicality, and patience with both your plants and local policy. My own steep learning curve was not unique. In researching and writing The Rooftop Growing Guide: How to Transform Your Roof into a Garden or Farm, I met with many rooftop growers reconfiguring their ground-level best practices in the new context of a sky-high site.
Here are few important questions I learned to answer along the way.
Are all rooftop gardens the same?
“Rooftop gardening” is an umbrella term. Rooftops can grow a wide range of plants, and they use many different systems to do so. Green roofs, container gardens, and greenhouses (which can use both in-soil and hydroponic-based growing systems) are distinct rooftop gardening spaces, each with their own catalogue of benefits and challenges.
New York City’s many rooftop gardens provide examples of each style of rooftop growing systems. Both the Eagle Street Rooftop Farm and the Brooklyn Grange are open-air, commercially-run green roof row farms. They are unique, as most green roofs do not grow edible plants.
There are different types of green roofs (modular, intensive, and extensive are the three basic divisions). All green roofs, no matter what types of plants they grow, serve to address the heat island effect and storm water runoff and they reduce energy costs for the building below by acting as an insulating layer atop the roof. These environmental benefits are not entirely unique to green roofs, but they address them more effectively than a container or greenhouse system can.
Green roof installation is most cost-effective when a roof’s size (10,000 square feet minimum) makes the costs of materials sourcing worth installation. For this reason, many aspiring rooftop growers turn to a second style of rooftop gardening: container-based systems. The rooftop gardens at Roberta’s in Bushwick; Print Restaurant in Tribeca, and North End Grill near Wall Street all use containers. Like all open air systems, with the right plants they can provide the environmental benefit of acting as a source of food and habitat for insects and birds.
Rooftop greenhouses are often more expensive to install than a green roof or container system, but provide year-round use for growers in four-season climates. Another benefit of greenhouse growing is that when installed with a hydroponics system, these gardens can be designed to use less water and grow more crops per square foot than other rooftop systems. Gotham Greens is a hydroponic greenhouse based for-profit business following this model, operating on rooftops both in Chicago and New York City. Since the 1970s, Eli Zabar’s Vinegar Factory on the Upper West Side hosts rooftop greenhouses with both hydroponic growing and in-soil container-based growing beds.
The value of each system depends on your goals as a grower. The cost, resources, and municipal policy in your area will also be a factor. How you plan to use the rooftop (during what season and what you’d like to grow) should also be considered. For example, Fifth Street Farm is a container-based open-air school garden used in the spring and fall by students of the Earth School in the Lower East Side, whereas P.S. 333 in the West Village installed a rooftop greenhouse for year-round hydroponic and aquaponic growing.
Roofs are designed, first and foremost, to protect the buildings below them. Before you put plants on your rooftop, make sure you’re working with a solid roof and that you have permission from the building’s owner as well as an understanding of your local building and fire code. Most municipalities have regulations for legal parapet height, setback (how far from the building’s edge you and your garden have to be) and how you access the roof.
For example, in New York City, if your rooftop has a parapet less than forty two inches high, you are not legally allowed on the rooftop. In Philadelphia, you must be able to access your rooftop through full door-sized egress (such as a headhouse) at the same level as the rooftop. Climbing up a fire escape or through a hatch are for emergencies only, not regular gardening use. In most cities, if you plan on selling edible crops, you’ll also need to check your local health code and zoning regulations.
I’m often asked how someone can determine if their roof can bear the weight of a garden. I am not a structural engineer, and weight bearing capacity is not a layperson’s question to answer.
However, familiarity with your climate’s precipitation averages and modes can tell you a little about the potential weight-bearing capacity of your building. Often, an area with little snow (Austin, Texas, for example) is likely to have fewer buildings with the weight bearing capacity of a snow-ready city (such as Denver, Colorado). In warm-weather cities such as New Orleans and Los Angeles, the rooftop gardens I visited were both atop outdoor parking garages, a structure designed to carry significant weight load. Swapping out cars for kale was an easy move.
Regardless of size or purpose, the buy-in of the building owner is crucial to any garden’s success. Successful rooftop gardens, particularly larger-scale projects, rely on the expertise of several types of professionals. These could include a structural engineer, a roofing contractor, an architect, a landscape architect, green roof or greenhouse professionals, and an expediter.
What do you want to grow?
You’ll want to choose a growing system based on your plant interests, climate, microclimate, local code, and budget. For example, a commercial grower in a four-season climate with an interest in growing year-round (and the money to back it) might start with a greenhouse system. Small-scale residential gardeners might want to go with a lower-budget, temporary container garden.
Keep the long view in mind. Take care, too, when installing expensive or permanent systems such as greenhouses and fixed planter boxes, if your interests or tenancy in the building is limited.
Check in with your goals helps every step of the way, from choosing growing media to selecting container size. For example, if you crave fresh peaches, don’t stick a peach tree in a five-gallon bucket (bad for the tree) or plant it straight in your green roof (bad for the roof). Focus on finding the right roof and growing system to accommodate the twenty-plus year relationship involved in raising a fruit tree.
What’s the microclimate like up there?
Rooftops are full of funky microclimates. Shadow projections, hot spots, damp zones, and wind all affect which plants will thrive in your new garden. Make a practice of observing how weather events like rainstorms and heat waves affect your roof. For example, does water pond or puddle, or do you have good drainage? Does the roof surface skyrocket to twenty degrees hotter than the pavement or nearby green spaces during the peak afternoon hours? Does the wind whip between nearby buildings and lacerate your rooftop when it picks up during summer storms? Do you have big bulkheads or AC units that cast shade? There are workarounds to all those quirks, but you have to identify them first.
Access to water is an important factor to consider, particularly if you’re growing vegetables.
One tip for container-based growers is to use sub-irrigated containers, which hold water in a built-in reservoir below the soil. These are not foolproof, but they’re certainly helpful for retaining water as well as preventing damage to the rooftop surface when puddles appear.
Familiarize yourself with your growing climate. Note the first and last frost-free dates, the average day length throughout the year, and your annual average precipitation. On a rooftop, this will help determine what you can grow and when, as plants need different temperatures for longer or shorter periods of the year to thrive.
How do I know when to start growing?
You’ve done your due diligence researching the different rooftop growing systems available to you and determined that you have safe and legal access to your rooftop. At last: It’s time to get planting. Now you can start to flip through seed catalogues and select crops with the untrammeled optimism particular to gardeners before they’ve faced their first crop failure.
Rooftop growing can be stressful for plants. This isn’t necessarily the fault of a rooftop, but more often the environment provided for their roots. Smaller soil volume, and less nutrient-rich soil can pose challenges. That said, within every failure are many opportunities for success.
Even in compromised growing situations, all plants will try to grow their roots, stems, and leaves. Because of that, edible leaf crops are a good go-to for starter gardens. For example, a fruit crop like peppers may fail to bear peppers for want of a pollinator or lack of nutrients, but collard greens will always be edible, even if they stop growing at a few inches high.
During the long daylight hours of midsummer, stressful growing conditions can also trigger seed production (i.e., bolting). Selecting plants with edible flowers and seeds are a good workaround. Herbs suit this parameter particularly well, as their flowers and seeds are often delicious. For example, if cilantro bolts, its seeds are coriander. Basil, sage, mint, thyme, oregano, and many other common herbs all produce edible flowers and seeds.
One of the most fascinating characteristics of the rooftop growing environment is the desiccating effect of wind on plants, which I’ve noticed tends to toughen them up quite a bit. On our roof, I’ve found that this is delicious on thyme, concentrating the flavor, but it bitters mint. I’ve forgone larger fruit such as water-loving watermelon for weedy cherry tomato plants, and I’ve opted for spicy pepper fruits over the eggplant I just can’t get to stay sweet.
Each rooftop ecosystem has its foibles. But the challenges are more than worth it. There is something of the sailor’s spirit in the view from a rooftop, looking out over the empty rooftops as uncharted waters to explore. A study conducted by the Urban Design Lab in 2011 suggested there are over 3,000 acres of available rooftop space in New York City suitable for rooftop farming. Imagine what the acres add up to worldwide. No green thumb can read a statistic like that and not feel the urge to get growing.