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Why You Can’t Put Real Dirt in Rooftop Gardens

The ACROS Fukuoka building in Japan, with a green roof. (Photo: yyama/shutterstock.com)

If you thought that cultivating a rooftop garden involved putting dirt on top of a building, think again.

Make the mistake of dumping dirt from the ground onto a green roof, and you’re likely to end up with soil compaction, dead plants, and leaks—or even a collapsed ceiling. Dirt is complicated, and when it comes to rooftops, farmers—and amateurs—need to be extra mindful of the beds they build for their little green darlings.

The dirt on roofs is not dirt at all, but “growing media,” a highly engineered blend of minerals and organic matter. This artificial, carefully manufactured product mimics and improves on the properties of natural soil, catering to an environment very different from the one on the ground. Growing media is somewhat similar to hydroponics, which is the process of growing plants without soil, using anything from baked clay pellets to ground coconut husks, pumice, wool made out of rocks, brick shards, rice hulls, sand, gravel, mulch, and even polystyrene packing peanuts.

Don’t put this on your roof and expect good things to happen. (Photo: Ianailic/pixabay)

To get on anyone’s roof, manufacturers of growing media have to meet a rigid set of requirements. Growing media needs to be sterile, stable, devoid of soil, and able to properly retain and drain water while remaining sufficiently aerated so that plants can breathe. It needs proper levels of nutrients, salt content, and pH, and it needs to respect the Goldilocks principle: light enough so that it doesn’t overburden the roof, and heavy enough so that it wont be dislocated by wind or water. 

Annie Novak, co-founder and head farmer of Brooklyn’s Eagle Street Rooftop Farm and author of The Rooftop Growing Guide: How to Transform Your Roof into a Vegetable Garden or Farm,  says to think of the large particulates (for example, expanded shale and clay) in green roof growing media as the columns of a building—holding up the growing media’s structure, but not adding a lot of weight. Besides particulates, green roof growing media can also include compost. The components included in growing media blends really vary, she says. For example, the growing media used by Goode Green to install the green roof at Eagle Street Rooftop Farm includes clay and shale particulates mixed with a compost composed of spent mushroom propagation materials, horse bedding, and other locally sourced materials. Potting soil, a common material for rooftop gardeners, is made up of sphagnum moss, perlite (made by air-puffing volcanic glass), vermiculite (a type of mineral particulate), and a touch of compost. Novak compares different compost compositions to cookies: though they all count as compost, there are different kinds: ginger snap, chocolate chip, snickerdoodle—the possibilities are endless. 

Expanded clay pellets, a type of hydroponic. (Photo: Lucis/WikiCommons CC BY-SA 3.0)

While rooftop farms and gardens only entered the commercial mainstream in the last decade, supported by modern techniques formulated and codified in the 1970s, Novak points out that green roofs can be traced way back to ancient Egypt and Pompeii. The oldest existing green roof, which holds seven oaks atop a medieval tower, dates to 14th- or 15th-century Italy.

Rooftop agriculture, also called “vertical farming,” is gaining ground as urbanites realize its potential. Columbia University’s Urban Design Lab has identified almost 5,000 acres of vacant land in New York City’s five boroughs that would be suitable for urban agriculture, the equivalent of six times the area of Central Park. Right now, there are only six rooftop farms in the city. “There’s a terrific amount of missed opportunity,” says Novak. 

The many components of farming and gardening can feel overwhelming to someone new to the game, which is why Novak likes to use baking and cooking analogies in explaining the details of dirt and growing media. “I don’t want anyone to think they can’t garden because they don’t know the vocabulary,” she says. “But most people know how to cook.” 

There’s a lot to learn, but with time, patience, and guidance, anyone can grow a rooftop garden—provided they don’t add dirt to the recipe.