BioEd Online

What colour is New York City? Many would say grey, the colour of cold, inanimate concrete. But not Jason Aloisio, a graduate student at the city's Fordham University. His eye catches all the green: lawns, gardens, overgrown empty lots and weeds in sidewalk cracks. He even sees the city's air as swirling with life. And to prove it, he has grown gardens on the city's rooftops without planting a single seed.

Aloisio started by exploring rooftops and inventorying the plants and animals he found. "I sifted through the leaves up there, bottled up insects, found trees growing in little crevices, even in garbage," he says. "I thought, if plants and animals can get up to these rooftops with nothing to grow on, what if we give them somewhere to grow?"

Aloisio presented his team's results on Monday at the annual meeting of the Ecological Society of America in Austin, Texas. He was able to piggyback his project on another group's experiment to grow native plants on green roofs, which had already been scheduled. The plots were set up in spring, but planting wasn't scheduled until autumn. So Aloisio laid out commercially available soil mixture designed for green roofs at two different depths — 10 cm and 15 cm — onto 2-by-4-metre plots on eight roofs across the city.

City plains

Over the weeks, the plots caught seeds carried by wind and birds. After just a few months, the most successful plots were jammed with plants, with biomass rivalling that of the prairies of the Great Plains. The plants were diverse, too, with 30 species showing up in total, one-third of them native to the area. Roofs contained an average of 12 species each, and 67 of the 85 plots were colonized by at least one species. Deep plots had more biomass than shallow plots.

The biggest surprise, says Aloisio, was that so many of the colonizers were edible (see ""). The most common plant across the plots, Amaranthus blitoides (known as mat amaranth or prostrate pigweed), is an edible plant often dismissed as a weed. Millet (Panicum miliaceum) also showed up, probably from birdseed, as did purslane (Portulaca oleracea), a succulent plant that Aloisio says is high in omega fatty acids and has a "sweet and salty flavour".

Aloisio is now trialling rooftop cultivation of various edible species of Amaranthus.

Nature roofs

Charlie Miller, founder of the company Roofmeadow in Philadelphia, Pennsylvania, says that Aloisio isn't the first to be curious about what might show up on an unseeded green roof. "That has been a sort of fad in Europe for the past six or seven years," he says. "In Germany, they call them Naturdächer — nature roofs."

Miller finds the idea more appealing in theory than in practice. "Our company would like to see projects that are highly biologically diverse and beautiful, and not completely random," he says. "On green roofs that have gone native because owners haven't maintained them, plant diversity collapses. Three or four species will displace everything else."

The director of the International Green Roof Association, Wolfgang Ansel, who is based in Nürtingen, Germany, says that whether a self-colonized roof is considered acceptable is all a matter of culture and expectations. "We have a tradition that it should be maintained and have low-growing succulents on the roof, Sedum varieties and some grasses. We don't expect the weeds."

For Aloisio, in addition to feeding his interest in what plants will survive in the exposed, shallow environment of a green roof, the project is about having some respect for the unheralded biota of New York City.

"I go around the city and wherever I am, I see plants growing in these tiny little crevices," says Aloisio. "They look like weeds, but they are still green."