Ava Chin

“The weeds are welcome here,” said Anasa Scott as we walked across a gravel rooftop at the City College of New York on a sunny day in Harlem, making our way around a solar panel and a satellite receiver to check out a series of south-facing soil beds that comprised the green roof.

With the proliferation of eco-roofs sprouting up across the city, I have long looked up at the tops of buildings and been curious about what tenacious weeds may be growing there. (When I was a child, a friend’s boyfriend had a giant ailanthus growing through the cracks in his parents’ East Village roof).

When Ms. Scott, general manager at the college’s Greenproofing.org, a consortium of faculty and engineering and economics students working on issues of environmental and business sustainability, informed me that she’d seen a weed thriving in their living roof, I was curious to identify it.

At first glance, all I saw in the plant beds spread out patchwork-quiltlike across the graveled rooftop (for a study in storm-water runoff management) were a sea of sedum — a succulent popular with gardeners as a ground cover — a few clumps of tall dried grass and what looked like the wizened remnants of lavender (an accumulation of plant life left by generations of students long-since graduated).

Ava Chin

Then, as my eyes adjusted in the sun, I noticed a bunch of weeds, including giant bunches of clover (Trifolium) large enough to grab by the handful; tiny, lemony-tasting wood sorrel (Oxalis), no larger than a dime and a nice treat in salads; and some small sprigs of mugwort (Artemisia vulgaris) nestled among old corn stalks, a carpet of hairy cap moss and what looked like mouse-ear cress with its pretty white flowers. In the southwest corner of the roof, a patch of clover had homesteaded itself into a pile of wood chips.

How do wild weeds like clover or wood sorrel get all the way to an urban rooftop eight stories high? “If they didn’t get in with the soil or the wood chips,” said Uli Lorimer, curator of the Native Flora Garden at the Brooklyn Botanic Garden, “then they could have gotten there through either wind dispersal, birds or seed banks.”

Ms. Scott thought the weeds might have entered the rooftop with the sedum, which was from another one of their green rooftops in the Bronx.

Whether on the ground or up in the eco-roof of a giant academic building, it’s difficult to tell this early in the season if the wood sorrel is Oxalis stricta (yellow wood sorrel) or Oxalis montana (with a lilac-colored flower) or whether the clover is red or white. Luckily, even though Greenproofing students will be laying down native flower seeds next, they will not be weeding, so at some point later in the season, I’ll return to check the plants when they flower.

“I’m all about the weeds,” said Ms. Scott, looking out at the botanicals growing in the fabricated shale that Greenproofing faculty and students used to fill the plant beds. “Why spend so much when the weeds are here anyway?”