Urban Heat Island of Manhattan

As New Yorkers, we’re all too familiar with the infamous ‘urban heat island‘ effect. To be fair, we’re often spoiled in our work as we traipse about, beating the heat in the idyllic, shaded, lush gardens we’re lucky to know, checking in on our sites as part of a robust maintenance portfolio. During install season, however, we put on our hats and bandanas, douse our heads from time to time with city water from the nearest hose bib, and generally try to beat the rooftop heat while we establish the infrastructure for future gardens. One such install found us planting perennial plugs in a semi-intensive green roof during the absolute hottest days of 2022. Fast forward a year later and we’re mired in even hotter New York City temperatures, but now working on a categorically cooler—in both senses of the word—green roof.

Cooling Down the Heights

Here are some photos from an install in July of 2022, where we were tasked with greening the breezeway connecting two apartment buildings in a lovely co-op already graced with lush courtyards and sidewalk gardens.

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We began by installing a root barrier to prevent any wily plants from entering the building without permission. Then, we established our edging and laid in drainage mat and separation fabric. Finally, it was time to haul dozens of bags of specially-formulated green roof soil (designed to provide a healthy, happy home for plants while minimizing weight and maximizing drainage) up our ladder, one at a time.

Once our layers were built and soil was installed, we set to work laying out a grid of native flowering perennials and grasses. We planted starter plugs by hand, also one at a time, and watered them in thoroughly to give them the best chance of surviving the summer and beating the heat.

Pollinators’ Paradise

The same roof, a year later, thriving in full to part-sun with an automatic drip irrigation system courtesy our friends at Lifesource. While it’s possible to install a green roof or rooftop garden without supplementary irrigation, your chances for success are greatly increased when you can target the root zone directly with strategically placed drip lines. We tend to insist on irrigation for all but the most drought-tolerant planting plans, or when our primary objective is simply rainwater capture and we’re sticking with that old familiar favorite, Sedum.

Our plantings for this particular project included a heady mix of native perennials, with all-star performances this year by Veronica spicata, Penstemon digitalis, Agastache ‘Blue Fortune’, Lobelia siphilitica, Eryngium yuccifolium, and several others:

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We’ve received great feedback from our clients and every resident we meet at this site so we know, aesthetically, we can mark it a success. Moreover, we can appreciate the obvious effects of creating a pollinator-friendly landscape in the middle of the city. The cooling effect is harder to measure directly when we’re knee-deep in perennials, but it’s one which is certainly observed anecdotally by our team during our maintenance visits. Tending this rooftop oasis in July of 2023 is a very different task than loading soil onto this roof was in July of 2022.

Planting it Forward

The EPA released a report (updated in June of this year) on the cooling effects and benefits of green roofs, specifically as they relate to urban heat islands. In it, they state that green roofs have the potential to cool rooftop temperatures by as much as 30-40 degrees Fahrenheit and that, in the aggregate, can combine to reduce ambient urban air temperatures by 5 degrees Fahrenheit. While 5 degrees may not seem like much relief when the thermometer is pushing the century mark, it’s certainly a significant change, and one we’re happy to be a part of when it comes to beating the heat.

Of course, we’re well aware (as our readers may be) that the benefits of greening your roof extend far beyond atmospheric changes. There is, notably, the immediate aesthetic improvement of an otherwise sterile landscape, the potential to capture stormwater and reduce sewer overflow, the energy savings to building owners, and the potential to protect and preserve your existing roof membrane, often adding years to its life. As we wipe the sweat from our brow and wish our new gardens the best of luck, it’s hard not to shuffle the cooling effect up the list. Perhaps it’s somewhat self-serving, but we like to think that every green roof we can plant this year makes for happier rooftop working conditions next year.