The Problem With Manhattan’s Green Roofs

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Mayor Michael Bloomberg has been trying to grow plants on the rooftops of Manhattan’s apartments for two years, as part of his PlaNYC initiative. Unfortunately, it’s not going so well. As students from a research team take soil samples of the Hempstead Plains growing on these green roofs, they are trying to determine how they can grow plants successfully so homeowners can reduce energy usage for heating and air conditioning; plus, they can potentially eliminate rainwater runoff.

The problems with these green roofs are varied, which is why the students are trying to find out how to improve the effectiveness and sustainability of these rooftops, for the benefit of all Manhattan residents. Here is a list of the perks and problems with these green roofs.

Problems:

  • Sedum is a common plant used on these green Manhattan roofs, but the problem is that sedum is no “greener” than tar or asphalt, which is commonly used on roofs today.
  • Does not absorb water efficiently, so this poses a problem for those that want to manage stormwater runoff.
  • Sedum absorbs heat, instead of reflecting it.
  • Sedum just is not performing well on these green roofs.
  • Adding a variety of species of plants helps maximize the benefits of these green roofs, so planting sedum-only roofs lowers the benefits.

Perks

  • As mentioned previously, these green roofs will allow the homeowner to manage stormwater runoff.
  • Green roofs can help regulate the temperature of the building.
  • Reduces the effects of heat-islands in the city

In order to maximize the perks and minimize the problems, scientists are looking into various species of plants and other types of living organisms that can help the green roofs thrive. An assistant professor at Barnard College, Krista McGuire, is looking into the use of fungi. One particular fungi called Pseudallescheria fimeti, was present in several roofs that were studied recently which seemed to have excellent results. These rooftops were thriving even though they were located in harsh environments that contained high levels of pollution. McGuire believes that these Manhattan roofs are missing this fungi. She’s hoping that her research findings will also help green rooftop companies figure out which species of plant will work best on which rooftop, for maximizing the benefits for the homeowners.

The growth of this fungi, however, depends on how much rain falls per year, the position of the roof it’s on, and the amount of pollution in the air.

“Plant species are adapting to new environments,” she says. “Without the fungi, the plants would not be able to grow and survive.” “In the long term, this information may help individuals decide which types of soil microbes to amend on their green roofs, so that they can maximize plant survival and minimize management,” she says.

Once the McGuire and the research team finds out which plants will grow where, hopefully they can find a solution to the problems affecting this environmental project. Green roofs and rooftop gardens are a great idea in theory, but only if they are given what they need to thrive.

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