As a Project Manager for the green roof firm Roofmeadow, I regularly travel around the country to perform construction oversight. In December I had the good fortune of overseeing construction at a hotel in Lower Manhattan, which sits one block from the Hudson River. From six stories up the view was unparalleled – and so was the wind.
During construction, the green roofs and raised bed production area experienced 50 mph winds. A much milder microclimate could be found at the street level, but up on the roof, temperature fluctuations and extreme wind conditions reigned.
Exposure is one of the most significant obstacles to rooftop farming. High winds cause winnowing (soil loss) and desiccation (soil drying), while temperature fluctuations can cause crops to bolt (flower) prematurely. Each roof experiences a slightly different microclimate, but some basic rules of exposure are as follows:
1| Mind the water: Rivers and other bodies of water in cities often act as wind corridors. Selecting a roof that is shielded from these channels can help to minimize extreme rooftop winds.
2| Stay low: Higher building stories generally experience greater wind speeds, which means that a farm will experience less stress on top of a 1-3 story building than on a taller structure.
3| Surround yourself: Take advantage of high neighboring buildings and taller segments of the farm’s own building, which can act as wind breaks. Positioning your farm directly south of a taller building wall can help to block gusts. The wall may also capture heat, which will warm the adjacent soil. Be sure to avoid taller buildings to the south that will cast shadows on your farm. Also keep in mind that building north of a vacant lot can be risky, because you never know if a taller building will be erected on that site.
4| Cover up: Temperature fluctuations can be minimized by covering your crop rows with shade cloth. This thin cloth is used regularly on ground-level farms, and it benefits crops by capturing the heat that is released by the plants and soil. Rooftop wind will fill the cloth like a sail, and so it’s best to build low hoop houses to frame the cloth. These hoop houses should be screwed or bolted to the sides of raised beds, or ballasted by the walkways between farm rows.