Dear EarthTalk: I've heard of green roofs, but what are "green walls?".

 
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Dear EarthTalk: I've heard of green roofs, but what are "green walls?"--P. Spencer, Alcoa, TN

Green walls (also known as biowalls, vertical gardens or vertical vegetated complex walls) are wall structures partly composed of or filled in with growing plant matter. More than just easy on the eyes, green walls work like green roofs by filtering air and water, soaking up carbon dioxide and helping lessen the "heat island" effect of urban areas while reducing air conditioning costs in their host buildings.

The self-proclaimed creator of the vertical garden concept, French botanist Patrick Blanc, pioneered the use of hydroponic cultivation techniques--plants grow in an irrigated mineral nutrient solution without the need for a soil substrate--to create large green wall installations in both residential settings and within larger public structures and even office buildings from Singapore to San Francisco and points in between.

Blanc's installations start by placing a metal frame on a load-bearing wall or structure. The frame supports a 10-millimeter-thick PVC plate, upon which are stapled two 3-millimeter-thick layers of polyamide felt. "These layers mimic cliff-growing mosses and support the roots of many plants," he says, adding that a network of pipes and valves provides a nutrient solution of dissolved minerals needed for plant growth. "The felt is soaked by capillary action with this nutrient solution, which flows down the wall by gravity."

"The roots of the plants take up the nutrients they need, and excess water is collected at the bottom of the wall by a gutter before being re-injected into the network of pipes: The system works in a closed circuit." Plants are chosen for their ability to grow in this type of environment and depending on available light.

"Each vertical garden is a unique wall composition of various types of plants that has to take into...

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