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Environmental News


From the Editors of E / The Environmental Magazine


Dear EarthTalk: I’ve heard of green roofs, but what are “green walls?”
—P. Spencer, Alcoa, TN

Green walls, or "vertical gardens," are walls partly composed of or filled in with live plant matter. They filter air and water, soak up carbon dioxide and help lessen the “heat island” effect of urban areas while reducing air conditioning costs in their host buildings. Pictured: a vertical garden at the Anataeum Hotel in London.

Photo © Niall Napier, Flickr

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 account the specific surroundings of the place in which it is created,” says landscape architect Michael Hellgren, who founded the firm Vertical Garden Design in 2004. “It is not only the colorful interplay between the plants on a ‘green wall’ that is fascinating, but also the appearance of the wall itself, which changes daily.”

Hellgren, who has designed and implemented large green walls in his home country of Sweden as well as in Spain, Portugal and Italy, among other locales, sources plants for his projects from various climate zones around the world. His favorites are so-called “lithophytes”: plants that can grow on rocks, branches and tree trunks without necessarily being rooted in soil. “Among other things these climbing plants have the enormous advantage of their roots acting as excellent natural drainage on the wall,” he adds.

While large “vertical gardens” are surely impressive, critics question the sustainability of such endeavors, given the energy inputs needed to run the pumps and other equipment used to maintain proper nutrient and air flows, and the emissions caused by the manufacture and transport of specialized materials. Also, larger green walls need more water than rain alone can provide, and thus don’t necessarily save water. But as the field matures, practitioners are finding wider arrays of plants to choose from that are better at taking care of themselves—and scaling back on inputs and supporting machinery with the hope that one day many of the walls will be self-sustaining gardens that cleanse our dirty air and compromised storm water.

CONTACTS: Patrick Blanc; Vertical Garden Design.



Dear EarthTalk: Thorium is a naturally occurring element that is supposedly more available, more efficient and safer to use than uranium for generating nuclear energy. Is this true and, if so, why haven’t we made the switch?
—Jane Westermann, Austin, TX

Advocates of thorium to power nuclear plants say that the element is safer than uranium, and that its waste cannot -- like the plutonium waste of uranium fission -- be re-formulated for nuclear weapons.

Photo © iStock Collection/Thinkstock

Thorium, a naturally occurring radioactive element found in abundance in the Earth’s crust all around the world, might well be a better fuel source than uranium for nuclear power generation for a variety of reasons. First and foremost, just one ton of the silvery metal can produce as much energy as 200 tons of uranium or 3.5 millions tons of coal, according to Nobel laureate Carlo Rubbia of the European Organization for Nuclear Research. Another advantage is that it comes out of the ground as a 100 percent pure, usable isotope. Unlike uranium, which contains only 0.7 percent fissionable material, thorium doesn’t require enrichment to be used in nuclear reactors. Also, the spent-fuel waste from thorium fission cannot be re-formulated for nuclear weapons like plutonium, the waste product of uranium-based fission.

Also, proponents say that thorium doesn’t require the high temperatures and mitigation equipment of uranium-based reactors. “The plants would be much smaller and less expensive,” Kirk Sorensen, a former NASA rocket engineer and now chief nuclear technologist at Teledyne Brown Engineering, told the UK’s Telegraph last year. “You wouldn’t need those huge containment domes because there’s no pressurized water in the reactor.” With no high temperatures, thorium reactors can’t “melt down” and release radiation.

“Once you start looking more closely, it blows your mind away,” adds Sorensen. “You can run civilization on thorium for hundreds of thousands of years, and it’s essentially free.” The advocacy-oriented Thorium Energy Alliance reports that there is “enough thorium in the U.S. alone to power the country at its current energy level for over 1,000 years.”

Nuclear researchers in the U.S. first contemplated using thorium as a nuclear energy feedstock back in the 1940s, but its lack of feasibility in making nuclear weapons put it on the back burner, where it has sat for the last six decades despite various attempts to revive the technology for practical use. In Russia, China and India, thorium reactors represent the next generation of nuclear power. India possesses about a quarter of the world’s thorium reserves. The country is working to develop a network of large thorium-based reactors, and plans to meet 30 percent of its electricity needs with thorium by 2050.

Many nuclear advocates and environmentalists alike don’t see thorium as the savior its supporters make it out to be. For one, uranium is still relatively easy to come by and inexpensive, and the nuclear industry is set up to run on it. Changing over to thorium would be expensive, and who knows what unforeseen problems may arise with full-scale deployment. Perhaps most important, some analysts worry that putting more eggs into humanity’s nuclear basket will surely further delay the transition to a truly green economy that runs on clean renewable energy from the sun, wind and other so-called alternative sources.

CONTACTS: CERN; Thorium Energy Alliance; Teledyne Brown Engineering.





GOT AN ENVIRONMENTAL QUESTION? Send it to: EarthTalk, c/o E/The Environmental Magazine, P.O. Box 5098, Westport, CT 06881; submit it at:, or e-mail: Read past columns at:


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