Enough with the vertical farming fantasies: There are still too many unanswered questions about the trendy practice In addition to not feeding the masses, vertical farming could have grave environmental and economic consequences

Five-plus years after the publication of Dickson Despommier's book The Vertical Farm: Feeding Ourselves and The World in the 21st Century, his dream — originally conceived as the production of food in the interior of tall urban buildings — is gaining momentum, despite many unanswered questions about its feasibility.

Although the fanciful skyscrapers depicted in countless architectural renderingsof vertical farms have never materialized in the real world, less ambitious indoor food-growing operations have been popping up in cities on every continent. And the buzz is growing even faster than the plants.

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There are projections that the vertical farming market will hit $4 billion by 2020. In a recent segment of NPR's Diane Rehm Show (in which I participated), several guests argued that vertical farming could revolutionize agriculture and even supply most of our food needs. The show's guest host, Maria Hinojosa, declared it "something big, different, and permanent."

However, in their efforts to develop a system that sustainably supplies cities with a large share of their food, theorists and practitioners of vertical farming face insurmountable obstacles. These include the limited range of crop species that can be grown; the tiny proportion of our population's total food needs that indoor crops could supply; the elite market being targeted; and the irrelevance of indoor agriculture to the lives and diets of people living in economically stressed rural regions where the bulk of our food is grown.

Meanwhile, looming largest among the many factors that will restrict the growth of vertical gardening (a term I believe is more apt than "vertical farming," given the potential scale and the types of food that can be produced) are its extraordinary energy requirements and heavy climate impact.

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Burning Daylight

No one would consider stacking photovoltaic solar panels one above the other. In such a system, only the top panel would produce electric current. The leaves of plants also need to be directly and strongly illuminated if they are to activate the photosynthesis that powers their growth.

If plants are living indoors, even if they are in an entirely glass-walled room, they can't capture enough sunlight to perform those functions. Plants nearest the windows will receive weak sideways illumination for part of the day, while interior plants will get much less than that; for both, the light intensity would be wholly inadequate to produce a significant quantity of food.

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Grown indoors, plants need much more intense artificial illumination in order to produce food than our eyes need in order to see. A real astronaut, for example, could not grow potatoes indoors on any planet under a sparse lighting setup like the one depicted in the film The Martian; however, the scene was adequately lit for filming and viewing.

Based on figures in a 2013 paper published by indoor plant-growth expert Toyoki Kozai of Japan's Chiba University and on the assumption of efficient LED lighting, I estimate that plants like potato or tomato that produce a fleshy food product require about 1,200 kilowatt-hours of electricity for each kilogram of edible tissue they produce, not counting the water stored in the food.

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That requirement approximates the annual electricity consumption of the average American home refrigerator — and that's a big energy bill to produce just two and a quarter pounds of food dry matter. This kind of thing could not be scaled up very far. At that rate, producing America's annual vegetable crop (not counting potatoes) in vertical systems under lights would require well over half of the electricity this country generates every year, and that would crank out 1.3 billion metric tons of carbon emissions per year. Heating and air-conditioning would add considerably to power demand and the emissions.

Given the energy hunger of indoor cropping systems, their designers typically assume that the product will be vegetables. Fresh produce has a high value per pound, and out of each 10 pounds of product harvested, 8 to 9½ pounds is water — and light is not needed to produce water.

In practice, the choice of food crops facing indoor gardeners is even narrower. Existing indoor systems all focus on growing leafy greens or herbs, because most of those plants' weight can be sold and eaten. If potatoes, tomatoes or green beans were being grown, much precious building space and costly electricity would go into producing inedible leaves, stems and roots.

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I suspect that much of the current interest in indoor gardening is stimulated by the success of indoor cannabis production. But that resource-intensive system, which in 2011 was already consuming as much electricity as two million American homes, is economically viable thanks to its product's $8-to-10-per-gram price. That compares with a fraction of a penny per gram for fresh vegetables or grains.

Warning: This Chard Is Not For Everyone

In Despommier's original vision for vertical food production, he was both wrong and right in his analysis of the problem to be solved. On one hand, he claimed we are running short of rural farmland. That is nonsense. In the United States, we produce 4,000 calories worth of food per resident daily, twice what's required. We have ample land; we just need to stop abusing the soil we have. So Despommier, recognizing that need, also presented a mostly on-target critique of industrial agriculture and the resultinbg soil degradation that results. On balance, his analysis was solid, but his proposed remedy — to take most or all rural land out of food production and move crops into cities — was doomed from the start.

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Don't get me wrong; I am all for urban cultivation of fresh produce. But it should be grown on sun-exposed plots of land and rooftops and “green walls” and in greenhouses. That would greatly improve the lives of urban-dwellers. But it would also leave the bulk of our 350 million acres of farmland untouched. Filling the sunlit land and rooftops in cities with crops could accommodate only a portion of U.S. vegetable production. And vegetables occupy only 3 percent of our total cropland.

Whether we were to use traditional or indoor gardening to urbanize food production (one consideration in making that choice: The indoor option would require tens of thousands of Empire State Buildings), we would still leave the vast bulk of our agriculture out on the rural landscape where it happens today.

Given that, suppose vertical gardeners were to backpedal to a goal much less sweeping than the salvation of America's farmland. Consider what it would take to provide fresh produce to just 15,000 city dwellers; that would be about 2 percent of the population of the District of Columbia.

That was the objective of a favorable 2013 analysis of vertical gardening by GIZ, a German engineering group. They estimated that the project would require a 150 x 150 square-foot building with 37 stories. It would cost a quarter billion dollars to construct and equip and would consume $7 million worth of electricity annually. Those estimates led them to conclude, “It is possible to grow only high value crops for consumers who have disposable income for such products.”

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In other words, such buildings would not serve 15,000 people of modest means. Were vertical gardeners ever to venture beyond leafy greens into growing a full range of vegetables, the sky-high inputs of capital, resources (especially electricity) and labor would limit the reach of their produce to a boutique market.

So the original aim of vertical cropping's proponents — to protect the soils of America's farm country by taking much of that land out of production­ — was well-meant but futile. It also completely ignored the people who live on farms and in rural communities across rural America. They produce the bulk of the country's food but often have even less access than city dwellers to good-quality food for their own families.

Small towns and rural areas are suffering disproportionately from food insecurity. The food bank network Feeding America estimates that while 43 percent of all U.S. counties are classified as rural, 62 percent of the counties with the worst rates of food insecurity among children are rural. The problem of poor access to affordable, nutritious food out here in flyover country is being addressed neither by the mainstream urban-food movement nor by vertical-gardening enthusiasts.

Much could be done to protect our lands while improving access to good food for all Americans, rural and urban. As a start, we could slow or stop the degradation of the nation's soils by eliminating the feedlot system of meat production and the scores of millions of acres of corn and soybeans that supply it. The land thus spared could be converted to soil-conserving pasture, rangeland or orchards as well as to a diverse array of food crops and cover crops that could be grown more sustainably. That could bring more income and better living conditions to rural areas while ensuring better nutrition for everyone, rural and urban. In the longer term, the replacement of annual grains and oilseeds with perennial food cropswill completely eliminate soil and water degradation in agriculture.

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Some forms of sun-and-soil-supported local food production that are being practiced in and around cities could also be implemented in rural America. Gardening has always been popular out here, and there's a lot more land on which to do it — enough land that not only vegetables but also staple foods like dry beans and grains can be produced for local consumption. That could be augmented in rural areas by community gardens and farms producing for local sale, and community canning centers and greenhouses that could make local food available year-round.

No Free Lunch, No Free Salad Bar

But even if funding and popular support can be found for all such good-food initiatives, the root causes of rural food insecurity will remain. Poverty, loss of family farms, the boarding up of small-town storefronts and ecological degradation are all attributable to exploitation by agribusiness, and they can't be fixed by growing salad greens under lights.

The roots of urban food insecurity are as economic and political as those out here in farm country. Sun-fed urban gardening, fruit and vegetable cultivation close to cities, and people's food initiatives all are important and need to keep expanding, but a still more profound economic transformation is needed.

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By claiming to be "revolutionary," vertical-gardening advocates are seeking to be viewed as part of such a transformation. But thanks to their hefty electric bills and limited crop range, they will have a hard time venturing beyond the elite market, let alone reducing their climate impact.

With the green façade crumbling, there is much loose talk in the vertical-gardening world about using renewable energy sources to power their grow-lights. That discussion spirals into some interesting circular logic: that we would use solar arrays and wind farms to convert sunlight's energy into electric current that would feed lamps that would convert a portion of the electrical energy into artificial sunlight to shine on plants so they can convert that light energy into food.

At each of those conversion points, there are big losses of energy and heavy infrastructure costs. It's about as wasteful as a system can be. Better to let crop plants do what they do best: capture cost-free, emissions-free sunlight for themselves, directly.

In other words, whatever source you use to power the lights for your vertical food-production system, you're giving a boost to greenhouse warming. Grow crops under lights, with climate control powered by fossil fuels, and you add directly to greenhouse emissions. Grow them under lights powered by wind turbines or solar arrays, and you consume green energy that could have been used instead to displace some of the fossil energy that's now running home refrigerators, air-conditioners, electronics, etc.

One also hears vertical enthusiasts argue, without providing any numbers, that they can compensate for the excess energy consumption required by electric lighting by growing their lettuce or basil close to markets rather than burning fuel to haul them across the country.

Reduced energy consumption for transportation is an excellent argument for urban gardening and farming within or close to cities, but it's no justification for indoor gardening. The climate impact of shipping food over long distances is significant, but the impact of energy-intensive food raising methods can be far larger than that. Dependence on artificial lighting in particular makes the impact of food production vastly larger than the impact of food transport.

Defenders of vertical gardening claim that it can produce much more food per acre of land per year than sun-and-soil agriculture. But not only are many of these comparisons exaggerated; they are also irrelevant. No matter how big the improvement in production per square foot per year, it will have no effect on the key number in vertical gardening's energy predicament: the quantity of photosynthetically active light required to produce each and every kilogram of plant tissue. That's a basic biochemical requirement. Increasing a food-production building's yield by stacking in more and more plants per floor or operating year-round only increases the demand for electric lighting.

The energy efficiency of lamps or production systems can be improved, but not infinitely, so indoor crops will always be deeply dependent on electricity and other industrial support. That means that with every kilogram of food we produce under artificial lighting, we will have passed up an opportunity to harvest free sunlight, and will thereby contribute to the Earth's warming.

In a country that desperately needs to achieve food justice, diversify agriculture, end soil abuse, abolish factory farming and slash greenhouse emissions, vertical gardening will have a net negative impact. Our country's food system, in cities and in the countryside, has been broken by our deep distortions of economic and political power. We need a broad-based political, economic and ecological uprising—and that will neither start nor finish in high-rise vegetable factories.