[1] In a windowless warehouse on the edge of a busy city, row upon row of lettuces glow under bands of pink and purple light, stacked one above another like books on a shelf. There is no soil here, no rain and no sunshine, yet the plants are flourishing. This is a vertical farm, and across the world more than 2,000 of them now grow food inside old factories, shipping containers and the basements of office towers. The idea is simple but striking: instead of spreading crops thinly across the countryside, farmers pile them upwards, squeezing an entire field into the footprint of a single room. What was once dismissed as a curiosity has, in barely fifteen years, become a serious branch of modern agriculture.

[2] The appeal of growing food indoors becomes clear the moment one considers how an ordinary farm works. A traditional field depends utterly on the weather, and a single storm, drought or frost can destroy a season's labour overnight. A vertical farm, sealed away from the sky, ignores the weather entirely. Its temperature, humidity and light are all controlled by machines, so the same crop can be grown in midwinter as in midsummer, and harvested again and again throughout the year. Because the plants are stacked in layers, a building can produce many times more food than an open field of the same size, and because that building can stand inside the city itself, the lettuce on a shop's shelf may have travelled only a few streets rather than several hundred miles.

[3] Water, too, is used with remarkable care. On a conventional farm much of the water poured onto the land simply soaks away or evaporates before the roots can drink it. Inside a vertical farm the same water is captured, cleaned and pumped round and round the system, so the plants are fed again and again from a supply that is barely topped up. By some measures these farms use as little as a twentieth of the water that a field would need to grow the same crop. There are no weeds to smother the seedlings and few insects to chew them, which means the farmer can do away almost completely with the pesticides and weedkillers that trouble so many people about ordinary food.

[4] The benefits, then, are real, but so are the obstacles, and the largest of them is the electricity bill. The sun pours its light onto a field for nothing, yet a vertical farm must manufacture its own daylight, burning through enormous quantities of power to keep its lamps glowing day and night. The pumps, fans and cooling units add still more to the cost, and in many places the price of all this energy is simply too high for the farm to make a profit. For this reason most vertical farms today grow only light, quick, leafy crops — salad leaves, herbs and soft fruits — that fetch a good price and are ready within weeks. Wheat, rice and potatoes, the crops that actually feed the world, remain stubbornly beyond their reach.

[5] Supporters answer that these difficulties are shrinking year by year. The lamps that light the plants are a special kind that wastes little energy, and they grow cheaper and more efficient with each passing season; some farms now run their machines on wind or solar power, trimming both their bills and their effect on the planet. Engineers are teaching computers to watch the crops constantly, adjusting the light and water for each tray so that nothing is wasted. As cities swell and good farmland grows scarce, they argue, the ability to grow fresh food beside the people who will eat it — clean, local and untouched by the weather — will only become more precious.

[6] Whether vertical farms will ever feed whole nations, or remain a clever way of supplying restaurants with fresh basil, no one yet knows. What seems certain is that the old, comfortable picture of farming — a green field under an open sky, worked by the changing seasons — is no longer the only one. Somewhere above us, in a tower of glowing shelves where it is forever a mild spring afternoon, the next harvest is already quietly rising, indifferent to the rain on the windows and the snow in the streets below.