FarmedHere maintains one of the nation’s largest vertical farms in the Chicago area, growing organic herbs, greens, microgreens and other crops. (Jennifer Kathryn Photography/FarmedHere LLC)

How can we feed a population that’s growing on a planet that isn’t? Grow up!

Outdoors, an acre of land can grow an acre of lettuce. Indoors, an acre of building with plants stacked floor to ceiling can grow many acres of lettuce. Which is why, in cities around the country, entrepreneurs are turning warehouses into vertical farms. They promise local produce, responsibly grown. Do they deliver?

There are big pluses to vertical farming, the most fundamental of which is its verticality. Traditional horizontal farming is limited by its two dimensions. But if you stack plants 10 or 100 high, that acre can do the work of 10 or 100 farmed acres. On top of that, the plants grow faster: You’re not limited to the hours of daily light the sun delivers, so you get even more lettuce per square foot.

Less land is a win.

Because indoor plants are fed by fertilizer either delivered through water (hydroponic) or misted directly onto dry roots (aeroponic), they get only what they need. There’s no extra, and there’s no runoff, which translates to no algae blooms in rivers, lakes and estuaries.

Less fertilizer is a win.

Then there’s less water. As that commodity is in increasingly short supply in many parts of the world, a system that can cut water use by up to 95 percent should command our attention.

Less water is a win.

Because the climate is controlled, and there’s no soil to harbor pests or disease, indoor farming requires few pesticides. Workers are exposed to fewer toxic substances, and there are no threats to honeybees or other desirable plants or animals.

Fewer chemicals is a win.

Lettuce grown indoors can also be fine-tuned nutritionally by adjusting the fertilizer, but studies comparing indoor and outdoor lettuce nutrition find little difference, so I’ll call that a wash.

Still, that’s four non-trivial wins, and they are part of the reason vertical farming seems to have captured the imagination of urban food growers and consumers.

But before you shell out for the microgreens, there are a couple of disadvantages. The first is that you’ll have to shell out a lot, and the second gets at the heart of the inevitable trade-off between planet and people: the carbon footprint.

If you farm the old-fashioned way, you take advantage of a reliable, eternal, gloriously free source of energy: the sun. Take your plants inside, and you have to provide that energy yourself.

In the world of agriculture, there are opinions about every kind of system for growing every kind of crop, so it’s refreshing that the pivotal issue of vertical farming — energy use — boils down to something more reliable: math.

There’s no getting around the fact that plants need a certain minimum amount of light. In vertical farms, that light generally is provided efficiently, but, even so, replacing the sun is an energy-intensive business. Louis Albright, director of Cornell University’s Controlled Environment Agriculture program, has run the numbers: Each kilogram of indoor lettuce has a climate cost of four kilograms of carbon dioxide. And that’s just for the lighting. Indoor farms often need humidity control, ventilation, heating, cooling or all of the above.


Farmworkers would be among the beneficiaries of indoor vertical farming, which would lessen their exposure to pesticides. (Eric Thayer/Reuters)

Let’s compare that with field-grown lettuce. Climate cost varies according to conditions, but the estimates I found indicate that indoor lettuce production has a carbon footprint some 7 to 20 times greater than that of outdoor lettuce production. Indoor lettuce is a carbon Sasquatch.

But wait! There are ways to make up some of that. Shipping lettuce (usually from California) also has a climate cost. If your lettuce is grown in a warehouse in a nearby city, you cut that way down. But transport savings aren’t even close to bridging the gap, unless your field-grown lettuce is being flown in. The carbon cost of air freight is so high that indoor farms would be a fine substitute.

We’re not done yet.

Lighting is getting more efficient, and that will help, but there are significant limits to improvement. A spokesman for Philips Lighting said the company expects that eventually its LEDs will become 10 percent more efficient, but not much more. Albright theorizes that something like 50 percent more is possible. The theoretical maximum is that all electricity flowing to the bulb is converted to light; right now, the best bulbs convert only half of it.

There’s another way to make lighting more efficient: Pump carbon dioxide into the air. Plants photosynthesize CO2, so if there’s more of it in the atmosphere, plants can grow better with the same amount of light. According to Albright, that’s another 20 percent savings.

Combine the lighting and CO2 savings, and you’re looking at something like a 40 percent efficiency improvement in the near term. Substantial, but not enough to make indoor farms climate-competitive.

For that, we need to look at the source of the energy. Not the source at the farm; even with perfect efficiency, solar panels on the roof of a warehouse can’t come anywhere near providing enough energy for stacks and stacks of plants. It’s the source at the power plant that matters.

The carbon footprint of your lettuce depends almost entirely on the carbon footprint of your electricity. If your farm is in coal country, the carbon cost is high. Natural gas, it’s lower. About a fifth of the electricity in this country is generated by nuclear plants, which have a carbon footprint close to zero, making indoor farms a clear win. And, as renewable energy sources such as solar and wind start to contribute more to the grid, the carbon cost of vertical farms will go down.

Racks of plants are stacked one above the other inside FarmedHere’s warehouse in the Chicago area. (Jennifer Kathryn Photography/FarmedHere LLC)

Nate Laurell thinks about that a lot. He’s the chief executive of FarmedHere, one of the nation’s largest vertical farms, growing organic basil, microgreens, arugula, kale and more in a warehouse outside Chicago. From a climate standpoint, going vertical is making a bet that renewable energy is coming. Laurell acknowledges the high carbon cost of his products today, but says that “reducing the carbon footprint of the grid is a solvable problem over time.” In the long run, he says, “electrifying agriculture” will be a climate win because the grid will go green faster than the farm.

I can see a future where the carbon cost of indoor lettuce comes down. Whether the dollar cost comes down commensurately is hard to predict. One of the problems with vertical farming is that it’s expensive. Maintaining a building, setting up hydroponic infrastructure and paying big-city rent or real estate taxes is a wallet-thinning enterprise. To date, all the vertical farms I’m familiar with grow herbs and greens, high-value crops that they sell to well-heeled urban consumers.

I asked Laurell whether that kind of farming can break out of the basil-for-rich-people model, and he said he’s confident that it can. Although his products are now priced to be competitive with other organic greens and herbs, he’s working to reduce that price point. “Our intent is to get to a point where you have cleaner food with less chemicals that gets to the grocery store within 24 hours at a price point that isn’t just for Whole Foods,” he says. He’s aiming to be competitive with conventionally grown lettuce and expects to get there within three to five years. “A high-priced organic product that’s sold to rich consumers isn’t that interesting of a business,” he says. “To me, the interesting business solves the problem of feeding people.” Laurell was unwilling to share details of his cost-cutting plan — understandable in a competitive industry — but I wish him luck.

The bottom line on vertical farms is that today, indoor lettuce has a huge climate cost, but it’s not hard to envision a world where a transformed energy grid changes that equation. For so many reasons, let’s hope that world comes soon.