I originally posted this in 2009, but was inspired to repost by this thought-provoking article by Tamar Haspel in the Washington Post: We need to feed a growing planet. Vegetables aren’t the answer.

I’m always surprised how many people who have done a little gardening think that makes them an expert on agriculture.  Donald Trump doesn’t have a monopoly on militant ignorance.

—————————2009——————————————

I hate to bash the New York Times twice in one week, but this is such a stupid idea that I hardly know where to start. Some guy thinks we should build multistory skyscraper “farms” in New York City. He claims that:

For every indoor acre farmed, some 10 to 20 outdoor acres of farmland could be allowed to return to their original ecological state (mostly hardwood forest). Abandoned farms do this free of charge, with no human help required.

What about the abandoned farmers? But I’m not really worried about them, because this is not going to happen, at least not on a scale that poses an economic threat to many farmers.

If hydroponics is as wonderful as claimed in this article, do you wonder why most farmers still grow stuff in soil, rather than covering their fields with hydroponic tanks? Hint: it’s not because they’re stupid.

Growing plants on the roof of a building — a “green roof” — poses various challenges, but at least a roof can get the same amount of rain and sunlight as a ground-level garden would, assuming no shading by nearby buildings. With a multilevel “vertical farm”, however, water and light must somehow be divided among the levels. OK, if the tower is taller than anything nearby, it can get some sunlight coming in sideways, but consider the geometry…

NYC is a bit more than 40 degrees (latitude) north of the equator. We can simplify our calculations, and bias our results slightly in favor of vertical farms, by calling it 45. Then, between the equinoxes (Mar. 21-Sept. 21, when noontime sun is vertical at the equator), the noontime sun over NYC is always at least 45 degrees above the horizon. If you don’t want any part of a one-acre “field” (about 200×200 ft) to be shaded by the next floor above, with the sun is at 45 degrees, then the next floor would need to be 200 feet up. (To take full advantage of the solar radiation that a farmer gets for free, you would actually need 400 ft spacing for when the midsummer, noontime sun reaches 60 degrees above the horizon. When the sun is below 45 degrees, closer spacing would do, but then the light is spread so thin that you don’t get much photosynthesis.)

How tall a building are we talking about? To suspend 20 acres of growing area over 1 acre of land, your 200×200 ft square tower would need to be 4000 ft tall, more than three times the height of the Empire State Building, with the sun at 45 degrees, or 8000 ft (more than a mile) if you want to capture all that midsummer noonday sun. What if the building were only 100 feet square? Then you could get 20 floors (but only 5 acres) in a building 2000 (or 4000) ft tall. But, the shorter the building, the farther it needs to be from any other buildings that might shade it.

Either way, the building would cost much more than the 5 to 20-acre farm it is intended to replace. My brother makes a living from a 20-acre organic vegetable and fruit farm, but he’s not making payments on the world’s tallest building. Of course, you could put the floors much closer together if you used artificial light, but then we’re talking about lots of hardware and electricity. (The drawing the accompanies the article shows floors roughly twice as far apart as they are across, but uses half of that height for solar panels. Assuming 5% efficiency, these could provide enough electricity to do nothing much, again assuming no shading by nearby buildings.)

What about some other shape? A pyramid might be good. The outer edge of each floor would get natural light and could be used to grow crops, while the inside of the pyramid could be artificially-lit office space. You still couldn’t increase your growing area much above the footprint of the building, without the shading problem discussed above, but it might be pretty. I suspect, however, that the author has a different kind of pyramid in mind. He writes:

(Disclosure: I’ve started a business to build vertical farms.)

Let me guess: the business model doesn’t depend on income from selling farm products, but on franchise fees or grants from governments and foundations for “demonstration projects” that demonstrate how gullible they are.