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In relation to a previous question: Would a medieval Arcology be possible? we established that without a water source it would not be posible to have a society.

We also established that we need a certain number of square miles per person to be able to sustain food production: How many people can you feed per square-kilometer of farmland?

Pack-animals used for travel between cities can be used for food source when an animal is no longer able to make the travel. This amount of meat can be a significantpart of the food intake.

The question: Would it be possible to have sufficient food production in a city, locked inside walls, given that every possible surface such as roofs, eg. tomato pots in windows, and basically every free surface was used for some kind of food production.

Archimedes screws or similar that are hand powered (maybe from child labor), to supply water to the roofs, and people are carrying buckets with water around the city, constantly watering.

Assume the cities inside the walls have houses so close, there is no room for gardens.

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  • $\begingroup$ Medieval times considered tomato's poison, so no to tomato pots :) This city would be relatively immune to the usual long term siege methodology of medieval armies, so it'd be highly desirable...can't find any historic precedence for you though, farming techniques don't give the returns you need at this time in human development. $\endgroup$ – Twelfth Apr 27 '15 at 17:19
  • $\begingroup$ Tomatos were not considered poison. That is a myth. They spread rapidly through Europe after being introduced from the Americas in the 1500s. $\endgroup$ – JDługosz Nov 7 '16 at 10:38
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The answers to "How many people can you feed per square-kilometer of farmland?" come to different conclusions, but they all are in the range of several hundred to 1000 m² to feed one person. Modern (!) hydroponics might reduce that to a 3rd. So let's be really, really optimistic and assume 100m².

How much roof area you have per person can easily be measured by how much living-space you have per person.

My apartment is 60 m². I live in a multi-story apartment building, but buildings with more than 2 floors were quite rare in the middle-age. So the roof area available for me would also be about 60 m².

60 m² per person would be quite decadent for the middle ages. It is even quite decadent for todays standards, because I am living alone. Even today a four-person family living in the city rarely has an apartment with more than 100 m². I would rather estimate the floor-area and by induction also the roof-area per person in a medieval town to be a lot less, more in the area of 20 m².

Sure, we excluded any public buildings, but we also did not take residential buildings with more than one floor into account, so these two factors should cancel out.

20 m² is simply not enough to feed the person living below it, even with high-tech methods, let alone medieval methods. The only way to increase the output would be with multi-story farming and artificial light sources, but the technology to create artificial UV light didn't exist back then.

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Hydroponics are a mostly modern technology precisely due to the light source requirements.

How advanced is your glassmaking industry? There could be a slight alternative there: plants can grow off mirrored light as long as the mirrors continue to reflect UV (most do), though you would need someone to adjust their facing constantly throughout the day. Mirrorkeepers would become an important position. However, the benefit is that you could (in theory), have a few large shafts with skylights that channel it underground to your mirrors. And then you have a weird-ass subterrean farm.

Mirrorcrafting for best effect and economy requires Renaissance level tech, though (for making modern plate-glass type mirrors, in any case), though you can have mirrors way before then, they're expensive and relegated to the rich usually in this period since they tend to be solid glass and silver.

However, if you're willing to allow this, you can combine it with cistern tech (with said cistern being fed by an aquaduct from somewhere, or you can simply fill it by capturing rain and aquifer water) to water your place and provide the water for the actual hydroponics. This, in combination with your rooftop gardens should be sufficient; though ideally there should be a secondary food source as well (draft animals don't tend to be great meals as they're bred for muscle rather than meat).

Its secondary; but this type of construction would also have the benefit of attracting some tourism due to it probably being unique ("You should see the underground gardens of X!").

As a further aside: what does your desert city export that brings the caravans to visit in the first place? They need some reason to bring stuff.

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One possibility not considered by those answering above is the use of vine agriculture. May it be possible to turn the vertical surfaces of buildings into productive surfaces. It would probably not greatly increase yields, but it would help. Green houses with multiple levels, thus allowing for tiered growing would, strictly speaking, have been possible, though glass was hard to make in large quantities. Insects and birds are other sources of food that could be available, though great quantities would be needed to make a significant impact on the caloric needs of a city.

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As an amateur gardener: while my limited hydroponics system uses a solar pump, using gravity feeds and screws would work. You only actually need to get the water to the top of the system a couple of times a day.

I'd wonder about the process by which a "purely" hydroponics system would get nutrients, though. Composting something out that would be chemically suitable as plant food and water soluble is probably complicated. (I buy mine in a jug at the store…)

Perhaps a more reasonable system might be raised bed gardening using tiers of soil with drip irrigation or clay pipes? (Saturating clay pipes underground leaches sufficient water into the soil through osmosis without as much evaporation as surface watering.)

There are great examples of tiering like this. I love the system where square planters are stacked at 45° rotation on each tier, leaving triangles for planting — you can load those up with, say, spinach or cabbages and stack them practically as high as you like, a couple of feet apart. Root vegetables can grow in trays stacked with a little more clearance.

One can pack in production by interplanting (pumpkins below, corn stalks with pea vines climbing them), layering them (squash vines tied overhead, spinach pots beneath them, watermelon vines down below) and the like.

Given access to New World and Far Eastern plants and an understanding of which plants "go together" (eg, don't over compete for the same nutrients), there really isn't much that a Victory Garden (Americanism for "home/personal vegetable garden") uses that requires much modern technology. The main production advantages at a small scale that we have over medieval farmers are consistent access to water (plants grow more "optimistic" when routinely watered as opposed to relying on weather) and being able to Google for what-goes-where.

If you're concerned more about sheer survival than variety in cooking, a few core crops (eg, potatoes, soy, rice, or amaranth as a protein base, with a selection of specific vegetables or fruits for minor nutrients, say, oranges, carrots, spinach) can be chosen to sustain a population on very little roof/balcony space. Supplementing those with medicinal/augmentation herbs (garlic, aloe, chillies, chamomile) wouldn't be too difficult.

If you want a less Vegan populace, rat, squirrel, rabbit, quail, peafowl, and chickens are all subject to being raised in inhumane stacked warrens and fed things that are indigestible by humans.

Also, there are many mushrooms that are edible and nutritious, and could be farmed indoors/in basements.

Of course, if this is European, so you'd have the problem of cold winters and food storage as well…

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