Babylon had hanging gardens, Mexico had (and has) floating gardens, neither of which hung or floated, but I’ll allow poetic license.
Extending back into the pre-Columbian Aztec era, and probably earlier, Mexico’s ‘floating’ chinampas don’t float, but do provide a marvelous example of designing-in season extension for your garden plot. That’s right, the folks in pre-Spanish Mexico famous for making bloody sacrifices and living in the middle of a lake were good at growing vegetables outside the constraints of the normal weather patterns and on plots of land that hadn’t existed before.
How? Well, if you are living in the middle of a lake, land is at a premium; why not build some ‘islands’ on which to grow food? Chinampas. While this innovative and labor intensive practice was previously understood to benefit the crops (translation: “be worth the effort”) because the lake irrigated the crops from below, Jason Turner’s 1999 dissertation work at UT Austin suggests another totally distinct reason. According to Turner, the real benefit was the microclimate created by the lake, which kept the plants a little bit warmer than the surrounding land air temperatures.
Where are the chinampas?
Mexico City is in a basin suspended high above sea level between several of mountain chains. Higher altitudes are colder, and cold air sinks, so despite being in the tropics, all the crop land in the basin housing Tenochtitlan (pre-Spanish Mexico City) is vulnerable to low temperatures (frosts on rare occasion, but a lot of tropical crops are ‘annoyed’ earlier than that.) Large bodies of water (Lake Texcoco, in this case) ameliorate this effect, which is why Burlington, Vermont, sitting just east of Lake Champlain, is distinctly warmer than Montpelier, Vermont, further south but miles inland.
Did the Aztecs (or the pre-Aztec culture, which may very well have started this technology) know about lake effects when they started building the chinampas? Well duh. They were living on an island in the middle of a huge lake/chain of lakes. (After the rainy season: one lake. After the dry season: 5 lakes.)
Those chinampas that still exist are largely in Lake Xochimilco (zoe-chee-mill-coe), which sat south of Lake Texcoco but connected to it. Xochimilco was south of the astonishing mid-lake dyke (just under 10 miles [15km] long) that the Aztecs built in the 1400s. The dam had the effect of controlling the seasonal flooding a bit and of decreasing the salinization of the southern portion of the lake. The northern portion got saltier, so fewer agricultural endeavors happened that direction.
How are chinampas built?
Lake Texcoco (et al) was not a deeply cleft mountain lake, it was a broad, shallow affair with lots of marshland. Marshes grow in 3 feet (1 m) or less of water, so not deep.
One version of how to build a chinampa: walk out into the marsh, pounding in some serious poles along the way. After about 300’, turn right, pound in poles for another 20’, turn right again, and pound poles the 300’ back. A little zip across to the starting pole, et voila! A 300 x 20’ rectangle. If the goal were just to create land, a large square chinampa would make more sense. Clearly the adjacency of the water and land was key.
Now weave reeds and vines and ropes and whatnot to make a huge mat. Huge! Use stones to sink the mat in the middle of the poles. Use more reeds and vines and ropes and whatnot to weave between the poles. Now there’s an oversized basket of sorts out in the marsh. That ‘basket’ gets filled with anything and everything: marsh bottom mud, human manure (Meso-America is the only site where the independent development of agriculture did not include domesticating animals), composted vegetation, straw, sticks, stones, and more dirt. In the end, the pile of dirt sticks up out of the lake bed not too far- maybe a foot or two.
There’s one more chore before the chinampas is ready for planting: willows. Willows are the sort of tree where a single stick can be poked into wet dirt and then BAM! There’s a new willow tree. In order to create a root system that could hold the chinampas together and provide lasting support, a tall, columnar variety of willow was “planted” (jammed into the dirt) along the edges. Once the willows are in, the chinampas is ready for plowing, a goat, a small house… really, whatever you want. And can fit on your canoe.
Set side by side by side, each chinampa is only accessible by canoe. This was not a novelty item- Mexico City used to be like Venice with some ‘streets’ actually being waterways. In chinampas territory, the canals for the canoes are long and quietly urban in their grid. When I visited Xochimilco, the chinampas were far higher or rather the water was far lower. This was unusual, but exceedingly interesting to experience.
In their heyday, the chinampa system provided 2/3 of the food for a bustling city of 200,000. Think we could do that again?