Floating gardens of Xochimilco, Mexico City

Mexico City is a very, very big city.

But in its south, there are farms on little islands. Farmers grow vegetables and flowers there. Canals of water go all around them.

Mexico City sits high in the Valley of Mexico, where a great lake called Texcoco once spread. The Aztec capital, Tenochtitlan, was built on an island in that lake. Later, most of the lake was drained, and the city grew over the valley.

About 28 kilometers south of the city center lies Xochimilco, where part of the old lake survives. Here, in a network of small canals, farmers still grow food on chinampas, long garden islands. Xochimilco is a borough of Mexico City, home to about 442,000 people in 2020.

UNESCO's 1987 World Heritage listing, Historic Centre of Mexico City and Xochimilco, joins two areas: the colonial city built over the ruins of Tenochtitlan, after the Spanish began draining the lake around it, and the lacustrine landscape of Xochimilco, 28 km to the south. There, on the edge of the residual lake of Xochimilco (the southern arm of the drained lake of Texcoco), UNESCO says some chinampas, or 'floating' gardens, can still be found, and parts of this half-natural, half-artificial landscape are an ecological reserve.

Xochimilco is one of Mexico City's territorial demarcations (boroughs), with 442,178 residents in the 2020 census (via Wikidata). UNESCO notes that the metropolitan area now fills nearly the whole valley and has entirely engulfed the remaining chinampas.

A chinampa is like a big, long garden bed in the water.

Farmers push in stakes made from willow branches. Then they fill the space with mud from the bottom of the lake and old plants. Water plants from the canals keep the soil rich.

Chinampas are called floating gardens. FAO describes how they are built: from mud from the lake bottom, branches and plants, inside a frame of stakes. First, farmers mark out the edges of a long, narrow plot. They cut big branches from ahuejote trees, a kind of native willow, to use as stakes.

Next, they fix a strong frame around the stakes. Then they fill it with two layers: one of plant material and one of mud. Farmers keep adding water plants, which feed the soil. Rows of ahuejote willows grow along the edges.

FAO describes chinampas as a kind of wetland raised-field agriculture: small islands in strips built with sediments from the lake bottom, branches and decaying vegetation, creating a web of channels. Its construction steps are: outlining the chinampa's perimeter with large cut ahuejote branches and stalks; building a robust structure fixed around the stakes; and forming the soil by filling it with two layers, one of organic matter and one of sludge.

Fertility is kept up by constant organic inputs, especially aquatic vegetation pulled from the canals. FAO notes that this also slows the eutrophication of the channels and lagoons, the build-up of nutrients that can choke still water. The knowledge is passed on orally in what FAO calls the chinampera culture, prevailing since Aztec times. UNESCO writes 'floating' gardens in quotation marks; FAO's description shows they are built up from sediment within staked edges.

Between the chinampas are canals full of water.

Canoes travel along them. Boats called trajineras carry people and goods too. The water also helps the plants grow.

The canals between the chinampas are part of the farm. They water the crops, and they are about 1.5 meters deep and 4 to 6 meters wide on average.

They are also roads. Canoes and flat boats called trajineras carry goods, farmers and visitors along them. In the rainy season, extra water from Mexico City is sent into the canals, so they help control floods. The water and the plants keep the air around southern Mexico City more humid.

FAO gives the irrigation channels an average depth of 1.5 m and widths of 4 to 6 m. They serve as transport routes for canoes and trajineras loaded with goods, farmers and tourists, and as flood control: in the rainy season, excess water diverted from Mexico City is held in them, so they work as regulating vessels.

The canals and wetlands, together with evapotranspiration from the vegetation, make the local microclimate of the city's south more humid, keep the soil more compact and reduce wind erosion. FAO describes the five chinampa zones as lying on permanent wetlands within a closed sub-basin and providing ecosystem services to the urban and peri-urban area.

Long ago, the main crop was corn.

Farmers also grew chiles, tomatoes and squash. Today they grow many more kinds of plants to sell in the big city.

For a long time, chinampas grew milpa: maize as the main crop, with Mesoamerican vegetables such as chile, tomato, squash and chilacayote, a kind of squash. Over the centuries, farmers added many other kinds of plants to sell in Mexico City's markets.

Chinampa farmers manage 51 domesticated plant species, according to FAO, plus 96 wild ones that they use, mostly as animal feed. Crops are planted together and rotated, and fields can be farmed all year.

FAO estimates that over the centuries the chinampas shifted from milpa production, with maize (Varadero chalqueño) as the main crop alongside chile, tomato, squash and chilacayote, to a diversified mix of species with different social and commercial value, adapted to the environment and to the demands of Mexico City's urban market.

Producers manage 51 domesticated species and 96 non-domesticated ones, used as fodder (67 percent), medicine (20 percent) and food (13 percent). Cultivation relies on agroforestry and intercropping, with multiple cropping and crop rotation, allowing intensive cultivation throughout the year. FAO also notes that the canals and ponds, often covered with aquatic plants, give shelter to birds and small animals.

The city has grown all around the chinampas.

People are working to keep the water clean. A rare animal called the axolotl lives in these canals. Clean water helps it too.

Mexico City has grown so big that it now surrounds the chinampas. UNESCO says the landscape has become vulnerable to environmental changes.

Xochimilco is also one of the last wild homes of the axolotl, a salamander that is critically endangered. The San Diego Zoo Wildlife Alliance describes projects that make refuges for axolotls by putting water filters in Xochimilco's canals, and says farmers are increasingly using traditional methods without pesticides or fertilizers. In 2017, FAO named the chinampa system a Globally Important Agricultural Heritage System.

UNESCO's criterion (v) describes Xochimilco's lacustrine landscape as vulnerable under the impact of environmental changes, and its integrity statement notes the metropolitan area has engulfed the chinampas. FAO has listed the Chinampas Agricultural System in Mexico City as a GIAHS since 2017, citing its productivity, ecological importance and long adaptation to hydrological constraints and the city's food demand.

The axolotl (Ambystoma mexicanum), critically endangered on the IUCN Red List, is native to lakes Xochimilco and Chalco. The San Diego Zoo Wildlife Alliance says its biggest threats are urbanization and pollution, and describes aquatic refuges made by installing water filters in Xochimilco's canals, alongside farmers increasingly embracing traditional practices free of pesticides and fertilizers.