Investigating Chinampa Farming
How the Aztec Empire fed the burgeoning population of its capital, Tenochtitlan, has long intrigued researchers. Most of Tenochtitlans estimated 150,000 to 200,000 inhabitants at the time of Spanish contact were not food producers. The system, known as chinampas, of draining swamps and building up fields in the shallow Basin of Mexico lakebeds, was a remarkable form of intensive agriculture that Jeffrey Parsons of the University of Michigan suggests provided one-half to two-thirds of the food consumed in Tenochtitlan.
At the time of Spanish contact, shallow lakes covered approximately 1000 km2 of the Basin of Mexico. Archaeological surveys show that large expanses of the lakes were converted into chinampas. Today the lakes are almost completely drained and covered by urban growth, but a few pockets of chinampa agriculture survive, including the popular tourist attraction, the Floating Gardens of Xochimilco.
Several ethnohistoric and ethnographic sources describe a range of building techniques, agricultural practices, and crops grown on chinampas. In general, chinampas are rectangular fields 2 to 4 m wide and 20 to 40 m long, surrounded on three or four sides by canals. Chinampa farmers pile up layers of vegetation and mud or dirt to raise the field surface to about 1 m above the water level.
This type of construction and the labor-intensive methods of chinampa agriculture help overcome the main limits to agriculture in the Basin of Mexico: variable rainfall, frosts, and soil fertility. The proximity of the field surface to the water table usually provides adequate soil moisture for crops. If not, irrigation water is readily available in the canals. The water also ameliorates nighttime temperatures, reducing the chance of frosts. In the past, soil fertility was maintained by periodically adding vegetation, household refuse, and organic-rich silt dredged up from the canals to the field surface. Among their labor-intensive practices, chinampa farmers plant seeds in specially prepared seedbeds, later transplanting the seedlings into the fields. In its most intensive form, cultivation is yearround.
We know little about the origins and development of chinampa agriculture. Although some researchers suggest that chinampa agriculture began as early as the Formative period, around 1400 bc, no fields have been securely dated prior to the Early Aztec/Middle Postclassic period (ad 11501350). In 1981, Jeffrey Parsons directed excavations at several archaeological sites in the Chalco and Xochimilco lakebeds to explore the development of chinampa agriculture.
My research uses plant remains excavated from an Early Aztec site (Ch-Az-195) to address the issue of the development of chinampa agriculture and, more specifically, to address questions about the economy and land use at what, from its location in the lakebed, appears to be a small hamlet associated with chinampa farming.
Ch-Az-195 is a small mound that lies approximately 1 km southeast of Xico Island and 1.8 km from the eastern shore of Lake Chalco. Eight small mounds on top may be households and one larger rectangular mound may be a public building. One excavation unit revealed remarkably rich and finely stratified deposits that extended down 370 cm before hitting the high water table. This area of the site served at different times as a midden of domestic refuse, an area of fill, and a habitation or patio area. The unusually wet conditions in the lower two-thirds of this excavation unit prevented the decay of plant matter and preserved a rich array of plant remains.
These plants represent a number of activities at the site: food production and preparation, collection of useful weedy plants, and craft production. Food plants include the major cultigens (maize, beans, squash, and chile), all of which are recorded as growing in chinampas. Fruit trees are represented by the hard pits of Mexican cherry, Mexican hawthorn, and prickly pear. These generally grow on the Piedmont slopes ringing the lakes, although some may have grown on Xico Island. The abundance of seeds from plants exploited for their edible green leaves (quelites) show that these plants were readily available and probably eaten. These grow as field weeds on chinampas. Many of the plants recovered from Ch-Az-195 have recorded medicinal uses. Various grasses and bulrush were used for baskets, mats, and other items. Cotton (the only plant found at the site that does not grow in the Basin of Mexico) and maguey were used for clothing. The charcoalprimarily oak and pine collected from the Piedmont forestsrepresents wood that served as fuel, building material, and material for canoes and tools.
The excellent preservation at Ch-Az-195 provides information on plant use and farming activities over the approximately two hundredyear span of the occupation. The plant remains suggest it was a farming site rather than a mound built in the lakebed solely for fishing or mat making. Chinampa crops occur in all levels, and the presence of maize stalks suggests that at least some maize was grown nearby and not imported as cobs or kernels to the site. In addition, the abundance of seeds from edible greens from the lowest levels of the site implies that these common field weeds grew on fields near the settlement or on the site. Not all of the plants, however, were necessarily grown on nearby chinampas. Perennial fruits, and perhaps even some crops, came from the lakeshore, the Piedmont slopes, or Xico Island. It is unclear to what extent the Ch-Az-195 farmers cultivated fields in these zones, collected in these zones, and/or traded for produce from these zones. Some evidence points to increased intensification of agriculture and a decrease in the use of Piedmont resources over the span of the site.
My research examines the diversity of plant use and farming at chinampa settlements. The data from Ch-Az-195, however, are not representative of the entire chinampa system. Plant use and agriculture will vary with local environmental, economic, social, and political conditions and with the population characteristics of a particular settlement. To document and understand this variation, I am planning additional excavations from other locations and time periods.
Virginia Popper is director of the Paleoethnobotany Lab at the Cotsen Institute of Archaeology at UCLA. Backdirt editors can be reached at firstname.lastname@example.org