Tropical High-Altitude Growing Conditions

TROPICAL HIGH-ALTITUDE GROWING CONDITIONS seem quite similar to the temperate-zone summer growing season. North American workers in the lowland tropics expect to “relearn” agriculture in their new settings, since the climates and crops of the lowland tropics are so different from temperate regions. But at higher elevations in the tropics, where cooler temperatures favor Brassicas, apples, and other familiar “temperate” plants over “tropical” crops, they feel right at home–but should they? We asked our network to identify unique factors in growing food at high altitudes in the tropics.

Latitude (distance from the equator) is very important when discussing altitude (height above sea level). While “high elevation” crops may be found at 1500 m in Central America at 15°N, the same plants may start growing at a much higher altitude in Ecuador at 0° latitude. You must adjust for your latitude when reading “altitude limitations” given for plants, such as tree species recommended for reforestation in a certain area.

Microclimates and climatic instability are very important aspects of growing food in the highlands. The complex ecological mosaic of mountain ranges and valleys determines wind and rain patterns, leads to marked temperature differences within a small area, and creates many microclimates which farmers can identify and use in their planning. The topography also produces wide variation in soil type and fertility and phenomena such as localized frost pockets. Paul Bueker in Ecuador writes that “microclimates are somewhat unstable over time. Rainy and dry seasons, hail, frost, and pest cycles are harder to predict than in temperate climates. There is an extra degree of randomness in the growing conditions and therefore less certainty for agricultural investment. The variations are not extreme, but they are sufficient to cause real problems in agriculture.” Farmers in the Andean highlands cope with this variability and uncertainty by staggering plantings through time, planting in several plots at different altitudes, and maintaining a diversity of crops.

Alfredo Petrov works with gardening and reforestation in Bolivia between 2700 m and 4100 m (at about 18°S). “At high altitudes, our sunlight is extremely intense: the atmosphere is thinner and so absorbs less of the shorter wavelengths like UV, and the sun angle is higher [than in the temperate zone], resulting in less shade, higher soil surface temperatures, and more evaporation. The intense sunlight is not a problem in temperate lowlands, regardless of temperature. The sun is great for solar energy, but young seedlings can get fried by the burning light. Two things that help combat this are artificial semi-shade and walls or thick hedges to reduce wind and evaporation. We use a black net for partial shade in our tree nurseries; results were poor before we started using this. Even native tree seedlings in the wild have a much higher mortality out in the open than in the shade of their mother tree. It was found in the high-altitude deserts of California and Arizona that the most important factor in growth of cultivated plants is walls for wind protection.”

Intense sunlight and other climatic factors can result in extreme weathering of exposed soils in the highlands. Bare soil at high altitudes can quickly lose its fertility. Hillside fields are also particularly vulnerable to erosion, and heavy rains and strong winds can quickly carry the soil off a slope. Soil protection and conservation techniques, such as creating living barriers along contour lines or planting trees, may be priorities in mountainous areas. Windbreaks and soil cover are more critical in the tropics than in temperate areas which have snow cover for several months of the year.

Many workers new to the tropical highlands are thrilled to discover a very pleasant climate with temperatures that could produce food year-round (with sufficient water, which is often a seasonal problem). They discover that crop yields are not directly comparable to temperate areas. Short days result in less photosynthesis, and together with the relatively low average temperatures, the same crops may take much longer to mature at high elevations in the tropics than in temperate areas. Alfredo Petrov notes that vegetables do not reach the sizes they do at higher latitudes, even with optimal fertilizer and watering. (At the other extreme, huge vegetables are grown in Alaska.)

Some crops require long or short days to flower, so plants must be selected for their adaptation to tropical daylengths and grown in the proper window for seed production. Plants which can flower regardless of the hours of light are called ‘day-neutral,’ such as tomatoes, peppers, eggplants, sweet peas, artichokes, and most cucurbits. Photoperiodsensitive plants are classified as long-day or short-day. Long-day plants are those which flower at some time during the long days (in reality, short nights), including spinach, sugar beets, radishes, Chinese cabbage, and most onions and carrots of temperate origin. Short-day plants only flower and set seed during short days. Examples include pigeon peas, chayote, roselle, amaranth, jícama, and most winged beans. (Daylength also affects plant growth responses other than flowering. For example, short days favor root and tuber growth in many tropical crops: white and sweet potatoes, taro, yam, jícama, Jerusalem artichoke, and cassava.)

In addition, many temperate plants are not adapted to the daily temperature extremes common in the highlands. Alfredo Petrov tells of a treeless Bolivian village at 4000 m in which a Canadian student initiated a tree-planting project. Since winter night temperatures sink to -20° C in that area, the student tested Siberian elm trees, which withstand temperatures even lower than -20° C in their native range. However, the village’s temperature usually reaches +20° C by 11 AM. All the trees died, perhaps due to the temperature fluctuations which never allowed them to go dormant.

Many traditional highland foods, such as quinoa in the Andes and buckwheat in Asia, are well adapted to high altitude conditions and are exceptionally nutritious. However, the temperatures may simply be too cold at very high altitudes to grow the variety and quantity of foods needed for optimal nutrition. In addressing the cold limitations, Food for the Hungry-Bolivia has had great success with mini-greenhouses in the highlands at 3000 - 4200 m. Greenhouses enable people to grow vegetables and other crops under plastic where previously there was little food production, despite the abundant sunlight. In Ecuador at 2900 m, I [LSM] found that the biointensive vegetable gardens, which were planted in beds filled 1 m deep with undecomposed organic material, did not show the frost damage evident in adjacent gardens planted in normal soil. The heat generated by the 'composting’ in the beds apparently protected the plants from cold damage.

Most highland crops do not thrive in ECHO’s climate, so we do not specialize in crops suitable for cooler, highland areas. A university in a highland zone may provide good information on plants adapted to your conditions. At present, we do not know of any one organization which connects people working in highland agriculture worldwide, although there are many interested individuals. ECHO is building a list of resource groups for people working in highland regions; if you work in the mountains, please write us with information about your organization and services.

The seed company High Altitude Gardens [online via http://secure.seedstrust.com/] specializes in frost tolerant, quick-maturing varieties which do well in cold climates. The cover crops resource CIDICCO has a bulletin on “Using Legumes in Traditional High Altitude Farming Systems”. Online at http://www.cidicco.hn/. 

A great introduction to crops adapted to the highlands is Lost Crops of the Incas (EDN 29-1). It is out of print, but is available to read online at http://www.nap.edu/openbook.php?isbn=030904264X