On Mt. Makiling near the University of the Philippines at Los Banos, farmers plant Leucaena leucocephala first to shade out the noxious grass, Imperata cylindrica. When the grass is eradicated they plant yams near the base of the Leucaena. In their initial stage of growth yams do better under partial shade. The canopy of the Leucaena promotes the early growth of these yams. When the yam begins to convert energy to tuber growth, bark is removed from around the trunk of the Leucaena in a band at about 40 cm from the ground. This kills the canopy of the living tree, causing the leaves fall to the ground. The leaves increase the organic content of the soil, and full sunlight is available to the yam vines. The stem provides a sturdy livng pole to support the yam. Girdling the trees requires much less labor than felling trees and planting poles. When girdled near the ground, regrowth will not shade the yams.
Beneath the girdle, the Leucaena copices, providing material for mulch or fodder. One strong sucker can be left to produce another tree. After the yams have been harvested, the sucker is saved for planting, and the remainder of the pole can be cut and used as firewood or construction material.
In the Philippines the yam pole is a part of a complex system which includes the intercroping of onions, garlic and squash which diversify the farm's production while minimizing insect and disease problems. The use of this tree as a support pole is only one of the tree's many functions: its deep tap root system draws up nutrients from the subsoil; its leaves add organic matter to the soil; and the tree's nitrogen fixing ability improves soil fertility. The use of Leucaena as yam poles and its interaction within the system has created some of the most productive hillside farms in the Philippines. Leucaena trees are not the only potential living yam pole tree, other nitrogen-fixing and fast-growing trees such as Gliricidia and Calliandria would probably work as well.
Note: We would appreciate hearing from any of our readers who have recent experience with such techniques as the one described in this article.
For a copy of the USAID Paper on Living Yam Poles write to:
Michael D. Benge
Rm. 515-D, SA-18
Agency for International Development
Washington, D.C. 20523