As a plant, tea was discovered under the great canopy of forest as an under storied plant species and was believed to be able to thrive well under shade. From this basic knowledge it was assumed that tea plants are grown ideally in shade environment. Most of the tea plantations therefore maintain a partial shade condition. From the beginning of the tea cultivation in Assam Sau tree (Albizzia chinensis) was grown in tea plantations and considered as tea fertilizing tree. During the early period, tea cultivation in Assam was carried out clearing the forest where there was abundance of Albizzia chinensis trees. Some Albizzia tree were also retained in the new clearings with a feeling that a canopy of forest trees might be beneficial for the growth of tea bushes, particularly of bushes of Assam race. Thus, planting of shade trees among tea bushes became a practice in the plains of N.E.India. Albizzia chinensis which is a legumewas the first tree to be used for shading tree. Other leguminous species such as Albizzia odoratissima, Dalbergia assamica, Erythrina indica etc. were also introduced as shade trees.
From the very beginning and more particularly by the first quarter of the last century, most tea planters appear to have accepted shade as a regular feature of tea estates in N.E. India and a tradition of growing shade trees for successful tea culture was build up.
Spread of Shade-Tree Culture
The shade-tree culture of N.E. India spread to other tea growing countries and regions. Sri Lanka, Indonesia and South India also started planting shade trees in tea plantations. When tea cultivation was started in Africa, planters followed the practice of old tea growing countries and started planting shade trees, particularly in the warmer belts.
In South India and Sri Lanka, the species of shade trees used and the method of their planting are quite different. They use extensively the non-leguminous Grevillia robusta along with some leguminous genera like Erythrina and Gliricidia. These species provide a lower canopy of shade and it is mainly used as a provider of mulch. These trees are lopped two or three times in a year and loppings are used as mulch. Unlike N.E. India, they do not keep the shade trees continuously. In N.E. India shade trees are maintained to provide more or less a continuous cover for the tea bushes at a height of 6-8 m above allowing sufficient light to filter through the feathery foliage of shade trees. Thus, the term shade tree as used in Sri Lanka or South India is a misnomer. It may rather be described as ‘green manure tree’.
In African countries, Grevillia robusta is the most common species although other species like Albizzia gummifera, A.adianthifolia, Gliricidia maculate are grown.
In the northern latitudes of Russia, Japan, Iran and other countries where temperature drops very low during winter, shade trees are not grown. Shade trees are also considered unnecessary in tropical and sub-tropical belts at higher elevations where the temperature remains low.