Trenches combined with living hedges or grassed lines are less labour-intensive method that is practiced in the highlands of Rwanda. The method involves digging trenches which have grass-stabilized banks, or simply planting grass strips and vegetative barriers across the slope to reduce runoff. This method, locally referred to as ‘progressive terracing’, is more adaptable by individual farmers across the country. In the study conducted by Kagabo et al. (2013) in the highlands of Buberuka Region of Rwanda, it was noted that net increment in yields and returns on investment increased with the age of terraces, since the capital costs of terracing were easily recovered.
These terraces also showed more resilience, some being over 30 years old and still effective. Along the contour band, soil from the upper parts of the slope is removed and deposited above by creating a series of discontinuous trenches in order to extend the flat terrain. Over 5-10 years, the terraces become enlarged and form a terrace along the contour, such that the slopes are transformed over time into level bench terraces (but never %).
The main purpose of this technology is to reduce runoff and soil erosion on the slope and to improve soil quality and soil moisture retention. Grass strips or living hedgerows are intended to both trap sediments and facilitate the slow formation of terraces.
The establishment of trenches combined with living hedges or grassed lines is less labor intensive and requires less skilled labor. Regular maintenance banks on which living hedges are planted, to seasonally stream living hedges to minimize the
competition of nutrients and water with plants. To efficiently use these terraces it is advisable to introduce intensive and rentable cropping systems or agroforestry with fruits and forage trees.
This technology was reported to be very resilient as trenches combined with living hedges or grassed lines of 20+ year old are still effective in controlling soil erosion. However, the soil quality on these formed terraces between alleys does not
homogenize 100% over the course of time. Large soil fertility gradients with marked spatial difference in both soil quality and crop yield from their upper parts downwards of the terraces is observed. The soil in the lower parts of the terraces showed as much as 57% more organic carbon content and 31% more available phosphorous than the soil in the upper parts of the terraces (Kagabo et al. 2013).