Ecological characteristics influence farmer selection of on-farm plant density and bunch mass of low input East African highland banana (Musa spp.) cropping systems

East African highland bananas (Musa spp., AAA-EA group) are a primary food and cash crop for smallholders in Rwanda and much of the East African highlands. Their production generally declines over time due to poor farm management and declining soil fertility. Farmers believe that among the bunch mass maintaining factors, plant density management offers some prospect. They often decrease banana mat (i.e. a single mother plant with interconnected suckers) density in an effort to increase bunch size, but the effectiveness and profitability of this practice has not been studied. In addition, not much research has been executed on the influence of climatic and edaphic factors on variations in on-farm plant density. An on-farm survey was conducted in contrasting agro-ecological sites of Rwanda (Ruhengeri, Rusizi, Karongi, Butare, Ruhango, Kibungo and Bugesera) to determine existing densities and their relationship to bunch mass. A plant density assessment method was used that measures the average distance of five mats to their respective nearest four mats to calculate average mat spacing. Plant density positively correlated with surplus/deficit water supply (i.e. difference between rainfall and water demand by bananas) (r2 = 0.62), with highest plant densities (>1500 mats ha−1) found in high rainfall areas (>1200 mm yr−1) with water surplus (218–508 mm yr−1) and lowest plant densities (1000–1400 mats ha−1) found in lower rainfall areas (1000–1200 mm yr−1) with water deficit (from −223 to −119 mm yr−1). Heaviest bunches (18.1–20.8 kg fresh mass plant−1) were found at lowest plant densities and medium sized bunches (14.7–15.5 kg) at highest plant densities. Lower soil and banana leaf nutrient contents (especially N, K, Ca and Mg) were observed on weathered soils (Acrisols) and were associated with smaller bunch mass in comparison to fertile soils (Andosols, Nitisols). Farmers tended to reduce mat densities (i) if they wanted to intercrop bearing in mind site characteristics, and (ii) to increase bunch mass to adapt to market preferences. The plant densities generally recommended by extension bodies (3 m × 3 m or 2 m × 3 m; i.e. 1111 and 1666 mats ha−1, respectively) are seldom practiced by farmers, nor do they seem to be very appropriate, as higher densities seem productive in areas with high rainfall and relatively good soil fertility.

► On-farm banana plant density and bunch mass varied with agro-ecological classes.
► Variation in on-farm plant density was driven by water supply and soil fertility.
► Plant density positively correlated with surplus/deficit water supply (r2 = 0.62).
► Small sized bunches were observed on weathered in comparison to fertile soils.
► Farmer selection of plant density depended on intercrop or bunch size.