1 Introduction

Onion (Allium cepa L., family Alliaceae) is one of the most important vegetable crops cultivated worldwide because of its high economic value and widespread use in household consumption and the food industry (Griffiths et al., 2002; Khokhar, 2014). Global onion production is close to 100 million tonnes annually, with South and Southeast Asia accounting for a significant share (FAOSTAT, 2023). In Nepal, onion is grown on approximately 19,346 hectares, producing about 334,047 metric tonnes each year. However, the average productivity remains around 17-18 t/ha, which is lower than the global average of 20-22 t/ha (MoALD, 2023). Improving onion productivity is therefore an important priority for meeting domestic demand and reducing dependence on imports.

One of the major factors limiting onion yield is its shallow and fibrous root system, which makes thef20 crop highly sensitive to soil moisture stress and temperature fluctuations (Mutetwa and Mtaita, 2014). In the tropical Terai region of Nepal, high temperatures during the bulbing stage increase soil water loss, reduce nutrient uptake, and ultimately restrict bulb development (Brewster, 2008). Since many smallholder farmers have limited access to reliable irrigation, practices that conserve soil moisture are essential for sustaining onion production.

Mulching is an effective agronomic practice for improving the soil environment. By covering the soil surface, mulch reduces evaporation, moderates soil temperature, suppresses weed growth, and minimizes soil erosion (Bhardwaj, 2013; El-Beltagi et al., 2022). Organic mulches, such as rice straw, banana leaves, and sawdust, also improve soil structure and fertility as they decompose, whereas black polyethylene mulch provides greater short-term moisture conservation and soil warming (Ham et al., 1993; Lamont, 2005; Ramakrishna et al., 2006; Nair and Ngouajio, 2012).

Although the benefits of mulching have been reported for several vegetable crops, comparative studies evaluating different mulching materials for onion under the Terai conditions of Nepal are still limited. Most previous research has focused on crops such as garlic, potato, and leafy vegetables, with little information on the performance of locally available organic mulches compared with black polyethylene under the same field conditions (Tarara, 2000; Yimer, 2020). Therefore, this study was undertaken to evaluate the effect of different mulching materials on the growth and yield of onion under the agro-climatic conditions of Nepal's Terai region.

This study was therefore designed to: (i) evaluate the effects of four mulching materials and an unmulched control on growth dynamics across four developmental stages; (ii) assess their influence on bulb yield components and total fresh yield; and (iii) identify the most suitable mulching strategy for onion production in Bardiya District, Nepal.

2 Materials and Methods

2.1 Experimental site

The experiment was conducted at Shree Janata Secondary School, Bansgadhi Municipality, Bardiya District, Lumbini Province, Nepal (28.26° N, 81.52° E; approximately 150 m above sea level) during the rabi season from November 2024 to April 2025. The site has a subtropical monsoon climate with an average annual rainfall of approximately 1,600-1,900 mm, most of which occurs between June and September. During the crop-growing period, the mean monthly temperature ranged from approximately 11 °C in December to 35 °C in April.

The experimental field consisted of sandy loam soil with good drainage and a slightly acidic to neutral reaction (pH 6.0-7.2), which is considered suitable for onion cultivation. Based on the regional soil characteristics of Bansgadhi, Bardiya, the cultivated soils generally contain 1.5%-3.0% soil organic matter, 150-300 kg/ha available nitrogen, 15-35 kg/ha available phosphorus, 180-350 kg/ha available potassium, and have a bulk density of 1.3-1.6 g/cm³. These soil properties indicate moderate fertility suitable for vegetable production. The experimental field had previously been cultivated with seasonal cereal crops and had not received any mulch treatment before the establishment of the experiment.

2.2 Experimental design and treatments

The experiment was laid out in a Randomized Complete Block Design (RCBD) with five treatments and three replications, resulting in fifteen experimental plots, each measuring 5.0 m².

The treatments were as follows: T₁: Control (no mulch); T₂: Black polyethylene mulch (25 μm thickness); T₃: Rice straw mulch (5 cm thick); T₄: Banana leaf mulch (5 cm thick); T₅: Sawdust mulch (5 cm thick).

The organic mulching materials were collected locally and applied immediately after transplanting to maintain a uniform layer over the soil surface. Black polyethylene mulch was perforated at the designated planting positions before transplanting.

Forty-five-day-old seedlings of the locally grown onion cultivar 'Nasik Red' were transplanted on 20 November 2024 at a spacing of 15 cm × 10 cm (between rows × within rows). An inter-plot spacing of 0.5 m and an inter-block spacing of 1.0 m were maintained throughout the experiment.

2.3 Crop management

The field was ploughed twice to a depth of approximately 20 cm, followed by harrowing and leveling seven days before transplanting. Fertilizers were applied at the recommended rate of 80:60:60 kg N:P₂O₅:K₂O/ha using urea, diammonium phosphate (DAP), and muriate of potash (MOP). The full doses of phosphorus and potassium and 50% of the nitrogen were applied as basal fertilizer at transplanting, while the remaining nitrogen was top-dressed in two equal splits at three and six weeks after transplanting (WAT).

Flood irrigation was used throughout the experiment. All plots received the same amount of irrigation water at five-day intervals to ensure uniform water availability among treatments. Soil moisture was not monitored using soil moisture sensors, tensiometers, or the gravimetric method; therefore, the effects of mulching on soil moisture were inferred from crop growth and yield responses rather than direct soil moisture measurements.

Manual weeding was carried out at three and six weeks after transplanting in the control plots, whereas no additional weeding was required in the mulched plots because the mulch effectively suppressed weed growth.

2.4 Observations and measurements

Five representative plants were randomly selected from the central two rows of each plot and tagged after transplanting for recording growth observations. Plant height (measured from the soil surface to the tip of the longest leaf), leaf length (base to tip of the longest fully expanded leaf), and the number of leaves per plant were recorded at 30, 60, 90, and 120 days after transplanting (DAT).

At harvest, stem diameter (measured 2 cm above the bulb neck), bulb fresh weight, bulb length, equatorial bulb diameter, and root length were measured using a digital Vernier caliper (±0.01 mm) and a digital balance (±0.1 g). Total bulb yield was determined from the fresh weight of all harvested bulbs in each plot and converted to tonnes per hectare (t/ha).

2.5 Statistical analysis

The recorded data were subjected to analysis of variance (ANOVA) appropriate for a Randomized Complete Block Design (RCBD) using RStudio version 4.3.1. Before conducting ANOVA, the assumptions of normality and homogeneity of variance were verified using the Shapiro-Wilk and Levene's tests, respectively. Treatment means were compared using Duncan's Multiple Range Test (DMRT) at the 5% level of significance. The coefficient of variation (CV%) was calculated to assess the precision of the experiment.

3 Results and Analysis

3.1 Plant height

Plant height increased progressively across all treatments from 30 to 90 DAT, with a characteristic decline at 120 DAT reflecting assimilate remobilization from foliar tissues to the developing bulb during senescence (Brewster, 2008). No significant treatment differences were detected at 30 DAT (Table 1). Significant effects were evident from 60 DAT onward.

Rice straw mulch recorded the tallest plants at 60 DAT (44.58 cm), 90 DAT (67.93 cm), and 120 DAT (63.41 cm), closely followed by plastic mulch (44.07, 65.28, and 61.83 cm, respectively). The control consistently produced the shortest plants across all stages. The superiority of rice straw mulch for plant height is attributable to its dual role in moisture retention and progressive release of nitrogen and other nutrients as decomposition proceeds, augmenting the inorganic fertilizer program (Baten et al., 1995; Ramakrishna et al., 2006). These findings are consistent with Islam et al. (2007) and Mutetwa and Mtaita (2014), who documented superior plant height responses under straw mulch compared with bare soil in Allium crops.

3.2 Number of leaves per plant

Leaf count per plant was significantly influenced by mulching at all observation stages (Table 2). Plastic mulch consistently recorded the greatest leaf numbers: 4.73 (30 DAT), 7.00 (60 DAT), 9.27 (90 DAT), and 9.00 (120 DAT). The control produced the fewest leaves at every stage. Plastic mulch maintains stable, slightly elevated soil temperatures that accelerate meristematic activity and shorten plastochron intervals, thereby increasing leaf production rates (Tarara, 2000; Lamont, 2005). Rice straw mulch was statistically comparable to plastic mulch at 90 and 120 DAT, suggesting its nutrient-release profile increasingly supports rapid leaf development as the crop approaches the grand growth phase. These trends mirror findings of Kabir et al. (2013), who reported plastic mulch superiority for early-stage leaf development with straw mulch increasingly competitive at later stages.

3.3 Leaf length

Leaf length increased from transplanting through 90 DAT before declining at 120 DAT as assimilates were redirected to bulb storage organs (Table 3). Rice straw mulch consistently produced the longest leaves at all stages (25.88, 46.32, 58.14, and 42.37 cm at 30, 60, 90, and 120 DAT). Plastic mulch was statistically at par at all stages. The enhancement under rice straw mulch is attributed to improved nitrogen availability from mineralizing organic matter, which elevates leaf chlorophyll content and photosynthetic efficiency, sustaining leaf expansion (Baten et al., 1995; Nair and Ngouajio, 2012). The control consistently produced the shortest leaves, reflecting competitive water stress and nutrient immobilization associated with soil crusting in unmulched plots.

3.4 Yield and yield-associated parameters

All yield parameters except root length were significantly affected by mulching treatments (Table 4). The absence of a statistically significant mulch effect on root length (p > 0.05) likely reflects high intra-block variability and the sensitivity of onion root architecture to micro-scale irrigation uniformity (Larkin, 2020).

3.4.1 Stem diameter

Plastic mulch produced the widest stem diameter (1.41 cm), followed by rice straw mulch (1.28 cm), while the control recorded the smallest (0.89 cm). Pseudostem girth is closely correlated with overall plant vigor, vascular bundle density, and capacity for water and solute transport to the bulb (Ahmad et al., 2022). Banana leaf and sawdust mulches produced intermediate values (1.11 and 1.06 cm, respectively), reflecting their lower thermal insulation and slower nutrient contribution relative to plastic and straw mulches.

3.4.2 Bulb weight

Fresh bulb weight differed significantly among treatments (p < 0.001). Plastic mulch achieved the highest value (111.47 g), followed by rice straw mulch (95.84 g), banana leaf mulch (80.93 g), sawdust mulch (76.58 g), and the control (61.72 g). The 80.7% increase in bulb weight under plastic mulch relative to the control underscores the critical importance of consistent moisture supply and thermal regulation during the bulbing phase. These results corroborate Islam et al. (2007) and Najafabadi et al. (2012), who documented substantial bulb weight advantages under polyethylene mulch. The competitive performance of rice straw mulch reflects the combined benefit of moisture conservation and gradual nitrogen supply during storage organ filling.

3.4.3 Bulb length and diameter

Sawdust mulch produced the longest bulbs (5.19 cm), though statistically at par with rice straw mulch (4.97 cm). Elongated bulb form under sawdust is attributable to its high porosity and low bulk density when applied as mulch, improving aeration and reducing mechanical resistance to radial bulb expansion (Haque et al., 2003). Bulb diameter was greatest under plastic mulch (7.08 cm), followed by rice straw mulch (6.53 cm), and was lowest in the control (5.21 cm). Equatorial diameter is a primary determinant of commercial grade in onion markets, making the plastic mulch advantage economically significant (Najafabadi et al., 2012).

3.4.4 Total yield

Total fresh yield showed the strongest treatment differentiation (Table 4). Plastic mulch yielded 35.27 t/ha 91.5% advantage over the control (18.42 t/ha)—and was significantly superior to all other treatments. Rice straw mulch (29.54 t/ha) was the next highest, statistically distinct from banana leaf mulch (22.14 t/ha), sawdust mulch (23.87 t/ha), and the control. The superior yield under plastic mulch is a cumulative outcome of consistently higher leaf area index, improved photosynthetic duration, enhanced nutrient uptake efficiency, and reduced weed competition across the entire growth cycle (Lamont, 2005; El-Beltagi et al., 2022). The yield hierarchy plastic > rice straw > sawdust > banana leaf > control is consistent with reports from comparable subtropical environments (Ramakrishna et al., 2006; Mutetwa and Mtaita, 2014).

4 Discussion

4.1 Effect of mulching on vegetative growth

The results clearly showed that mulching improved vegetative growth compared with the unmulched control. Rice straw mulch produced the tallest plants and the longest leaves, while plastic mulch resulted in the highest number of leaves. These improvements suggest that mulching created a more favorable environment for onion growth during the growing season. Mulch generally reduces water loss from the soil surface and suppresses weed growth, allowing plants to use available water and nutrients more efficiently. Since soil moisture and temperature were not measured in this experiment, these factors cannot be confirmed directly, but they are likely to have contributed to the observed differences.

The better performance of rice straw mulch may also be related to its gradual decomposition during the cropping period. As the mulch breaks down, it can improve the soil surface environment and provide a small amount of nutrients to the crop. Similar responses have been reported by Ramakrishna et al. (2006), who found that straw mulch improved crop growth by conserving soil moisture and enhancing soil conditions. Likewise, Mutetwa and Mtaita (2014) reported improved vegetative growth of onion under organic mulching compared with bare soil.

4.2 Effect of mulching on bulb development and yield

The improvement in vegetative growth was reflected in bulb development and final yield. Plastic mulch produced the highest bulb weight, bulb diameter, stem diameter, and total bulb yield, whereas the control consistently recorded the lowest values. Onion bulb development depends largely on the plant's ability to produce and transfer photosynthates from the leaves to the bulb. Therefore, treatments that supported better leaf growth were also able to produce larger bulbs and higher yields.

Rice straw mulch also performed well and produced significantly higher yield than the control. Although its yield was lower than that of plastic mulch, the difference was relatively small considering that rice straw is an inexpensive and locally available material. This suggests that rice straw can be a practical alternative for farmers who wish to improve onion production without relying on plastic mulch.

Root length was not significantly affected by the treatments. This indicates that the increase in yield was more closely related to improvements in above-ground growth and bulb development than to changes in root elongation.

4.3 Comparison with previous studies

The findings of this study agree with several earlier reports showing positive effects of mulching on onion production. Islam et al. (2007) and Najafabadi et al. (2012) also reported higher bulb yield under plastic mulch than under bare soil. Similarly, Ramakrishna et al. (2006) found that organic mulches improved crop performance by reducing evaporation and creating more favorable soil conditions.

However, the magnitude of yield improvement observed in the present study differs from some previous reports. Such differences are expected because crop response to mulching depends on several factors, including climate, soil type, irrigation method, cultivar, and mulch characteristics. In this experiment, onion was grown under the subtropical conditions of western Nepal using flood irrigation, whereas many previous studies were conducted under different environmental conditions and irrigation systems. These differences may explain the variation in yield response among studies.

4.4 Agronomic implications

Among the tested mulching materials, plastic mulch was the most effective for maximizing onion yield. Nevertheless, its use also raises concerns about the disposal of polyethylene after harvest and the possibility of plastic residues remaining in agricultural fields.

Rice straw mulch offered a good balance between productivity and sustainability. Although it produced a lower yield than plastic mulch, it substantially outperformed the control and other organic mulches. Because rice straw is readily available in the Terai region and decomposes naturally, it can serve as a practical and environmentally friendly option for smallholder farmers. Therefore, farmers seeking maximum yield may prefer plastic mulch, whereas those aiming for a low-cost and sustainable production system may find rice straw mulch to be a suitable alternative.

4.5 Limitations and future research

The findings of this study should be interpreted in light of several limitations. The experiment was conducted at a single location during one growing season, and soil moisture, soil temperature, nutrient dynamics, and weed biomass were not measured directly. Consequently, the mechanisms responsible for the beneficial effects of mulching could not be verified experimentally.

Future research should evaluate these mulching materials across different locations and growing seasons to determine whether the observed responses remain consistent under varying environmental conditions. Measuring soil moisture, soil temperature, and nutrient availability would help explain the processes responsible for improved crop performance. In addition, economic analyses and assessments of the environmental impacts of plastic mulch would provide useful information for developing practical recommendations for onion growers in Nepal.

5 Conclusion

This study provides comprehensive evidence that mulching materials significantly influence the growth trajectory, bulb development, and marketable yield of onion under subtropical conditions in Chitwan, Nepal. Plastic mulch consistently maximized yield parameters, achieving a total fresh yield of 35.27 t/ha nearly double the control yield of 18.42 t/ha with the highest bulb weight (111.47 g), bulb diameter (7.08 cm), and stem diameter (1.41 cm). Rice straw mulch emerged as the strongest-performing organic alternative, particularly for plant height and leaf elongation, with a total yield of 29.54 t/ha representing a 60.4% improvement over the control.

Sawdust mulch demonstrated a specific advantage in promoting bulb elongation (5.19 cm), potentially valuable in markets favouring elongated bulb forms. Banana leaf mulch and the unmulched control consistently underperformed across all parameters. Given documented concerns regarding microplastic accumulation, soil contamination, and disposal challenges associated with long-term polyethylene mulch use, rice straw mulch is recommended as the preferred sustainable option for smallholder onion production in Nepal’s Terai region. Where maximum productivity is the primary objective and plastic waste is responsibly managed, plastic mulch remains the most effective single intervention.

Future research should investigate the economic feasibility and cost-benefit ratios of mulching systems at different farm scales, long-term effects of organic mulches on soil health and microbial communities, performance across multiple onion varieties, and the potential of biodegradable plastic mulches as transitional alternatives to polyethylene.

Authors’ contributions

BK conceived and designed the study, conducted the research, interpreted the results, and prepared the original manuscript. AG performed the data analysis, assisted with data interpretation, and critically reviewed the manuscript. SC managed the experimental field, supervised field operations, and collected the experimental data. All authors read and approved the final manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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