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Does pot size matter for lettuce?
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Does the size of the pot or grow bag change how much lettuce you harvest? In an open-field trial in Indonesia, lettuce in the largest of three grow bags gave 170.41 g of shoot per plant, against 36.69 g in the smallest.
Balcony and rooftop growers are often tempted to squeeze more plants into smaller containers. Researchers at Universitas Sriwijaya in South Sumatra set out to measure what that costs a lettuce crop, and to find the smallest volume of growing medium that still gives good growth and yield (Ria et al., 2026). They grew green lettuce in three sizes of polybag (grow bag), outdoors in the lowland tropics.
What this means for your grow
In this trial, more root room meant much bigger lettuce: the largest bags gave the most leaves, the most leaf area and the heaviest plants. Smaller bags dried out faster and ran hotter at midday. The authors still say the smallest size can be used in urban growing because it saves space, provided the plants get more frequent, well-timed watering. These results come from one cultivar under a single set of open-field conditions in the lowland tropics, and the authors caution that the findings may not apply everywhere.
What did the team do?
Seedlings of the cultivar ‘Grand Rapids’ were raised in trays for 14 days, then moved into polybags of three sizes: 15 × 20 cm (about 1.43 L), 20 × 25 cm (about 3.18 L) and 25 × 30 cm (about 5.97 L). Each plant was watered once a day in the late afternoon and fed a diluted liquid organic fertiliser once a week.
The team counted leaves, measured canopy and leaf area, and on a single day logged soil temperature, soil moisture, air temperature and leaf temperature every two hours from 08:00 to 16:00. The plants were harvested 42 days after transplanting and weighed.
What did they find?
- Bigger bags, much bigger plants. Shoot fresh weight was 36.69 g per plant in the smallest bags, 93.12 g in the medium bags and 170.41 g in the largest.
- More leaves and leaf area. Plants in the largest bags averaged 28.44 leaves and 3868.95 cm² of total leaf area, against 12.50 leaves and 1023.12 cm² in the smallest.
- Bigger roots. Root length was 25.00 cm in the largest bags and 7.76 cm in the smallest.
- Small bags dried out and heated up. As soil temperature rose from approximately 27 °C to 32 °C, soil moisture fell. Around midday, leaf and soil temperatures rose more sharply than air temperature, particularly in the smaller bags, while plants in larger bags kept lower and more stable leaf temperatures.
- Leaves were not wetter, just heavier. Leaf water content did not differ significantly between bag sizes, so the authors conclude the extra weight came mainly from more dry matter.
- A handy leaf-area trick. Leaf area was predicted closely by multiplying leaf length by leaf width and then by 0.8019 (R² = 0.9882), which gives growers and researchers a way to estimate leaf area without cutting leaves.
What could it mean?
The authors conclude that a larger growing medium volume improves the root-zone microclimate, with lower soil and leaf temperatures and better moisture retention, and so supports vegetative growth and yield. They add that the smallest volume can still be recommended for urban farming because it is more space-efficient and allows higher planting density, as long as irrigation is managed well, particularly with more frequent and well-timed watering. The study focuses on container growing in urban farming, and giving lettuce roots more room, or watering small containers more often, is something you could try with your own containers.
What were the limitations?
- The authors note the study was run under a single environmental condition with one lettuce cultivar, so the findings may not generalise, and further work across climates, cultivars and irrigation regimes is needed.
- Root-zone temperature and moisture were measured on a single day.
- Some percentages in the paper’s abstract do not match its tables, so this post uses the figures from the tables.
Trial details
- Where: Indonesia. Open field in Jakabaring, Palembang, South Sumatra (104°46ʹ44ʺ E; 3°01ʹ35ʺ S), a lowland tropical climate region. Universitas Sriwijaya.
- When: September to November 2025.
- Crop: green lettuce (Lactuca sativa L.) cv. Grand Rapids. Seeds soaked in warm water at 60 °C before sowing; transplanted 14 days after planting; harvested 42 days after transplanting.
- System and substrate: seedling trays filled with ultisol (loam-textured soil) and chicken manure at 2:1 (v/v); plants then grown in polybags of 15 × 20 cm (V1, about 1.43 L), 20 × 25 cm (V2, about 3.18 L) and 25 × 30 cm (V3, about 5.97 L), one seedling per bag.
- Watering and feeding: watered once daily in the late afternoon. From 7 days after transplanting, 100 mL per plant once a week of a liquid organic fertiliser diluted to 2 mL per litre (10.04% organic C, 2.05% N + P2O5 + K2O, 0.54% organic N, pH 5.44, total Fe 332.21 ppm, Mn 38.8 ppm, B 74.20 ppm, Mo 2.39 ppm).
- Climate: outdoors; temperature, rainfall and air humidity for the period are shown as a figure in the paper. Root-zone measurements at about 5 cm deep and about 7 cm from the stem, every two hours from 08:00 to 16:00 on one day.
- Design: randomised complete block design, three bag sizes, four replications of six plants each (72 plants).
- Statistics: ANOVA with Least Significant Difference test at P < 0.05, in RStudio.
- Results (smallest, medium, largest bag): shoot fresh weight 36.69, 93.12 and 170.41 g; leaf number 12.50, 19.11 and 28.44; total leaf area 1023.12, 2558.94 and 3868.95 cm²; leaf dry weight 2.82, 5.42 and 9.89 g; root length 7.76, 18.30 and 25.00 cm; root dry weight 0.38, 0.84 and 1.67 g. Leaf water content 90.61, 92.67 and 92.40% (not significantly different).
- Conflicts of interest: the authors declare none.
- Publication: Advances in Horticultural Science 40(3): 237-245, Firenze University Press, published online 26 October 2026 (peer reviewed), https://doi.org/10.36253/ahsc-19438. Licence: CC BY 4.0.
- Not reported in the paper: the medium used to fill the polybags, light levels, the amount of water given each day, and funding.
Links
- The study: Morpho-physiological and yield responses of green lettuce to growing media volumes in urban cultivation
- The article page at Firenze University Press: Advances in Horticultural Science
References
- Ria, R. P., Lakitan, B., & Gustiar, F. (2026). Morpho-physiological and yield responses of green lettuce to growing media volumes in urban cultivation. Advances in Horticultural Science, 40(3), 237-245. https://doi.org/10.36253/ahsc-19438