Thick then thin – a novel approach to reducing plant spacing variability in small-seeded vegetable crops

Establishment outcomes can be highly variable in of small-seeded vegetable crops, reducing crop value through lower yields and higher product variability. We conducted a proof-of-concept study to explore the potential to achieve targeted plant populations consistently and to reduce variability in plant spacing by sowing additional seed and thinning emerged plants. In one onion crop, replicated plots were sown at half the standard within-row plant spacing (72 mm). Eight weeks after sowing, plant spacing was recorded in each plot, and then thinned using templates that identified plants to be removed based on spacing from the last retained plant (plant distance thinning), or at fixed distances from the start of the plot (fixed distance thinning). Plant-distance thinning treatments reduced within-row spacing variability (coefficient of variation (CV) 75% pre-thinning to 34% post-thinning). The mean spacing (74 mm) was similar to that of the control plots (73 mm; CV 61%). Fixed-distance thinning treatments increased the mean and CV to 115 mm and 84% respectively. A survey of six commercial onion fields showed that they varied significantly in within-row plant spacing variance (< 0.001). Sowing more thickly, and then thinning small-seeded crops like onions shows promise in reducing plant spacing variability, extending the existing use of this approach from crops such as lettuce where automated thinning is being developed. The next consideration is measuring the impact of thinning treatments on yield and variability and considering automated methods for thinning of tightly spaced crops.