Operational Performance of a Staggered Multi-Crop Mini-Garden for Space Crop Production Using Zero-Discharge Reusable Root Modules

Future International Space Station (ISS) mini-gardens must provide fresh produce repeatedly while maintaining water and nutrient conditions within a small, reusable root-zone volume. This study evaluated the operational performance of staggered planting and selective harvesting over four months in a ground-based Utah Reusable Root Module (URRM) simulator operated as a zero-discharge system. Five independent root modules (RMs), providing a total planted area of approximately 0.40 m², were planted with leafy greens, bonsai basil, and dwarf tomato. The system produced 4,299 g of fresh edible biomass, with produce available on 77 of the 100 days within the harvest period. Daily water use remained below approximately 2 L d⁻¹ for most of the trial, and peak demand occurred at different times among RMs. RMs containing established tomato plants behind newly planted leafy greens showed lower water-use peaks and biomass production, suggesting incomplete use of module capacity. Lettuce performed well during the first cycle but was less consistent in subsequent cycles, whereas mizuna and kale provided more consistent production under reused-media conditions. Bulk electrical conductivity (bulk EC) declined during periods of rapid crop growth, indicating that nutrient depletion, rather than salinity accumulation, was the main root-zone risk under zero-discharge operation. These results indicate that ISS mini-garden operations can distribute harvests and water demand over time, but crop calendars should coordinate planting dates, crop architecture, harvest timing, residue management, reused-media constraints, and root-zone nutrient monitoring across independently managed modules.