Spinach (Spinacia oleracea) is a nutritionally and commercially significant crop grown in controlled environments; however, its seeds are difficult to germinate in soilless culture. Excess moisture in the root zone can inhibit germination and impede seedling establishment. To address this, we conducted an experiment with a randomized complete block design (three replications) to identify the optimal moisture content for spinach ‘Space’ germination. In each of 63 sealed containers, we placed 20 seeds on a double-layered paper towel pre-moistened with varying amounts of reverse-osmosis water. We quantified visible germination percentages daily in response to seven moisture indices [moisture mass ÷ (paper mass moisture mass)] ranging from 50% to 94% over an 11-day period. Air temperature and relative humidity were 22.12 ± 0.02 °C and 34.35% ± 8.80%, respectively. The optimal moisture index was 80%, which resulted in the highest cumulative visible germination percentage (92%, which is close to the labelled 93% on the seed package). Deviating from the optimal moisture index in either direction decreased the germination percentage to 0%–22% and delayed germination by up to 5 days. The response of the cumulative visible seed germination percentage to the moisture index followed a normal distribution. The daily new visible germination percentage peaked on day 3 under most moisture indices; it was 55% under the optimal moisture index and < 24% under the other moisture indices. In conclusion, a moisture index of 80% resulted in the highest germination percentage (92%) and the fastest germination time (2–5 days) in spinach ‘Space’, whereas lower or higher moisture indices caused poorer (0%–67%), delayed, and less uniform germination. Once transplanted, the spinach seeds germinated with this moistened-paper method under the optimal moisture index adapt and perform well in soilless substrates, including rockwool, which is notoriously difficult to germinate spinach seeds in.
