Uniform seedling emergence is crucial for economically viable spinach (Spinacia oleracea) leaf production. However, non-uniform seed germination occurs due to variation in moisture sensitivity between and within spinach seed lots. To test moisture sensitivity, we developed a floating germination system with fixed distances between germination papers and the water table, so that moisture levels could be standardised. We tested germination performance of different cultivar seed lots, with one seed lot fractioned in different seed sizes, and of seeds with an intact, open or removed pericarp. At a high moisture level, smaller seeds germinated better than larger seeds, and seeds without a pericarp or with an open pericarp germinated better than intact seeds. Further, the pericarp of smaller seeds was thinner than the pericarp of larger seeds. A lower temperature or increased oxygen level resulted in higher germination rates for differently-sized seeds at high moisture levels. In conclusion, the sensitivity of spinach seed germination to moisture is influenced by seed size, hence pericarp thickness and intactness, and is driven by the oxygen availability to the seed embryo. To determine the full germination potential of spinach seed lots, we recommend a standardised low moisture level in addition to a temperature of 15 or 20°C.