To investigate the impact of earthworm manure supplementation on cucumber seedling growth in cocopeat, various concentrations of earthworm manure were prepared in 20% volume increments. Six treatment groups were established: 100% cocopeat (CK), T1 (20% earthworm manure + 80% cocopeat), T2 (40% earthworm manure + 60% cocopeat), T3 (60% earthworm manure + 40% cocopeat), T4 (80% earthworm manure + 20% cocopeat), and T5 (100% earthworm manure). Seed germination and seedling growth were evaluated for each treatment, along with an assessment of the physicochemical characteristics of the seedling substrate, to ascertain the optimal earthworm manure dosage. Findings revealed that the optimal range for earthworm manure supplementation was between 40% and 80%, with the total porosity of the earthworm manure emerging as the primary constraint on seedling growth. Based on the study findings, additional enhancements to the physicochemical properties of the seedling substrate composed of “earthworm manure-cocopeat” are advised to optimize cucumber.