Castor (Ricinus communis) is a non-edible seed cultivated for the highly-valued oil extracted from its seed. Uneven germination and seedling emergence is a frequent problem for the establishment of castor fields. This study tested the hypothesis that castor can be selected for fast germination using the method of single-seed selection. Two-hundred castor seeds cv. BRS Energia were incubated at 22 °C in germination paper, and the 10 seeds that germinated faster were selected, planted, and cultivated to produce seeds. The harvested seeds were subjected to an additional cycle of single-seed selection for fast germination, and the gemination ability was sequentially tested along 8 months after harvest. It was observed that freshly harvested seeds have reduced germination ability at suboptimal temperature (22 °C), but they germinated better when exposed to near-optimal temperature (30 °C) or the germination ability gradually increased over time. The selection for fast-germinating seeds unintentionally selected seeds that germinate quickly in response to sodium hypochlorite (NaClO) which was used for disinfection, and that sensibility was effectively transferred to the descendants. Other highly saline substances (NaCl and KCl) elicited germination on the previously-selected NaClO-sensitive genotypes. The study was not conclusive if two cycles of single-seed selection obtained substantial improvement of the germination ability of freshly harvested seeds under suboptimal temperature. •Castor seed germination is highly uneven.•Single-seed selection was effective for improving germination of castor seed.•Castor germination ability is very low on freshly harvested seeds.•Castor seed germination ability increases as the seed is stored.•Castor genotypes were unintentionally selected for germination elicited by NaClO.