Seeds of Cleome (Cleome gynandra L.) are characterised by low germination and may require after-ripening. The dormancy characteristics and optimum conditions for seed germination of this species have not been explained. The objective of this study was to investigate the effects of temperature, light and pre-germination treatments (chilling, scarification, hydration and germination in the presence of KNO3 or GA3) on the germination of Cleome seeds. Seeds were germinated for 10 days and the final count of germination was used to determine seed performance. The highest germination percentage (60% and 80%, for a 2-year old and a 1-year old seed lot, respectively) of untreated seeds was achieved when alternating temperatures of 20–30°C (16 h—8 h) in the dark or constant 30°C in the dark were used. Among the pre-germination treatments, only scarification (puncturing of seeds at the radicle end) improved germination. Seeds were found to be negatively photoblastic, and the phenomenon was more pronounced when the germination test was undertaken at 20°C and at light exposures of 12 h day−1 or longer. Germination of photo-inhibited seeds was, however, improved by treatment with GA3. It is recommended that the germination of Cleome should be performed under conditions of darkness and either alternating 20–30°C or continuous 30°C.