Long-term monitoring of the phenology of Sargassum hemiphyllum in Hong Kong over the last 16 years, albeit not continuously, indicated a predictable timing of the onset of its various growth stages. However, in recent 5 years, some anomalies in winter sea surface temperature (SST) in Hong Kong were observed. The winter SST could be much colder (13–14 vs. 17 °C of mean winter temperature), or the winter period could be prolonged. In 2012, the winter was exceptionally long (>3 months) and the SST started to rise only in May, a delay of more than a month compared to earlier records. A consequence of this is the delay in the onset of reproductive structures in S. hemiphyllum from late February/early March in normal years to mid-May in 2012. Laboratory experiments indicated that a drop in temperature, followed by a rapid rise, could trigger receptacle formation in S. hemiphyllum. But, this may not always be necessary especially for thalli which were already in the rapid growing stage (RGS). RGS thalli continuously exposed to 14 °C (−4 °C of ambient) and 18 °C (ambient) temperature could also develop receptacles, whereas those exposed to 22 °C (+4 °C of ambient) and 26 °C (+8 °C of ambient) did not survive. On one hand, this indicated the need of Sargassum thalli to accumulate enough resources prior to the formation of receptacles, as in the case of those in the slow growing stage (SGS) that responded more positively to slightly elevated temperature than the ambient (22 vs. 18 °C). On the other hand, it also showed differential sensitivity of growth stages to temperature change. Between SGS and RGS, it would appear that RGS could become a bottleneck for continuous survival of the population under the projected global warming scenarios.