Hetty is right, most plumeria go dormant in winter even here in the tropics. The few that don't do not grow much and just manage to keep the leaves and blooms they have through the winter. I don't believe that in a favorable climate cold is an influence and it would seem that the hours of light only need to vary a small amount. We only change from 11 hours of sunlight in the darkest time of winter to 13 hours in the middle of summer and yet the plumerias know when to lose their leaves. This makes me think that even a slight shortening of daylight could trigger them.
