Plumeria's do not HAVE to go dormant. you can let them grow through the winter but they will grow slower than in the summer so really its up to you to decide to either to have your plumeria go dormant or to keep giving it sun and having it grow. also make sure that the greenhouse stays warm enough for the plumeria's to not get frost damage.

Dawson

Unless you have a Singapore variety (evergreen), your plumeria will probably go dormant. The dormant process is actually a response to the dry season of the Caribbean triggered by cool temps and sun exposure. Even in the tropics, most plumeria have a short dormancy period in Jan/Feb. A few varieties may or may not drop leaves but most will.

You can keep plumerias growing (and blooming) in the winter by giving them more than 14 hours of supplemental light a day + warm temps. This tricks them into thinking it's still the growing season. In lieu of that, they will likely drop most of their leaves by the end of January.

None of mine went fully dormant last winter and they were fine this year. The ones that didn't go dormant last year are dormant now though. I have a couple others putting out leaves after cutting them off a month ago but they'll give up eventually and go to sleep!