I'm definitely not a Plumeria expert yet, but I grow mine indoors with my Orchids and other plants, so here's my 2 cents on winter.

Plants are pretty smart when it comes to seasonal cycles and are really only concerned about one thing. How much energy does it have to expend to continue absorbing energy to grow. If the cost of producing energy becomes greater than energy received, then the plant/tree will go dormant to preserve resources for the next growing season. Some areas will give the plant ample sunlight throughout the year, but the temperature drop will signal the plant to do dormant because the lower temp will require the plant to expend energy faster than it can absorb it.

I think if your garage temp drops below, say 50-60 degrees, that may signal the plant to go dormant for the winter. Not a bad thing, because it will still concentrate on root growth, but it will not want to force leaf growth in the colder environment. With my orchids, I have to let them experience a 15 degree temp drop in the fall so they understand it's time for winter blooms. That means letting my growing area drop to around 50 degrees at night. I suspect my Plumeria will go dormant when that time comes.

Your plants should certainly stay alive, but they will go dormant. They will drop leaves even before you start to bring them into the garage and then they will continue to drop leaves throughout the winter until, probably by January, most will be bare.

Cuttings are best rooted over the summer months.

Newly-rooted cuttings will likely drop leaves over winter.

What is going to be your average temperature in your garage? Temperature should definitely not go below 55 degrees -- and should probably be at least 60 for new cuttings.

If you set your artificial lights to perhaps 12-14 hours, you may get some blooming in established plants.

I would strongly suggest you look into a quality led! It will give you a much better spectrum of ligh & many more lumes to increase growth. Think of it like this.......t5 = keep your plumie growing. LED = keep your plumies THTIVING!! You will also get much better color!

On the down side, LEDs are more expensive.

Anyway, something to think about. Hope this helps.

Peace & happy growing,
Jason

All newly rooted plants should be kept in the house to be safe. They're at their most vulnerable that first winter. Second year plants should be kept in a heated garage at a minimum.

You can cluster your plants and wrap the whole group with sheets to preserve the warmth from the light; it certainly won't hurt. But I wouldn't try to keep them going all winter because as Dave said they will go dormant below about 65-70 degrees. Mine in the warm house will even go dormant.

Your best bet may be to just start them off early next spring with lights, say around the end of February, so they have a longer growing season (or shorter dormant season).