[hi there...
plumeria are deciduoa plants and so are meant to lose all leaves and flowers in the winter months.
as the weather starts warming up again, you will see the tips of the plumeria start to grow odd little lumps (the new leaves) THIS is the time to fertilize.
in winter it is dormant... asleep, so no point fertilising....as it wakes up from it's slumber, you want to give it a boost so it wakes right up!!!
fertiliser should be given regularly (every two weeks for a liquid fert.) all through spring and summer, then start slowing down (every 3 - 4 weeks) as autumn gets going.... it's leaves will start to drop around autumn anyway.
happy growing
plumeria lover from down south,
Renay