If you have an indoor environment they like, leave them in it all the time - instead of shifting them back and forth.

Think Honolulu - that's the kind of climate they are commonly grown in. One where 59F is a vicious cold snap.

They've been going outside every day above 50F for at least a few hours since february. Natural lighting is so much better and I don't get that much natural light in my small apartment. I believe they are doing so well because they are going outside and I do not think they would be happy inside all the time. Now they are in the two gallon containers, they can no longer sit on the windowsill.

Going outside / inside during this same time has not hurt my cacao, cinnamon, or cloves and they are ultra tropical - much more tender than the plumeria. The plumeria will stay outside if / when the temperature allows. If the weather had cooperated, they would be outside now allowing my ultra tropicals to have a better spot under the lights when they are not outside.

House plants are much better off staying in the house all the time. Even in climates with suitable hot summers the plant has to endure a change every time it is moved. And it can bring in bugs and slugs when it is brought back into the house in fall.

Unless they burn or turn yellow on the tips you don't really know that your tropical plants are not being affected adversely by the outdoor stints - maybe they are now growing more slowly than they would if they had been left in the house. Individual leaves do not adjust to sudden changes in conditions, that is how it is possible for plants to become damaged by them. Hardening off is something that is demonstrated by new growth made in the new environment.

If you intend to keep them going there you will still have to arrange suitable permanent quarters for the plumerias whether they are stuck outside for the summer or not.

I bring a lot of tropicals back and forth each year, and I do find that they sometimes bring some nasty bugs back in with them that they wouldn't have had if I left them indoors. Plumeria though really do need the very highest full direct sun we can give them here, and indoors during the growing season just won't cut it. Even in a south window they get leggy and rarely branch, growing too tall & spindly if they are left in all year.
Pure cold hardiness, they are just fine outside now (I have 2 out). I have left them outdoors with no damage right up to the 2nd frost of the year at around Nov. 1. They do tend to get started very slowly in this weather though, both from growing slower in the cool weather, and from pouting about too-wet roots from this climate.
My usual spring goal is to get them as root bound as possible before taking them out, then they do fine outside on May 1, but this year I was a slacker and they are just barely puttering along (rainy cool May). If you've already been putting them out days, and they are healthy with good roots, I would probably leave them out now. It would be even better if you can put them where they'll receive full sun and reflected heat from a concrete or brick wall or patio.
- Tom

Thanks, Tom.

They get full sun except for about an hour late morning. All of them are on a wooden deck that reflects lots of heat so when the sun is out, it's much warmer than the thermometer says.

Didn't know if plumeria were tropical or sub tropical. I grow mostly edibles and have vanilla orchids, cacao (cocoa), cloves, carob, macadamia, cinnamon (zeylanicum), black pepper (piper nigrum), blue lilly pilly, etc. and most of those need to come in when it gets below 60F (and that's stretching it for a couple of them)