I would expect your plumeria to overwinter well in that environment. They will probably go dormant, losing all their leaves, but will perk up again in the spring.

Mike

What's the minimum temp for them to keep their leaves on??
Brittany

Brittany,
It is not only temperature, it is also light. Plumeria respond to shorter days by going dormant. Most of the info I have seen suggests 16 hours of light (daylight augmented by growlights). I would suspect that keeping them at 60 with the lighting should keep the leaves. However, they may still slow their growth. Be careful of diseases that may occur in a greenhouse. Some advocate spraying for fungus regularly. Keep the air circulating.

I usually let mine go dormant, using the greenhouse just to protect them from frosts/freezes. I will keep new seedlings and newly rooted cuttings in the house under a growlight the first year. After that, they go in the greenhouse.

Mike

S,
I have been growing them in Kansas City for almost 12 years now, in greenhouses and under HID lighting. Here are some things I've learned. Take what you like and leave the rest.
Greenhouse the best! lol They act like different plants in cold climates. Better to ask people who grow them near you -my opinion, not intended to offend. People in warm climates experience near effortlessness with them. Their experiences are much like ours are with peonies or easy roses. Our winters are long and much colder and this is stressful on them. They are way out of their natural range/habitat here in the temperates.

First of all, if it is a first year cutting, 50F is too cold-even in a greenhouse. Unless it is a strong cv then you risk "withering dieback." First year cuttings, in general, are not strong enough to handle stress of our long winters. Best solution for the first year is bottom heat -even a greenhouse. A recommended temp for the first year is 80F in the root zone. You will have enough light in the greenhouse, but 50F is too cold in the root zone -even for mature trees up here.

They will survive down to mid thirties here outside(I had several in the ground this year until 11/15 however I knew a few things based on experience so it was not as effortless as it sounds-the trick is I had them up against the concrete foundation where the soil was still around 60F six inches down.) With our humidity(which CA doesn't have) you risk rot and cold temperature stress in pots-especially with first year cuttings. They don't like cold and wet/humidity. My advice for cold growers(anyone below zone 7 or 8) for what it is worth, is to not let them get below 60 in the root zone until you know what you are doing-even as trees. The operative phrase being until you know what you are doing- know the signs of stress, the cv strength, how to cheat the climate etc. Many of us in cold climates have soil thermometers. I have several trees over 10 feet tall now and a solid collection of 100 trees(more if you count seedlings.) Good luck. If you are ever in KC next summer come by and see the forest. Enough blathering out of me. Hope I didn't offend.