Pentas ARE a perennial. They are a tender perennial. I honestly don't know about overwintering them in a dark garage, though. Some quick reading on the internet says yes to this, after a severe cut back of the stems. Sounds like you have three options....take cuttings and grow them inside, cut your plant back and bring it inside to continue to grow, or cut the plant back and stick it in the garage.

I've had one bloom all winter long with a grow light.

If they are the type of perennial that can go dormant and grow back from the ground, but just not in a place as cold as where you are, like wax Begonias, I would be fairly confident about trying to store them in dormancy. If they are like Coleus that just can't take frost at all, this would not work. A few sources I checked said a few light frosts is about as much as Pentas can take, it doesn't go dormant. Maybe using a hanging basket could be a way to keep this plant in some light without using "down" space.

was the season long enough for seeds to set???

ken

I'd put them in a pot and bring them inside and keep them watered. I don't think they will survive being kept dry in a dark garage all winter. They're more of a tropical perennial, not a hardy perennial like you're used to. I know they will die pretty fast if they aren't kept watered, there are lots of them growing where I am, and they bloom most of the year, but then, we don't usually get hard freezes very often.