I'm in zone 7b and according to the research I've done the only costus which is reportedly hardy to my zone is costus speciosus. I have several costus out now that are different sizes, some are taller than me, a couple are between 2-4 feet. I want to leave them all out probably just to see if they'll make it. now my question is, is it a really bad idea to cut back some of the canes? I can't help but think they're probably gonna die anyway, it'd be a heck of a lot of cuttings, but I don't want to leave open wounds that might compromise their ability to make it through the winter
I have speciosus & var. speciosus (have some of both taller than me,,,it's actually budding even, and a small var. one)
spicatus --same, taller than me, budding
arabicus variegata, 4-5 feet, I reeeeeally doubt it has a chance of being hardy, and a smaller one too
Indo wax ginger--I have a monster (pictured...it's big, no blooms of course :)
In my experience, with the slightly cooler temps in the fall, cutting them back makes the plant think it is springtime. This can cause a growth spurt, then , winter sets in, and there is no stored energy in the bulb, and leaves it vunerable to the cold.
This happened to me several times after hurricane seasons. I lost more plants from the cold than the hurricane itself. When the hurricanes hit in Aug or Sep, they usually begin to put out more growths immediately, which leaves it more vunerable to the cold. The next spring they didn't come back from dormancy.
This is just my experience. Happened with most of my curcumas, var. speciosus, and costus barbados, costus green mountain, and others, I can't remember, because I didn't replace them. LOL!!
This plant has been growing in my yard for over 10 years. I am in zone 9. It freezes to the ground almost every year, even here. All it takes is a single night below 32F and all the canes will turn to mush. Cutting it back now won't make a difference for you either way, because in your zone, you will lose them all anyway. None of your topgrowth will make it through the winter in your zone unless the sun goes supernova.
The word "hardy" applied to this ginger refers only to the underground rhizome mass, not to the topgrowth. You can cut it back as much as you want, when the rest freezes later cover with a nice layer of mulch and wait til spring.