I have had them outside in pots for several years now. By the end of the winter, the leaves have all turned brown and most will pull away from the crown easily. The first year or two, this led me to believe they were dead, but being slow to replant pots in the spring, I have been pleasantly surprised to see that they came back. I have even divided them once. I know they are marginal, and even more so for you, but, as I said, they can surprise you. If I were you, I would leave them alone until it really warms up. Who knows? Maybe you've gotten lucky.

Don't have 'Woodlanders Red', but do grow 'Woodlanders Hardy', also a red flowering one. Has survived in near 0°F temperatures and remained evergreen. I moved it this past spring, to a full sun area to promote better flower production. Will see if that works! Don't grow Cordyline 'Red Star', but a number of gardeners in my Zone do. It is promoted as hardy to 5°F and to remain evergreen, if located in a sheltered, SW location. The location that I would like to plant a few, is in my "Arctic" microclimate zone and I'm sure they wouldn't survive there! The only suitable location that I have, is presently occupied with a large colony of Georgia Asters and those are maintained for a local hive of European Honeybees, which descend upon them in hordes during the fall! I couldn't think of disturbing those.

You may want to ask on the tropical plants forum too...

I live in the UK and see Cordyline australis everywhere. Indivisa isn't so common though, I know there are some in very wet parts of the UK like some gardens in Northern Ireland and Devon. Occasionally we have a cold winter and there's a mass die off inland, but it's no big deal really. Most come back very strongly from the roots, especially if they were a large, mature plant before they died in the winter they reach their former size in a couple of years and come back very fast from a strong root system with fresh, new growth. What actually kills them is when you see well-meaning but misguided people wrapping them up in winter with bubble wrap and frost fleece. It's the worst thing to do in my opinion, these things hate to be soaking wet in winter and the frost protection people put round them just makes them rot. I don't protect my ones that are in the ground. If they die back they dieback, the soil doesn't freeze deep enough in Britain to kill them in most cases. Container ones i just put in a cold greenhouse to keep them from dying from the damp of winter which is the real cause of death. I have noticed a lot of variation in the plants grown here. Some are very narrow leaved, but mature plants are usually wide leaved, not sure if the leaves get wider over time or different varieties are tougher. I suspect the narrow leaved plants are the "Atlantic green" variety whereas the wider leaved ones are generic australis with maybe some indivisa introgression, although according to something i read somewhere the narrow leaved varieties are meant to be hardier. I've also never seen a mature, large plant of the red, pink or variegated varieties though i know some examples exist, they're probably less hardy and were perhaps selected from stock from the North island.