For Cordyline austalis, the authors mentioned in the article have published a host of papers over the last decade exploring various attributes as predictors of plant hardiness. To save space, I have just provided the Google link to their names as associated with Cordyline australis..

As to propagation, here are some resources. The first is from mature plants in NZ and the second link has some description about trials of Cordyline hardiness in the UK as well as propagation suggestions.