You may want to read the North Carolina State University publication, Plant Propagation by Stem Cuttings: Instructions for the Home Gardener.
It gives information on propagating rhododendrons from both semi-hardwood and hardwood cuttings.
Excerpt: "The four main types of stem cuttings are herbaceous, softwood, semi-hardwood, and hardwood. These terms reflect the growth stage of the stock plant, which is one of the most important factors influencing whether or not cuttings will root."
