Conventional wisdom is that hybrids are hardier than their parents. First generation hybrids are sometimes hardier than either parent, but some hardy parents don't transmit this tendency. Now as we get to generations well beyond the first generation, this tendency has to become smaller or we would be able to grow rhododendrons in Antarctica. In fact the hardiest parents seldom result in a hybrid as hardy as the hardiest parent. However sometimes two not-so-hardy parents will produce a hardier offspring, especially if they both have a hardy parent in their lineage.

One key to hardiness in the Eastern US is for the tissue to be fully matured before killing frosts. They must also remain dormant until the last killing frost is past even though there may be warm spells before then. For this reason some plants that are hardy in the north are not hardy in the south. Also they become hardier as they grow older reaching close to full hardiness in 3 years.

One rhododendron that transmits hardiness to its offspring is R. catawbiense, but, according to David Leach, its offspring are never as hardy as the parent. For this reason selections of the species are hardier than the hybrids. R. brachycarpum tends to transmit its hardiness to its hybrids. R. maximum is another species that can transmit hardiness to its hybrids.

The Finnish super-hardy rhododendrons are all bred with R. brachycarpum subsp. tigerstedtii as a parent. It is thought to be the best plant to transmit hardiness to its hybrids and still retain the good characteristics of the other parent.

One example where hardiness doesn't come through is in the breeding of yellows. Breeding a hardy with a yellow never results in a plant nearly as hardy as the hardy parent. Many hybridizers have spent their life trying to do this.

rhodyman: thanks for your reply. In my original question, I guess I was thinking only in terms of first generation hybrids created from two different species. Once we consider hybrid to hybrid breeding, all of the genetic factors influencing hardiness would come into play for however many generations and/or species involved in the parent hybrids.

Provenance of the species is another factor I had not really thought about. If a species ranges over a wide geographical area there is often a good deal of variation in hardiness. The species fortuneii used extensively by Joseph Gable and others is a good example.

My sense of the hardy yellow problem has always been that yellow coloration in most of the hybrids that have been developed depends on species wardii somewhere in the background. The transmission of the yellow color seems linked to the lack of hardiness in wardii. Species aureum has been used, but the yellow turns out a cream color as in Serendipity. Capistrano, though, is a true deep yellow. Capistrano's root problems in warmer climates could be eliminated by grafting - nothing wrong with the flower.