JARS v37n4 - Root Hardiness of Rhododendron and Azaleas

Root Hardiness of Rhododendron and Azaleas
Al Reid, Finwood, N.J.

Although several scientific tests have been made, many people are either unaware of or ignorant of the information published from these tests. All results show that above ground plant parts will survive much lower temperatures than their roots.
One study done at Cornell University by Drs. Good and Steponkus, supported in part by the A.R.S., was first published in July 1977. The results showed that 'Hino Crimson' was killed at root temperature of +19°F, an Exbury Hybrid at +17°F, and 'Purple Gem' at +15°F.
Another study done at the University of Massachusetts published in 1976, was done over a five year period. Results showed both 'Hinodegiri' and an Exbury Hybrid were killed at root temperature of -12.2°C, and 'P.J.M.' at -23.3°C.
Other studies produced similar results, that is, that roots of Ericaceae are not nearly as cold hardy as their tops. Variations in degrees of killing temperatures are probably due to some plants being acclimated artificially and others naturally.
There is an excellent article on Cold-Hardiness, plus other information on Rhododendrons and Azaleas in the Quarterly Bulletin Volume 34, Winter 1980, Number 1, Page 20, a reprint from the Rosebay.
If you have plants in raised beds or containers, be advised they will need extra protection to survive cold winters. If you don't have a deep cold frame, I would suggest planting them in the ground or use a deep mulch.
Do not be fooled by exceptions. I have had azaleas with poor roots that I could not sell, but tossed on top of the trash pile. Some of these survived and bloomed the next spring while healthy bushes, balled and burlaped, fifteen feet away on top of the ground were stone dead.