More On Heat Tolerance
Carl A. Deul, Northridge, California
Recently quite a few very knowledgeable people in the Mid-Atlantic region have proposed that there is a direct correlation between heat tolerance and hardiness in rhododendrons. I would like to caution our eastern colleagues not to rush to a hasty conclusion on this point. Perhaps we are dealing with more than two genetic factors involved with what we call heat tolerance. Some observations we have made in Southern California should be considered before any conclusions on this matter are drawn.
Although we have periods of daytime summer heat which range from 110° to 115° F, the heat is dry and our evenings cool off and drop below 90° F. The dry heat allows rhododendrons to transpire and cool themselves. In addition, evaporation of water near the root zone cools the plants even more.1 This set of conditions creates less stress for rhododendrons than the high humidity and relentless heat waves of the Mid-Atlantic region. We have found that there are very few rhododendrons, once established, that can not endure our summer conditions.
The "kicker" is our lack of winter cooling. We will often have brief periods of hot dry winds (80° to 90° F) during October to November and again in January. The fall and winter hot spells really take their toll on many varieties. The ability of a variety to endure our winter conditions seems to have very little to do with the heat tolerance or hardiness of a rhododendron. Interestingly enough, these qualities seem to be more related to the "toughness"' of the American species R. catawbiense and R. maximum; most of those that tolerate our winters seem to have hybrids of these species somewhere in their ancestry.
Perhaps what people are observing as a correlation between hardiness and heat tolerance is due to a toughness factor. I would venture to say that wherever this correlation is being observed in a hybrid, we are dealing with a parent which has a great deal of toughness.
I haven't extensively verified this conclusion, but in every case where a species lacks both heat tolerance and hardiness, it has been growing in a particular ecological pitch whose climate has not changed much for a very long period of time. The toughest, most hardy and heat tolerant species have been pushed about during the previous climate cycles between ice age and tropical.
The synthesis of my proposition is that perhaps what we are calling heat tolerance should be called endurance and toughness. In addition, there could be more than two dominate genetic factors which are responsible for heat tolerance.
I would like to enter an additional tidbit of information: We have reason to believe that the Malesian Rhododendrons are the most heat tolerant of any segment of the genus. These plants are, not hardy much below 30° F; obviously there is no correlation between heat tolerance and hardiness in these rhododendrons.
I realize that I have posed more questions than answers in this article; my main intent was to provoke more thought on the subject. Perhaps additional insight can be added in the future as people in the warmer and dryer areas do more selective breeding and release the results of their programs.
1 Root Zone Temperatures were recorded between 60°-70° F, one inch below the surface, when the air temperatures were greater than 110° F.