Hardiness of Rhododendrons
Rhododendron ABC's #6
Now that we are familiar with the terms species and hybrids and have found that there are hundreds of species in the Genus and thousands of hybrids from interbreeding of the many species, it is time to turn our attention to some of the horticultural aspects of rhododendron collecting.
Let us imagine that we have been affected with at least a little bit of what can (and may) become a lifelong infection known as rhododendronitis. What to do? First of all, infectees should come to grips with climatic factors in their particular region.
We know that rhododendrons have developed in and adapted to various and climatically different regions and, as a result, respond in diverse ways to fluctuations in growing conditions. Some have developed where frosts seldom, if ever, occur; some in regions where temperatures go below freezing but where frosts do happen; and, happily, there are many types that shrug off frosts and can withstand long and severe bouts of cold weather. So, pay attention! and always try to find out the "hardiness rating" of plants in which you are interested.
Hardiness is generally expressed as some sort of number that indicates the lowest temperature at which a plant can normally survive. Other factors such as dryness, heat and sun tolerance, disease and pest resistance and exposure to excessive air movements are also important, but coldness is the most important consideration for the plant's long-term survival.
The Royal Horticultural Society of the UK has for many years used a scale of H-1, H-2, H-3 and H-4 when rating plants for hardiness: H-1 plants can't stand frosts and must be wintered in warm-houses; H-2 plants can stand about 10' of frost (down to about 22° F) for short periods; H-3 plants should be hardy in regions where the lowest temperatures are of relatively short duration; H-4 plants are for regions where the lowest temperatures are as low as or lower than 10° F. Unfortunately, this does not tell us whether an H-4 plant will survive 0 degrees or -10 degrees or lower. Generally speaking, an H-4 plant should survive most winters in the Pacific Northwest. However, in the Central States and the Northeast, one needs to find out the experiences of other growers with the particular plant. It is important to recognize right away that the H-1 to H-3 plants will not survive cold winters such as those experienced near the Great Lakes or New England. The American Rhododendron Society has abandoned a number rating and uses the lowest temperature that a plant can tolerate to rate its hardiness.
After hardiness, then what? You have selected a plant that has sufficient cold resistance, so now you must get it and set it in an appropriate spot. (I recommend that you buy or obtain your plant from a reputable grower with experience in your climatic region).
The best results will be with newly dug plants. The next best will be with plants grown in containers. The plants most likely to give trouble will be those that are balled and wrapped in burlap or plastic. These latter may be starved for water or may have been frozen in the wrapped condition. Plants whose root balls have been solidly frozen give particularly disconcerting results. The plant right after planting may look okay, put out new growth, but then gradually (or sometimes rapidly) wilt and expire several months or even longer after planting.
This also holds true for evergreen (has leaves all year) and deciduous (drops leaves in winter) azaleas.
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