As we enter into our second year in the test garden with the new Southgate rhododendrons (presented as an answer for Deep South gardeners), what follows is an introduction to leading causes of rhododendron mortality.
Introduction to Rhododendron Woes: Ill-spent Youth
So, when I was younger, I went through the inevitable phase of falling in love with rhododendrons, a particular folly for those who live in areas where summer temperatures do not fall below 52 degrees F. Inevitably (after a number of tries), the rhododendrons I planted died, plain and simple. For all of you who have admired rhododendrons in the garden centers every spring and succumbed to a purchase, I am here to tell you: It is (mostly) not your fault.
Why Rhododendrons Fail, Short Course:
For those of us who visit areas with cool summer nights (think Appalachian mountains), it can be disheartening to see rhododendrons majestically covering the landscape of seemingly every home one passes, the memory fresh of our own pitiful plants withering away down on the Piedmont (or Coastal Plain).
There are two primary disease threats to rhododendrons (and these threats strike fast and hard like assassins in the night): Phytophthora and Botryosphaeria dothidea. Rhododendrons being rhododendrons, there are other threats (leaf gall, petal blight, on and on), but these two fungi are the real killers. Clemson University has a good write-up on the primary dangers to rhododendrons.
These diseases need three things to thrive: moisture, warmth, and something else that escapes me for now. The key is the warmth part. In areas where night-time summer temperatures fall below 50 degrees F. or so, the fungus' life cycle is interrupted. It is important that rhododendrons be planted correctly no matter where you live; if you live in an area where the climate will not be of any help in breaking the fungus' life cycle, then you must be meticulous in helping on the issue of moisture sitting around the roots. i.e. you must plant correctly.
Our part in why rhododendrons fail: We plant them too deeply (and they are often pot-bound as well).
If you are foolhardy enough to buy a rhododendron for your garden and you live in the Deep South, then I both pity you and admire your pluck. Rhododendrons need a good supply of moisture but excellent drainage; this seeming contradiction is solved by having extremely friable soil that receives regular rainfall.
The great mystery for many gardeners who lose rhododendrons is that they can't figure out whether the problem was too much water or too little. The thing they really need to figure out is how to plant a rhododendron properly; plant a rhododendron with the goal of providing a place that prevents water sitting around roots.
How to Plant A Rhododendron Properly
I'm gonna keep this short and simple: When planting a rhododendron, ignore almost everything you've learned when it comes to planting in the garden (big deep hole). Rhododendrons and azaleas have fibrous roots that grow very close to the surface. Dig a big hole in the clay of the Piedmont for a rhododendron, and you've dug a drowning pit.
Here's what you do: Dig a hole roughly 1/2 to 2/3 the depth of the root ball. Set the rhododendron in the planting hole and then fill with a mix specific to rhododendrons and azaleas (acidic and loose). Fill in area around the rhododendron to height of original rootball. You will need to grade the inevitable mound gently away to avoid erosion problems. (Bonus: the method works with Oakleaf Hydrangeas as well, who also will not tolerate wet feet.).
My description may not suffice, so here is an excellent write-up on how to plant a rhododendron from the American Rhododendron Society.
Caring for a pot-bound (a.k.a. root-bound) Rhododendron
Sudden death of rhododendrons 2-3 years after planting is often because they were pot-bound from the start. If you buy a plant and roots encircle the rootball, you should address this issue. The Rhododendron Society of Canada (Niagara region) has a good guide to the care and planting of pot-bound rhododendrons.