Stress - Is It Ever Good for Rhododendrons?
As every gardener knows, watering is much more an art form than a science. With various soils, humidity, exposures and microclimates most of us gardeners are content to set the clock on a water timer and hope we guessed correctly. Add to this the many climates we have, giving exact advice on watering is problematic.
But many times the advice is given that "mild drought stress in late summer or early fall will initiate the onset of dormancy causing a rhododendron to cease growth and reduce metabolic activity" and therefore "harden" the plant for winter protection. Our observations over a twenty-year time period show just the opposite.
Climate-wise we live on the southern end of the Willamette Valley in Oregon - the Umpqua Valley to be exact. Portland is on the north end. Springs are long and sometimes rainy. Summers have an average 85°F to 95°F (29°C to 35°C) and are dry. Fall usually turns into Indian summers with an early freeze followed by warm weather. Rain can start at anytime between September and late November. We do not get Portland’s summer or heavy winter rainfall. As you can imagine we need to water from mid spring to mid fall.
Once the plants do not need as much water because of cool temperatures, short days or early rain we limit our watering. But we never intentionally put any drought stress on our plants.
Our flower buds develop during the months of July, August and September. If water is withheld to the point of drought stress in late summer or early fall late budding plants will not form flowers. And many flower buds will desiccate. What we do have to watch out for - and this we have to remind our customers - is not to fertilize past early June. We can have a second flush of growth which will still be tender by the first frost. But this is caused by late fertilization, not late irrigation.
Last fall we had a record breaking 11°F (-13°C) the first few days of November. Of our 600 rhododendrons in our garden over 200 had some frozen flower buds. A half dozen plants lost all their buds. All our customers had similar experiences, many losing all their flowers. The people with the most damage had dry rhododendrons. Winter hardiness does not seem to correlate with "drought stress."
Rhododendrons are evergreen; they do not go into dormancy. We buy our nursery plants from the Oregon coast and plant (heel) them in western red cedar bark in late fall. By early spring their roots have grown 1 to 3 inches into the bark. They are not dormant in the winter. That is why Oregon State University recommends fertilizing rhododendrons in late fall. Their roots grow all winter long. Fertilizing in late fall/early winter puts nitrogen into the leaves and stems of the plant which helps prevent damage from winter freezing, something that could not be done if the plants were dormant. They do need to harden off before severe freezing. They need to slow down their metabolic activity. But this is not dormancy. Proper use of fertilizer (using it before June) and the cool weather slow down their activity, not "mild drought stress."
Mid winter hardiness also needs a certain amount of moisture in the root ball. Dry plants freeze at higher temperatures than moist plants. We discovered this with one of our hard freezes many years ago. It was 0°F (-18°C) for almost a week. Plants that were under eaves and other dry spots froze. Plants that were 3 feet away and the same varieties and moist did not. All our plants are under fir trees. None had winter sun exposure. The only factor was how dry they were. This principle has proved true year after year.
Stress never seems to be a positive factor for the plant. Many people claim that a stressed rhododendron will bloom at a younger age than a vigorous one. And certainly those gnarly department store plants seem to bear this out, especially those plants grown by large wholesale plant factories in Oregon and Washington and earmarked for the East Coast trade. They always have tiny crowded root systems and extra thick stems. They show every sign of little irrigation, over fertilization, compacted soils and sun stress. Yes, they have been stressed, and they are fully budded. And you wonder how it was that they have survived, let alone budded up so well. Have they budded because of the stress? Is stress a good thing? Or has the rhododendron reached a point that the ratio of plant mass is much greater than the root ball and therefore considers itself to be “mature”? This state would have occurred in another year or two of proper cultivation. If we knew the answer, we would know why rhododendrons bloom.
So where did this concept of stress producing early dormancy, and dormancy protecting plants from freezing come from? Answers to that question could go a long way to explain its validity. Perhaps some ARS historians can give us the answer. It may be a question of climate. It may be necessary in some areas. But not in our climate.