Tips for Beginners: Watering Rhododendrons
Rick Peterson
Federal Way, Washington
Reprinted from the Rhododendron Species Foundation newsletter, January 1996

Rhododendron species grow in very diverse habitats, but one condition most of these regions share is available moisture throughout the growing season. Those species native to the Himalayan mountains are drenched by summer monsoons from June to October. The north and west-facing slopes of the Caucasus Mountains of Turkey are home to several species, and this area receives heavy rainfall distributed throughout the year. In the areas of eastern Canada and the United States rainfall is generally uniform during the year, although there are occasional extended intervals of dry weather during the summer months. A small number of rhododendrons have evolved a tolerance for periods of drought, but these are the exceptions.

For species in cultivation, when there are prolonged intervals without rainfall during the growing season, supplemental watering is necessary, using either hand-set sprinklers or irrigation systems. The key to watering rhododendrons is to achieve a balance between keeping the root zone moist, but not saturated. Where drainage is less than perfect, or summer drought conditions prevail, it is essential to monitor soil moisture. Water thoroughly and as deeply as possible. Allow sufficient time for partial drying so that oxygen can penetrate the soil. Where drainage is poor, rhododendrons intolerant of this condition will suffer. They should be moved or the drainage improved by either raising the plants above the wet soil level, such as onto a berm, or planting them in raised beds.

During the growing season rhododendrons require adequate water from the time the leaves begin to emerge until they are mature; failure to do so will often result in smaller, misshapen foliage. Be attentive the first spring and summer after a rhododendron is planted. Until the roots begin to move into the surrounding soil, they will use water from within the root ball; therefore, it is important that the root ball be monitored for moisture as well as the area around the new plant.

The ideal time of day to water is in the early morning, when the air is often calm and the temperature is cool. There will be less evaporation because of these factors, and thus less waste of a precious resource. Under some circumstances, however, it may be necessary to water at midday when the air temperature may be very high, particularly during the summer. Despite a widely held misconception, watering at this time does not increase the potential for leaf scorch. In fact, watering with overhead sprinklers actually lowers the temperature of both the air and leaves. Also, the humidity surrounding the plants is increased and this lessens the risk of sunburn.

In late summer or early autumn, some very mild drought stress may be beneficial to initiate the onset of dormancy, i.e., causing a rhododendron to cease growth and reduce metabolic activity for the winter season. For example, in the Puget Sound region of Washington State, the growing season can extend into October or later if temperatures remain mild and the fall rainy season has begun. Occasionally in November the temperature will suddenly plummet low enough to cause injury to plants that have not reached dormancy.

Rick Peterson is the Co-Executive Director and Garden Manager of the Rhododendron Species Botanical Garden in Federal Way, Wash.