JARS v36n1 - Light

Bob Wright, Tacoma, WA

Reprinted from Tacoma Chapter Newsletter

When you make your first purchase of rhododendrons, if you are like the majority of the rest of us, you asked the person you bought them from how they should be grown. The most frequent responses to that question provide information about drainage, humus in the soil, soil pH, planting depth, fertilizers and winter protection. It is not too unusual to be told to avoid planting them anywhere with a southern exposure or where they are exposed to direct summer sunlight. With all this information in their heads, many persons have been known to plant their new prize in a foundation planting on the north side of their home or under the Birch tree that is also newly planted in their street side yard. Several years later, they come to the conclusion that rhododendrons are difficult to keep alive or are very difficult to bring into bloom. If they had been given information about necessary light levels and competition with the roots of other plants, they may have come to other conclusions.
In their native habitats some of the large-leafed specimens will grow where they do not receive much direct sunlight, but they will be growing where they are exposed to high levels of light. This has been suggested as one reason why so many of those species which grow in mature forests are successful epiphytes. By growing on other trees they are able to find enough light to survive and bloom very successfully. Rhododendrons which do survive in insufficient light levels rarely do more than produce vegetative growth. Plants which do not bloom are not successful in reproduction and therefore contribute nothing to the succeeding generations in that location.
High levels of direct sun light can be harmful. If insufficient water is available, exposure to direct sunlight can raise the temperature within the leaf high enough to cause dehydration, damaging the leaves. Exposure to high levels of direct sunlight during the time the ground is frozen can also cause damage from dehydration. It makes good sense then to put your plants where they can have adequate water supplies during the year and where they are protected on clear days during the middle of summer and during the winter.
One way of sheltering the plantings would be to plant them in association with other trees, provided they do not produce too much shade or have invasive roots which would compete with the roots of the rhododendrons for nutrients and water. An extra advantage of planting in this way is that the companion plants can break up airflow and that can prevent wind dehydration. Some trees which produce heavy shade can be thinned to allow more light to penetrate to the planting beds. The lower limbs of conifers can be removed to allow more light around the base of the tree while the crown provides protection from too much direct sunlight. Some conifers, such as Douglas Fir and Hemlock in our area have very shallow root systems so plantings under them should be far enough from the trunk to avoid this problem.
Deciduous trees with large leaves can produce problems when they drop their leaves. Dwarf alpines can be smothered by a heavy leaf drop. Soggy leaves on any rhododendron can cause that portion of the plant to rot and could be the cause of death of the plant. Alpines probably should be grown in an almost open setting anyway to approximate their natural environment.
Peter Cox (DWARF RHODODENDRONS) and David Leach (RHODODENDRONS OF THE WORLD) discuss these problems in more detail and suggest companion trees which can be used in landscaping a city lot which are compatible with your favorite plants.