Growing Rhododendrons and Azaleas

The Culture of Rhododendrons

Michael Dirr | 1998

Editor’s Note: In the note that follows Dr. Dirr discusses the single greatest cause of Rhododendron death among newly planted shrubs. In a Question and Answer period at the Rhododendron exhibit at Canada Blooms, 1999, 80% of all questions posed were of the form .... “I planted a new Rhododendron last spring. It seemed to grow well for a while and then the leaves started to hang limp late in the summer. I watered it and it helped a bit. This year there are a few small blooms and the plant is not growing. What is happening? They must be hard to grow!!!”.... Dr. Dirr’s ovbservations in the first three paragraphs below, are consistant with our own and other members’ experiences!!! BEWARE OF ROOT BOUND CONTAINER PLANTS!!!

Comments of Rhododendron Culture

Having tracked rhododendrons and azaleas from the Midwest to East Coast to the southeastern United States, I can unequivocally state that inadequate drainage is the most prominent factor limiting growth. If poor drainage does not directly kill plants, it predisposes them to insects and diseases such as root rot.

They have fine silk-like roots, generally without root hairs, and are easy to transplant balled and burlapped or from a container; the root mass is usually profuse, which allows for successful planting; numerous plants are container-grown and the roots form a web (almost fabric-like) at the interface of the medium/container; if plants are left in the containers too long the root mass is so thick and matted it must be cut; ideally make vertical slits with a knife; this will insure contact with the soil when planted and, hopefully, root penetration into the native soil.

I have observed azaleas and rhododendrons newly transplanted into a landscape that were wilted or dead; a simple tug on the plant and removal from the ground showed the plant was removed from the container and planted as a solid mass; water uptake was severely limited as the plantng hole became dry; I suspect more one gallon azaleas die this way than any other cause.

Provide light shade in the North, even heavier shade in the South; have noticed tremendous lacebug infestations on plants exposed to full sun; the State Botanical Garden has a fine collection of ironclad R. catawbiense hybrids that were planted on a slope with the root balls above the soil and covered with pine bark; over the years they have prospered, but have received fertilizer and water as well as an occasional insect spray. Rhododendrons are extremely sensitive to salinity, high pH (chlorosis), and winter injury.

With emphasis, I recommend gardeners contact their local Extension offices for state or regional guides to the best rhododendrons and azaleas. Almost all states produce some type of guide. Recently, Rhododendrons in Alabama, Circular ANR-1 51, crossed my desk. Therein, 31 rhododendrons with hardiness, color, height, and characteristics are discussed. This is a logical starting point for those who simply want to test the Rhododendron waters. Be aware that many rhododendrons are grown in the Pacific Northwest and shipped East. The cultivars may or may not be adapted. Do your homework!

Diseases and Insects: Botryosphaeria canker, crown rot, dieback, dampening-off, azalea petal blight azalea gall, leaf spots, leaf scorch, powdery mildew, rust, shoot blight, shoestring root rot, wilt, rhododendron aphid, azalea stem borer, azalea leaf tier, black vine and strawberry weevils, giant homet Japanese beetle, asiatic garden beetle, lace bugs (in the Southeast, lace bugs are terrific problems with the small leaf evergreen azaleas, although they appear to infest any plant with the generic name of Rhododendron; Dr. S. Kristine Braman, Entomology, University of Georgia, has an active program to determine resistance, et al.; J. Environ. Hod. 10(1):40~3 (1992) discusses resistance in deciduous azaleas); red-banded leafhopper, azalea leaf miner, rhododendron tip midge, mites, mealybugs, pitted ambrosia beetle, rhododendron borer, scales, thrips, rhododendron whitefly, nematodes, stem girdling caused by woodpeckers. Rhododendrons are troubled by many pests and their culture is often fraught with difficulty. Good cultural practices will reduce the incidence of disease and insect damage.

Landscape Value: Like all rhododendrons a nice plant for the shrub border, groupings, massing, foundations; should be sited in a slightly shaded area and out of strong winter sun and wind; survived -20°F in my Illinois garden and flowered nicely.