rhododendron bloom

Rhododendron in bloom

Updated: May 24, 2021

Key points

  • Rhododendrons and azaleas offer a wide range of flower colors in the spring and have become popular landscape plants in Maryland. 
  • All rhododendrons and azaleas are members of the genus Rhododendron. There are evergreen and deciduous forms of both.
  • Rhododendrons and azaleas prefer similar growing conditions: cool, moist, well-drained, acidic soil (pH between 4.5 and 5.5), partial sunlight, and protection from strong winds. Azaleas will generally tolerate drier conditions than rhododendrons.
  • Avoid planting these shrubs in poorly drained compacted soils, low areas that collect water runoff, and locations near downspouts. These sites will often cause root rot issues.
  • Other related landscape plants that require similar growing conditions include andromeda, mountain laurel, leucothoe, and blueberries.
  • Where growing conditions are unsuitable, these shrubs are susceptible to the disorders and diseases described below.

Abiotic problems

Leaf chlorosis (yellowing leaves)

Chlorosis symptoms, caused by a deficiency of iron, appear as yellow leaves with prominent green veins. These symptoms are usually caused by high soil pH. 

soil test can determine if the pH is too high. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts but is in an unusable form due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways.

Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. Some fertilizers such as ammonium sulfate will also acidify the soil. Foliar sprays of iron sulfate or chelated iron can reduce symptoms.

Chlorosis caused by magnesium deficiency is initially the same as iron but progresses to form reddish-purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. 

Other causes of chlorosis include poor root growth, root rot, root damage caused by over-fertilization or excessive deep cultivation, soil nematodes, and poor drainage.

Winter injury

Low temperatures can cause bark splits near the base of the stem, damaged flower buds, and browning on the edges (marginal necrosis) of the leaves. Both bark splits and flower bud damage can be caused by a sudden temperature drop in the fall before new growth has hardened off, cold temperatures after dormancy have broken in the spring, or after a winter thaw. Winter hardiness will also influence the likelihood of winter damage of marginally hardy varieties. 

Marginal leaf browning that results from drying winter winds is generally called “winter burn”. Tissue necrosis is caused by the removal of water in the leaves faster than the plant can replace it through root uptake from frozen water in the soil. Winter burn can be reduced by placing plants in locations less exposed to winter winds. Physical barriers, placed about 18 inches from the plants on the windward side, made from materials such as burlap or plastic, can also lessen winter wind damage by reducing wind velocity.

Diseases of Rhododendrons and Azaleas

Powdery Mildew

This disease is usually more common on deciduous azaleas compared to evergreen plants. 
Young plants grown in heavy shade are the most seriously affected by this disease. 
Infected plants appear to be covered with a powdery white substance on the leaves. 
The disease is more severe during periods of cool, moist weather. 
These fungi produce spores on the surface of the infected leaves which are spread by wind currents to surrounding leaf tissue. 
These fungi overwinter in the bud scales for initiation of infection next season.

MANAGEMENT
Maintain proper plant spacing to ensure good air circulation. 
Check the label registration on horticultural oil products and registered fungicides for powdery mildew control listings.

By Dr. David L. Clement, University of Maryland Extension Specialist, Home and Garden Information Center. Revised 4/2020.

References

Johnson, W.T. and H.H. Lyon. 1991. Insects That Feed on Trees and Shrubs, 2nd Ed. Ithaca, NY: Comstock Publishing Associates, Cornell University Press. 560 pp.

Pirone, P.P. 1978. Diseases & Pests of Ornamental Plants, 5th Ed. New York: John Wiley & Sons. 566 pp.

Sinclair, W.A. H.H. Lyon, and W.T. Johnson. 1987. Diseases of Trees and Shrubs. Ithaca, N.Y.: Comstock Publishing Associates, Cornell University Press. 574 pp.