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Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Rhododendrons: Spring color in shady areas

David Trinklein
University of Missouri
(573) 882-9631

Published: May 3, 2019


For sheer "flower power" few shrubs in the landscape can match the colorful elegance of rhododendrons. Available in a wide array of colors including white, pink, carmine red, lavender, deep purple and blue, their flowers are borne in terminal clusters that may reach twelve inches in diameter. Well-suited for shady landscapes, these attractive evergreen shrubs usually reach their peak of beauty in early May in Missouri.

The term rhododendron is derived from the Greek words rhódon (rose) and déndron (tree). Literally interpreted, therefore, rhododendron means "rose tree." Although some of the rhododendrons we grow might have rose-colored flowers, they are not roses. Additionally, most of the types we grow are shrubs, not trees. The genus Rhododendron is a member of the Ericaceae plant family and contains over 1000 species. A goodly number of species are native to North America.

rhododendron flower

Years ago, rhododendron and its "look alike" azalea were classified in two separate genera. Today, they are grouped together, with azalea being considered a subgenus of rhododendron. Most species and hybrids of azaleas flower earlier than rhododendrons, but there are other ways to distinguish one from the other. Most true rhododendrons are evergreen. In contrast, most azaleas shed most or all of their leaves in the fall. Also, true rhododendrons have ten or more stamens in their flowers, while azaleas have five. There are other differences between the two spring-flowering shrubs, but the growing conditions required by both are the same.

Although considered to be shade plants, rhododendrons should not be planted in deep shade. The latter will result in a lack of flower production. There often is a fine line of distinction between too much shade which will reducing flowering and too much sun which gives rise to leaf burn. Ideally, rhododendrons should be located in filtered shade, out of the wind.

Proper soil preparation is a must for rhododendrons to thrive. They have fine root systems that grow near the surface of the soil and are not able to penetrate heavy, tight soils. For best results, prepare the soil deeply and add about 50 percent organic matter such as peat moss, compost or leaf mold. Good soil drainage is very important. Raised beds or soil mounds can be used to facilitate the latter, if organic matter alone is not sufficient.

Rhododendrons require an acid soil, with a pH between 5.0 and 5.5 being ideal. Soils having received a lot of lime in previous years may need to have their pH lowered before planting rhododendrons. This also holds true for garden soils that have been irrigated with alkaline water for a number of years. The importance of an acid soil is so critical that testing the soil before planting would be a good investment of time and resources.

For plants already in place, soil pH can be lowered gradually by adding iron sulfate or agricultural sulfur. The latter is finely ground elemental sulfur that has been pressed into small pellets using bentonite clay as a bonding agent. Adding about one and one-fourth cups of iron sulfate or two and one-half pounds of agricultural sulfur per l00 square feet of garden will lower soil acidity by one pH unit.

In contrast, in rare cases where soil acidity is too low, use ground limestone to raise the pH to about 5.0. In either case, do not try to change in the soil's acidity too hurriedly. If in doubt, add less of the material being used to change soil acidity and make a second application in the fall or following spring.

Rhododendrons should be planted no deeper than the level they were grown. Whether container grown or field grown and balled and burlaped, the ball of soil around the roots should be set slightly higher at planting time to allow for settling. A mulch of pine needles, hardwood chips or aged sawdust over the soil will lower maintenance requirements. Wait until after flowering has been completed before applying fertilizer. Fertilizers formulated to have an acid reaction in the soil are available commercially and marketed as rhododendron and azalea food. Read and follow label directions carefully. Excessive fertilization can lead to root damage.

Watering is important during hot, dry summer periods to prevent damage to the shallow root system. A generous layer of mulch can minimize the water problem, but cannot substitute for additional water during periods of drought. In early fall, pull the mulch back from the base of the stems to allow hardening of the bark before the advent of winter. Push the mulch back against the stems again about the time the first hard freeze is predicted.

There is a wide variation in the winter hardiness of rhododendrons. Screens that provide shade during the late part of the day and protection from winter winds are helpful in overwintering plants successfully. The evergreen leaves of rhododendrons should be exposed to light during the winter. Therefore, try to avoid covering them when providing winter protection.

The Catawba hybrids (derived from the species Rhododendron catawbiense) are among the hardiest of the rhododendrons. Cultivars suited to our climate include 'Nova Zembla', 'Roseum Elegans', 'Catawbiense Album' and 'Catawbiense Boursault'. Rhododendrons known as PJM hybrids are also good choices for Missouri. The American Rhododendron Society maintains a searchable database on its webpage (http://www.rhododendron.org/search_intro.asp) where cultivars can be identified by traits such as winter hardiness, flower color, bloom date and mature plant height.

Once established, rhododendrons require little care other than watering, management of soil acidity and monitoring for pests. The latter includes mites, scale and an occasional borer. Other problems include root rot (in poorly drained soils), winter dieback, and diseases such as leaf blight, canker and powdery mildew.

Rhododendrons find many uses in the landscape. In large settings, they look especially elegant in the filtered shade of towering oaks, pines or other large trees. Additionally, they make good foundation plants in shaded areas or can be tucked into smaller gardens.

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