Rhododendron

Brian Barth
rhododendron close up

Rhododendrons are evergreen shrubs native to the moist temperate woodlands of the world. They bear stunning flower clusters in spring and are exceptional in foliage and form.

Clarification of Names

There is often confusion when it comes to the names rhododendron and azalea.

Rhododendron leaves
Rhododendron leaves
  • Rhododendron is the botanical name for a group of plants that includes the plants that are called, in common language, rhododendrons and azaleas.
  • Thus, all azaleas are considered rhododendrons, botanically speaking.

However, the plants that are commonly referred to as rhododendrons differ from azaleas in that they have large, thick, glossy leathery leaves. Azaleas' leaves are thin, non-glossy and rarely over three or four inches long; rhododendron leaves are generally between five and eight inches long.

Also, all rhododendrons are evergreen, while azaleas may be either evergreen or deciduous.

Rhododendron Overview

rhododedndron in flower

Dwarf rhododendrons may be as little as three feet tall and wide, while other varieties grow into small trees. However, the vast majority of rhodies available in nurseries mature in the five to 10 foot range.

The individual flowers are about two inches in length, though they grow in large clusters, which are tucked elegantly behind the oversize leaves. Flower color varies among shades of red, white, purple and pink. The shaggy bark is very attractive, as is the compact, rounded growth habit.

Toxicity

It is important to note that rhododendrons are quite toxic and should not be planted where livestock or small, unsupervised children have access to them.

Environmental Preferences

Rhododendrons prefer cool, moist climates and are rarely successful in hot, arid places. As woodland plants, they require shade, though a bit of filtered light coming through the canopy above them is better than deep, dark shade.

Rhododendrons need the richest possible soil with perfect drainage and constant moisture. It also needs to be acidic - a pH between five and six is ideal.

Applications in Landscaping

rhododendron with shade plants

Rhododendrons are the quintessential woodland shrub and are perfect understory plantings for large deciduous trees. They can also be used where a shade-loving shrub is needed for a foundation planting or hedgerow, though they are not amenable to formal shearing (though they have such an attractive shape on their own, that it's not really an issue). Asian-themed gardens frequently make use of rhododendrons, as well.

The dwarf varieties are suitable for container culture and are a great way to add interest to a shady deck or patio.

Planting and Care

Fall is a great time to plant rhododendrons, as the roots will have plenty of time to get established before hot, dry weather comes. Early spring before the flowers open is also an acceptable planting time.

Soil Preparation

Rhododendrons should be planted on a low broad mound at least six inches above the surrounding grade. Spread three to four inches of compost over the mound and work it in to a depth of six inches. If the pH is above six (you can use a pH tester to determine this), use some form of acidifying agent in the soil, such as peat moss, pinestraw, leaf mold, sulfur or aluminum sulfate.

If planting rhododendrons in pots, make sure to use a potting soil that is specially blended for acid-loving plants, sometimes referred to as 'ericaceous' soil.

On-Going Maintenance

Monitor soil moisture very carefully to ensure that the root ball does not dry out. Rhododendrons have dense, fibrous root systems that suck up water quickly, so even if the surrounding soil seems moist, the roots may be dry. It's better to water them with a gentle spray or slow trickle because flooding them with a hose will likely result in the majority of the water rolling away rather than soaking into the root zone.

  • Mulch - A deep layer of mulch is essential to protect the shallow roots of rhododendron from overheating and drying out.
  • Fertilizer - If the planting soil is sufficiently rich, fertilizer is optional, but make sure to use one that is formulated for acid-loving plants. Adding a fresh layer of compost over the topsoil each fall is a great way to fertilize rhodies.
  • Cutting - Rhododendrons can be cut back if necessary to control their size, but it's best to avoid altering their natural form if at all possible.

Pests and Disease

Volumes could be filled with information about the pests and diseases that attack rhododendrons. The truth is, however, that cultivating proper growing conditions is the most feasible way to avoid ill health, as most of what attacks rhododendron can't easily be overcome once symptoms are present.

Soil that is not in the right pH range can cause a variety of nutrient deficiencies and make the plants vulnerable to other health problems. Insufficient moisture and boggy soil are also common culprits.

That being said, removing infected plant material is a good general hygiene practice. Minor issues like scale, aphids, and other insects are readily treated with insecticides and powdery mildew can often be taken care of with a fungicide treatment. Trying to identify other pests or diseases that show up is difficult at best, much less trying to find the right chemical cocktails to treat them. Many simply do not have any effective form of treatment.

Varieties

Plant size and flower color are the primary variables when selecting a rhododendron.

rhododendron flower buds
Dopey
  • 'Koichiro Wada' grows to about six feet in size and has deep pink buds that open into light pink flowers.
  • 'Midwinter' is an early blooming variety with purplish-pink flowers that grows to about five feet.
  • 'Cynthia' is one of the largest rhodies, growing into a 20 foot tree with rose-colored blossoms.
  • 'Dopey' is a red-flowering form that grows to about six feet in height.
  • 'Lavandula' is a dwarf variety growing about three feet tall and wide with purple blossoms.

Temperamental but Irresistible

Rhododendrons are the rose bushes of the shade - they are as finicky as can be, but hard not to love. If you live in warm climate, azaleas are a better option, but if you live in the right climate, rhodies are at the top of the list for the shade garden.

Rhododendron