Rhododendron and Azalea

The genus Rhododendron is a large and diverse group that includes not only the "rhodies" but also Azaleas. These shrubs are some of the most popular of all the shade tolerant flowering shrubs. Their huge clusters of blooms create a dazzling display during the spring and into early summer.

Beautiful Diversity

There is great diversity within this group with many different growth forms ranging from dwarf varieties that grow only a foot tall to the towering native giants that can reach up to 20 feet in the wild. Many varieties have beautiful deep green evergreen foliage and some are deciduous. Most require little pruning as they have a very attractive natural layered growth form. They are like the "perfect" garden shrub when planted in the right location!

Rhododendrons and azaleas are typically grown for their beautiful displays of flowers and they have been widely hybridized over the years to create an even greater diversity in flower color, foliage, and growth form.

Plant Wisely!

Rhododendrons thrive in moist, well-drained, acidic soil that is high in organic matter. The optimal pH for these shrubs is between 4.5 and 6.0. If your soil tests higher than 6.0, it is advisable to add a soil acidifier to bring the pH down to within the recommended range. See the section below for our recommendations for lowering the soil pH.

Wet feet are deadly for these plants! Be sure to plant them in an area that is well-drained as they are prone to root rot and other fungal diseases when the soil is constantly soggy.

In general, Rhodies and azaleas prefer bright dappled shade but there are some varieties that can tolerate full sun and some that will grow and bloom well in dense woodland shade.

They will not tolerate hot, dry, or windy conditions. Plant them in a location that is sheltered from high winds that can desiccate the foliage and cause damage especially during the winter.

Fertilize these plants with a slow-release organic fertilizer for acid-loving plants in the fall and again in the late winter or early spring. Espoma Holly-tone is a great choice for rhododendrons.

Pruning Rhododendrons

Pruning is generally not necessary except perhaps for light pruning to remove wayward branches and spent blooms or to maintain the desired shape or size. This light pruning should be done right after they finish blooming. To remove the spent flowers, carefully cut or pinch them off without damaging the small leaf buds that have formed just behind them.

If your Rhododendrons have gotten a bit too tall, bring them back down to size by cutting the stems back to just above a whorl of leaves. Be sure to prune carefully so that you maintain the natural layered look of these beautiful shrubs.

Severe pruning of very overgrown rhododendrons and azaleas is a great way to bring them down in size if they have grown too large for your space. Because these plants have dormant buds on the interior bare wood close to the main stem, they can be pruned heavily until just bare branches remain or you can even cut them back to one to three feet above the ground. This type of hard pruning should be done in late winter or early spring  while the plants are still dormant. Severe pruning like this is not always 100% successful but a very high percentage come back with beautiful new growth!

Acidifying your soil

It is a good idea to have your soil pH tested before planting your rhododendrons or azaleas. Providing the plants with the best growing conditions will help ensure their success in the garden.  

• Sulfur in some form is the most effective way to acidify your soil. Straight chemical or elemental sulfur is commonly used but it is slow to affect a change in soil pH.
  • Aluminum sulfate causes a more rapid decrease in soil pH but the aluminum can become toxic to plants if it is overused.
  • Iron sulfate adds iron to the soil as well as lowering pH.

• Organic amendments for lowering soil pH include peat moss, pine straw, or sphagnum moss. Though these amendments are beneficial to soil structure, they must be added in large quantities in order to affect pH and then the pH change is very slow.

If you have the right conditions, these beautiful shrubs are a "must have" in the garden!