Growing Rhododendrons & Azaleas in Your Garden

Photo by: Giles Barnard.

Zones:

Rhododendrons can be grown in Zones 4-9; however, the most variety is available in Zones 5-8.

Rhododendron types:

There are eight subgenera of rhododendrons to choose from—some are tall like trees, others are more bush-like, some are evergreen others are deciduous.

Here are the five most commonly sold types of rhododendrons:

• Azaleas (deciduous and evergreen)
• Species rhododendrons
• Elepidotes (meaning leaves without scales)
• Lepidotes (meaning leaves with scales)
• Vireya (tropical and often epiphytic)

Height/Spread:

The mature size of a rhododendron varies depending on the species and cultivar. Some are as small as 18-inches by 18-inches, while others grow to 20-feet by 20-feet. “It used to be that everything would be six feet and taller after ten years,” says Michael Martin Mills, former president of the Greater Philadelphia chapter of the ARS. “Nowadays, you can even find elepidotes that will be no more than three feet tall in that time.”

Bloom time:

Peak bloom generally occurs in midspring; however, some rhododendrons bloom as early as March and others bloom as late as July.

Flower color:

Rhododendrons offer a virtually unlimited palette of floral color. Their colorful flower clusters, known as trusses, come in many hues: pink, white, reddish-purple, deep crimson, blue, and yellow.

Exposure:

Rhododendrons do best in partial shade or filtered light, especially those with large leaves. Tiny-leaved alpine and dwarf species are best in bright sun.

Climate:

The damp, mild climate of the Pacific Northwest is perfect for growing rhododendrons; while, growing rhododendrons in colder climes, especially the Northeast, can be a challenge. “We often envy people out West because they can grow things we can't,” says Philadelphia's Michael Martin Mills. “A lot of Asian species and their hybrids can't survive our cold winters.”

Whether you live in a mild, cold or hot climate will determine what time of year is best for planting rhododendrons. Photo by: John Swithinbank / Alamy Stock Photo.

When to plant:

The American Rhododendron Society (ARS) recommends the following planting times based on your climate.

• Mild climates: Rhododendrons and azaleas can be planted year-round
• Cold climates: Early spring planting is best, with early fall planting a good second choice
• Hot climates: Fall planting allows the plant's root system to establish during the cooler months

Where to plant:

In addition to filtered shade and good drainage, pick a spot with protection from the wind. The ARS recommends that you avoid planting rhododendrons near concrete because it creates alkaline soil conditions that are detrimental to their development. Furthermore, don't plant “rhodies” near other surface roots that compete for space, water and nutrients

Soil:

Rhododendrons thrive in acidic soils (pH 6 and lower) that are light, well drained, and rich in organic matter. “They actually like to grow on top of the landscape, as opposed to deep in the ground,” says Jenkins Arboretum's Harold Sweetman. Amending the soil with organic matter such as leaf mulch or fine bark will help both the acidity and drainage.

Planting tips:

• Water your rhododendron thoroughly before planting
• Loosen the root system prior to placing the plant in the ground to stimulate new root growth
• Position the crown of the root ball a few inches higher than the surrounding soil
• Space plants according to their mature size
• Avoid smothering the trunk of your rhododendron with mulch

When deadheading, be careful not to remove the buds or shoots below the flower cluster. Photo by: Agencja FREE / Alamy Stock Photo.

Watering:

Due to their shallow, fine roots, rhododendrons require regular watering through dry periods. They will show signs of drought stress much sooner than plants with deeper roots. Keep the soil consistently damp, but don’t let it become soggy.

Mulching:

Mulch with compost, bark chips, or pine needles to prevent weeds, since hoeing can easily damage a rhododendron's surface roots. Mulching also helps retain moisture. Replenish the mulch annually, or as needed.

Pruning:

Rhododendron shrubs can be pruned to show off their sculptural trunks or to give an arched or cascading effect. Do your pruning right after they finish blooming and be careful not to cut off next year’s buds (rhododendrons bloom on old wood).

Fertilizing:

Proper soil preparation before planting, along with regular mulching with organic material means extra fertilizer is usually unnecessary. If you think your soil is no longer up to par, apply a fertilizer designed for acid-loving plants in late winter or early spring. If you’re having a specific problem, check out these recommendations from ARS.

Transplanting:

If your rhododendron has outgrown its spot, don’t worry, its shallow roots make for easy transplanting.

Diseases & pests:

The best way to prevent damage to your plants is to make sure they are properly planted and well maintained.

Rhododendrons are susceptible to the following diseases:

• Branch die-back
• Phytophthora and Armillaria root rot
• Powdery mildew
• Gall
• Petal blight

The following insects can be a problem:

• Lace bug
• Weevils
• Thrips
• Rhododendron borer
• Spider mites
• Aphids
• Azalea caterpillar
• Leafminers