The rhododendron—from the Greek rhodon (rose) and dendron (tree)—is an acid-loving woody shrub with colorful flower clusters.

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

DESIGNING WITH RHODIES

Here are a few tips for incorporating rhododendrons into your garden:

  • Choosing plants with staggered bloom times will guarantee a few months or more of splendid color.
  • Rhododendrons make wonderful companions to other acid-loving plants and trees, such as ferns and flowering dogwoods.
  • Many bulbs, like tulips, daffodils, and lily-of-the-valley, will bloom around the same time as a rhododendron without competing for water, nutrients, or space.

PICTURES OF RHODODENDRON VARIETIES

Swipe to view slides

Photo by: Cepreй PЫбин / 123RF.

‘SCINTILLATION’

Flower: Sweet pink flowers with greenish-yellow markings in throat.

Habit: A broad-branched shrub.

Height after ten years: Four feet.

Leaves: Evergreen leaves provide winter interest.

Hardy to: -15°F.

Photo by: Igor Grochev / Shutterstock.com.

‘CUNNINGHAM'S WHITE’

Flower: Brown-flecked blooms.

Habit: Dense.

Height after ten years: Four feet.

Leaves: Dark evergreen leaves.

Hardy to: -15°F.

Photo by: Laura Stone / Shutterstock.com.

P.J.M.

Flower: Reddish-purple trusses; heavy-blooming.

Habit: Upright and dense.

Height after ten years: Four feet.

Leaves: Glossy, elliptical leaves.

Hardy to: -25°F.

Photo By: Rob Cardillo.

'CINNAMON BEAR'

Flower: White bell-shaped blooms, about 20 per truss.

Habit: Compact.

Height after ten years: Two feet.

Leaves: New growth has white cast; at maturity, a heavy cinnamon-brown indumentum.

Hardy to: 0°F.

Photo By: Harold Greer.

'ROSEUM ELEGANS'

Flower: Funnel-shaped and lavender-pink with up to 20 blooms per truss.

Habit: Dense and spreading.

Height after ten years: Six feet.

Leaves: Dark olive green and rounded, with cupped edges.

Hardy to: -25°F.

Photo By: Rob Cardillo.

'PRINCESS ELIZABETH'

Flower: Eye-catching deep crimson blooms that turn paler at the base. Forms tall conical trusses.

Habit: Upright and taller than it is wide.

Height after ten years: Seven feet.

Leaves: Dark matte green.

Hardy to: -15°F.

Photo By: Rob Cardillo.

'BERG'S HARDY FORM'

Flower: Vibrant Dark shade of blue that appears in clusters of three and four.

Habit: Upright and compact.

Height after ten years: Four feet.

Leaves: Small (this is a lepidote) and dark green.

Hardy to: -10°F.

Photo By: Rob Cardillo.

'KEN JANECK'

Flower: Deep pink funnel-shaped blooms form on a large truss.

Habit: Compact and mounding.

Height after ten years: Three feet.

Leaves: Curved dark green with striking toast-colored indumentum.

Hardy to: -15°F.

FREQUENTLY ASKED QUESTIONS

What’s the difference between rhododendrons and azaleas?

Azaleas are actually a type of rhododendron. While they are very similar in shape to other rhododendrons, they can be differentiated by their hairy leaves and five stamens (instead of the usual ten).

Are rhododendrons poisonous?

According to the National Capital Poison Center, serious poisoning is unlikely when small pieces of rhododendron or azalea are eaten. However, life-threatening symptoms can occur when large amounts of these plants, or honey made from them, are consumed. The Center recommends watching children and pets closely when outdoors to prevent this from happening.

Why does my rhododendron have yellow leaves?

Wherever soil tests neutral to alkaline (i.e., yielding a pH reading higher than 7), rhododendrons will be hard to grow. When the pH is too high, their leaves turn yellow. Try lowering your soil’s pH by digging in organic matter. If this doesn’t work, try an above-ground growing mix for rhododendrons made from 50 percent well-rotted manure, 40 percent high-quality topsoil, and 10 percent shredded leaves, worked together with a 5-inch layer of peat moss.

Why doesn’t my rhododendron flower?

Failure to bloom is often caused by one of the following two factors: Cold weather kills their flower buds, or they aren’t getting enough direct sun. Pick a rhododendron variety that has been proven hardy in your area and plant it in a location with an eastern or southern exposure.

WHERE TO SEE RHODODENDRONS

Visit the following gardens to see rhododendrons at their finest:

  • The Rhododendron Species Foundation Botanical Garden, the world's largest collection of rhododendrons, covers 22 acres in a conifer forest just south of Seattle. The garden boasts more than 600 of the 1,000-plus identified species.
  • Crystal Springs Rhododendron Garden is just minutes from downtown Portland and contains an outstanding collection of rare species and hybrid rhododendrons and azaleas. Explore the seven-acre garden during April and May when the plants are in full bloom.
  • The annual Heritage Museums & Gardens Rhododendrons Festival in Cape Cod is a great place to see many varieties in bloom. Enjoy peaceful walks surrounded by majestic flowers and participate in activities with horticulture experts.
  • Rhododendron State Park in Fitzwilliam, New Hampshire features a 16-acre grove of Rhododendron maximum, which has a soft pink, almost white, flower and is the only elepidote species native to New England. Visit in July when the fragrant clusters of pink blossoms burst into bloom-immerse yourself in their perfume by following a trail that encircles the grove.
  • Jenkins Arboretum & Gardens in Devon, Pennsylvania features a diverse collection of over 5,000 rhododendrons and azaleas from around the world. The first blooms begin in late March and the last end in late July.

Related:

Best Flowering Shrubs
Pruning Shrubs & Perennials

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