A tree that the wasps love…

Elen DelSignore

Rhododendrons and azaleas, both from the genus Rhododendron, have long been mainstays of late spring because of their spectacular clusters of showy blooms and large green leaves that often last through winter.

The flowers are usually tubular-, funnel-, or bell-shaped—and often fragrant. The leaves of the smaller azalea are usually pointed and narrow; the leaves of the rhododendron are generally large and leathery.

These shrubs generally perform best if they are provided with moisture and shelter under trees. They prefer climates with adequate rainfall and moist summers. The two main azalea groups, evergreen and deciduous (varieties that drop their leaves in the fall) can be found in nearly ever part of North America, from the frosty Canadian plains to tropical Florida. The rhododendron types are fussier, preferring environments where it is neither too hot nor too cold (Zones 5 to 8). They need a certain amount of chilling to develop strong flower buds.

With thousands of varieties, there are rhododendrons and azaleas for just about every landscape situation. There are low-growing ground cover azaleas as well as plants that can grow up to 25 feet tall. Though most plants flower in the spring, there are also summer-blooming varieties that add color and charm to the garden.

Explore more tips for growing the best varieties of rhododendrons and azaleas.

Planting

• Most large-leaved varieties require dappled shade; avoid deep shade or full sun. A sunny spot that receives a few hours of shade is perfect.
• Soil should be well-drained, humus-rich, moist, and acidic (pH 4.5–6).
• Amend planting areas with compost, peat moss, or a substitute, and oak leaves to achieve ideal conditions.
• Azaleas and rhododendrons have shallow root systems and need moist soil and mulch to keep them from drying out.
• The best time to plant is in late spring or early fall.
• When shopping for plants, pay attention to when they flower. Early varieties can blossom in March, late ones into July or even the fall.
• Buy plants that are a deep green (not yellowed), not wilted, and well watered.  Check the soil in the container with your finger and avoid plants that are bone dry.
• Set new plants so that their top roots are at soil level or slightly below. If you plant them any deeper, the roots may rot.

Care

• Mulch plants annually with 2 to 5 inches of pine bark chips or pine needles to protect shallow roots, retain soil moisture, and keep the soil damp. A lack of water reduces flower-bud formation. (Keep mulch a few inches away from the trunk.)
• Fertilize azaleas and rhododendrons sparingly and only when flower buds swell in the early spring, even if they are fall bloomers. Heavy applications of fertilizer will burn the plants.
• Water plants during the summer if rainfall is less than 1 inch per week.
• After flowering, deadhead where practical, to promote vegetative growth rather than seed production. Remove dead flowers from rhododendrons carefully; next year’s buds are just under the old heads.
• In regions with severe winters, wrap evergreen rhododendrons with burlap in the fall.
• Transplant azaleas and rhododendrons whenever the ground is not frozen or waterlogged.

Warm-Weather Advisory (Zones 7 to 11)

• If your weather heats to above 90°F in spring, avoid white-flowered azaleas. Their thin petals shatter in the heat.
• Plant in a site that receives afternoon shade, especially in hot areas. In tropical zones, azaleas will bloom in full shade.
• Buy plants in 3-gallon cans rather than 1-gallon cans. They’re a better bargain in hot climates. Small plants, with their fewer roots, struggle in the hot late spring and summer.

Cold-Zone Reminders (Zones 3 to 6)

• Plant in full sun to increase flowers and avoid mildew problems. Shrubs need a minimum of 6 hours of full sun daily.
• Plan on the lee side of a windbreak. If subjected to cold, dry winds, their leaves and buds dry out and die.

Pruning

• Do not prune spring-flowering shrubs such as azaleas and rhododendrons.
• On young and old plants, simply snap off spent flower stalks by bending them over until they break away from their stems. Be careful not to damage growth buds at the base of each flower stalk.

Pests/Diseases

• Susceptible to vine weevil, whiteflies, leafhoppers, lacebugs, scale insects, caterpillars, aphids, powdery mildew, bud blast, rust, leafy gall, petal blight.
• If soil is not sufficiently acidic, root rot and lime-induced chlorosis could occur.

Recommended Varieties

There are more than 900 species of rhododendrons, which vary greatly. It may take a book to understand this wide world. You may also be interested in joining the American Rhododendron society which runs a database with information on more than 1,200 plants.  Find out more at www.rhododendron.org.

Here is a handful of varieties that we think you’ll enjoy:

• ‘Blue Diamond’ is a dwarf evergreen shrub that grows to 5 feet. It blooms mid-season with violet-blue flowers. Zones 7 to 9.
• ‘Cecile’ is a vigorous azalea that grows to 7 feet and produces dark salmon-pink buds in mid-season. Zones 5 to 8.
• ”Hydon Dawn’ is a low-growing rhododendron that actually tolerates full sun. Pale pink flowers bloom in mid-season. Zones 7 to 9.
• ‘Nova Zembla’ is an evergreen rhododendron that grows 5 to 10 feet tall and bears deep red flowers in late midseason. Zones 5 to 8.
• ‘Rosy Lights’ is an azalea that offers extra cold hardiness. It grows to 4 feet and bears deep purple-pink flowers. Zones 3 to 8.
• ‘Purple Gem’ is a dwarf rhododendron which grows to 2 feet and is an early season bloomer. It bears small, light purple flowers. Excellent in the front of a border or in a rock garden. Zones 5 to 8.

Wit & Wisdom

The glittering leaves of the rhododendrons
Balance and vibrate in the cool air;
While in the sky above them
White clouds chase each other.
–John Gould Fletcher