Rhododendrons

A tree that the wasps love...

Credit: Elen DelSignore
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Botanical name: Rhododendron

Plant type: Shrub

USDA Hardiness Zones: 4, 5, 6, 7, 8, 9

Sun exposure: Part Sun

Soil pH: Acidic


Flower color: Red, Pink, Yellow, Purple, White

Bloom time: Spring, Summer

Rhododendrons and azaleas, both from the genus Rhododendron, have long been mainstays of late spring because of its 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 for the smaller azalea are usually pointed and narrow; the leaves of the rhododendron are generally large and leathery.

This shrub generally performs 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 leave 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.

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

  • 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.

Special Features

  • Attracts Birds

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

Free E-Card

Send this e-card of a colorful rhododendron!  Or, browse more flower photos in our e-card gallery!

Comments

I live in Oregon and have a

By houfie on April 18

I live in Oregon and have a large number of rhodies. Most of them are doing great, with glossy leaves and profuse blooms. I have a couple, though, that look stressed. The leaves are drooping and have a brownish tinge. The buds have set, though, and look like they will bloom. I'm curious if I need to move them (both are in shade), amend the soil or fertilize. Many thanks for any advice.

I am thinking of planting a

By cia van orden on April 18

I am thinking of planting a rhod. in a largish pot in a shady spot in Sunset Zone 15. Northern Ca. Guerneville. I see many around the hood but in the ground.

I have had my rhododendron

By Atha Garland on April 14

I have had my rhododendron for 2-3 years now and they are still not blooming that is the problem

It can take 2-3 years after

By Almanac Staff on April 18

It can take 2-3 years after planting for rhododendrons to bloom. There are also many other reasons for a lack of flowers and some varieties bloom less than others. Cold temps. in the spring can kill new buds. Does you rhododendron get enough sun? Add compost to the soil around the bush and you may also want to test the soil (5.5 pH is recommended for rhododendrons). See our planting and care tips on this page for some more guidence.

What kind of fertilizer do

By Joan DiSarcina on April 6

What kind of fertilizer do you recommend for rhododendrons?

Very little fertilizer is

By Almanac Staff on April 7

Very little fertilizer is necessary for rhododendrons if you have proper soil preparation with organic matter and mulchingwith compost. They have low nutritional requirements.
You do want to ensure your soil acidity. The plants thrive best at a soil pH between 5.0 and 5.5.
Do not fertilize rhododendrons at planting; newly planted shrubs can be fertilized after they become established.
Some gardeners do an application in early spring. Ask your garden center for a fertilizer especially formulated for acid-loving plants and follow directions. Fertilizers supplying the ammonium form of nitrogen are best.
A fertilizer analysis similar to 6-10-4 applied at 2 pounds per 100 square feet to the soil surface is usually adequate. Cottonseed meal is also a good fertilizer. Do not apply any fertilizer after the first week in June. 

I have moved into a house

By new to this

I have moved into a house that has azaleas in the front of the house. There are 5 plants but all are sparse and leggy, I would like to know how I can bring them back to a full beautiful state, any suggestions?

After the blooms have faded,

By Almanac Staff

After the blooms have faded, you can prune the azaleas. Prune to shape and prune back the taller shoots to where they originate. Don't prune after July or you'll remove next year's flowers.

I just planted new

By Stacy Dohlman

I just planted new rhododendrons last fall. This winter A rabbit drastically pruned one back. All the leaves are gone and it is very short. Will it survive? Or should I plan on replacing it?

We'd go ahead and cut the

By Almanac Staff

We'd go ahead and cut the rhododendruns back, water, and fertilize them with aluminum sulfate, and see if they make it. You'll know soon.

I have older plant they have

By marjorie siegman

I have older plant they have become very leggy. Can I cut them way back? If so when and how? Thank you

Normally, rhododendron need

By Almanac Staff

Normally, rhododendron need very little pruning. However, they can become leggy as the years roll by. You can prune in the early spring. Here is a great preference page from the American Rhododendron Society. Pay attention to: When to prune (early spring), How to Start, and Pruning to Rejuvenate.

I just planted a Pieris

By Beaver Den

I just planted a Pieris Japonica Variegata in a very sheltered spot near my front porch. It doesn't get any direct sun there. I live in Ottawa, Ontario where we have very cold winters. The garden center neglected to tell me how particular these plants are. Do I have to use burlap every winter on this shrub? I am tempted to return it.

Your biggest issue is lack of

By Almanac Staff

Your biggest issue is lack of direct sun.  These plants need full sun or partial sun and shade. They'll grow without direct sun but they won't flower well. Growing in a protected site is good as they need to be sheltered from winds. A site sloping
to the north or east is best. Low areas should be avoided as should areas beneath shallow-rooted trees, including maple, ash, and elm.
Very little winter protection is needed if you have the right planting site. Cut back on watering in mid-September to help plant tissues mature before winter. However, a good watering before freeze in November is a good idea. If you have tree limbs (or an old Christmas tree), it can help to lay the boughs around the plants for added protection in wintertime. The rhododendron is a great winter plant because it keeps its lovely green leaves throughout the season.

Can a rhododendron be

By John J

Can a rhododendron be transplanted by digging a portion of the plant out and moving to new location? If so when?

Yes, rhodies transplant well

By Almanac Staff

Yes, rhodies transplant well if done properly. You need to get all the roots out. In terms of timing: Early spring transplanting is recommended for cooler climates.  Very late summer to late fall transplanting is preferred in warmer climates. Really, the only bad time is the hot periods of summer.

I have heard it is possible

By Gallatin

I have heard it is possible to start new Rhody shrubs by cutting and planting the pieces. Does this actually work? Do you need a root-growth powder to dip the ends into first?

Yes, you do need a rooting

By Almanac Staff

Yes, you do need a rooting hormone. The powder kind is most common. Here's a good reference page on how to root a hydrangea cutting: http://www.rhododendron.org/v48n4p201.htm

Rhodaderum got to big need to

By Vernon L Stallings

Rhodaderum got to big need to cut back when can I do it

In general, rhododendrons

By Almanac Staff

In general, rhododendrons don't need to be pruned, however, if they are getting too big, reduce the size with light pruning. Remove the flower clusters on rhododendrons as soon as flowering is complete. If it's just overgrown, you can actually prune it back to the stubs and it will recover in a few years and become a shapely young-looking plant. 

My azaleas (20 years old)

By KenP

My azaleas (20 years old) have never been pruned but have in the past couple of years been overrun my my neighbors ivy. The ivy is very difficult to control. And suggestions? Thanks, Ken

Do you mean English ivy? This

By Almanac Staff

Do you mean English ivy? This is a non-native plant which is an invasive that threatens other plants. Perhaps you can speak to your neighbor because it needs to be cut down before seed set and potential spread of this species. Chemical control measures for English Ivy can be
found in the Plant Conservation All
iance web site here: http://www.nps.gov/plants/alien/fact/hehe1.htm

we live in south and received

By A nonymous

we live in south and received a rhododendron for b,day should we wait until fall to plant it out it get real dry in summer. where should we keep it if we wait until fall under a shade tree or in the house thanks

In warm climates,

By Almanac Staff

In warm climates, rhododendrons are best planted in the fall. They require: excellent drainage, a soil pH between 4.5 and 6.0, and protection from hot afternoon sun so a high canopy of filtered shade is a good idea. Oaks and pines are a great companion. They must have a constant supply of moisture but never sit in stagnant water.

pruning rhodies

By Anonymous

do you mean NEVER prune? We have two large rhododendron bushes which are growing almost to roof height, one of which is also growing outward invading the driveway. i had heard that you must be careful when pruning rhodies, but would like some specific guidelines, if possible. Thanks!

You CAN cut them back but no

By Almanac Staff

You CAN cut them back but no need to routinely prune a rhododendron. According to the American Rhododendrum Society, "If a plant grows out over a walk or needs to be restricted for some reason, it may be pruned back moderately without fear that the plant as a whole will be damaged. It is often possible to do this pruning during the blooming season and have flowers for the house. Old leggy plants may need pruning, but often these are better replaced with smaller newer varieties. Old plants, however, can be cut back severely and still recover, although it may be a while before they bloom again."

Thanks

By mikeknox1952

I just wanted to drop you a line to thank you for your hospitality during my stay. I got this "rhodo" shot from the Old Farmer's Almanac site. I hope you like it.

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