MASON-DIXON CHAPTER, ARS

RHODODENDRON FAQ

This list contains some questions that are often asked about rhododendrons and azaleas, along with brief answers. Naturally, each situation is unique, so the answers may not be appropriate in every case. For more information, see our Readings page or consult with a local nurseryman.

A CAUTIONARY NOTE: This cultural information is aimed primarily toward the mid-Atlantic region and may not apply to all areas. For example, the deep south has a more serious problem with root rot, and raised beds containing a good deal of pine bark are often called for there.

RHODODENDRON vs. AZALEA

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What is the difference between rhododendrons and azaleas? What about deciduous azaleas?

Azaleas are rhododendrons, part of the Ericaceae or Heath family. Plants commonly called azaleas have thinner, more pointed, hairy leaves. The plants commonly called rhododendrons and azaleas are different enough, however, that they seldom cross-breed.

Deciduous azaleas lose all their leaves in winter and are genetically different enough from the evergreen types that they rarely cross breed.

   
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Are the cultural requirements for the two types different? Do deciduous azaleas differ in their requirements?

Cultural requirements are pretty much the same for all rhododendrons, including the deciduous azaleas.

   
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Can azaleas usually take more sun than rhododendrons?

Yes, azaleas can usually take more sun than rhododendrons without suffering leaf burn or desiccation.

   
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What do the terms lepidote and elepidote mean?

An elepidote rhododendron generally has larger leaves and no scales on the underside of leaves. Lepidotes have scaly leaves.

   
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How do you tell the difference between small-leafed rhododendrons and azaleas?

Small leaf rhododendrons generally have thicker, darker green leaves. Azaleas' leaves tend to be hairy.

 

PLANTS AND PLANTING

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Is it better to purchase plants locally rather than by mail order? What about the large chain stores?

As long as the cultivar is known and is one that is adapted to the region, it doesn't matter where it is purchased so long as it is a healthy plant.

   
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When is the best time to buy and plant new rhododendrons?

The best planting time for USDA hardiness zone 6 and south is the early fall; for zone 5 and north, the spring is better. (See the USDA zone map, to determine your zone. A draft version of an updated map is available here.)

   
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I recently bought some plants in containers that are root bound. What is the best way to prepare them for planting?

Loosen and remove some of the potting medium until root ends are pulled out of the mix and extended out into the soil when planted. Work soil around extended roots with your hands, packing with feet will tear roots.

   
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I have read conflicting things about preparing the soil for planting. Some sources say to add organic matter like peat moss, while others say it is better not to use soil amendments. Which way is best?

Most plants grow better in soils high in organic matter. Amend a 3-4 foot wide area no deeper than the root ball. Dig into the soil 3 inches of sphagnum peat moss and fine pine bark (a 50-50 mix). Organic matter improves soil drainage of excess water, while holding moisture for better growth.

   
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What about moving established plants?

Large plants can be moved as long as a good fibrous root system is present. (Click here to learn the story of moving a really large rhody.)

   
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I bought some plants on sale this fall, but don't want to plant them until spring, as the location where they are going has a large number of spring bulbs. What is the best way to keep them over the winter?

Plants can be held over winter outside in the shade with roots buried with soil or mulch. Do not allow them to dry out. In a case like this, it may be better to use the sale plants in an area where they can be planted in the fall and purchase others in the spring to plant among the bulbs.

   

GROWING TIPS

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When is the best time to fertilize azaleas and rhododendrons? What about rooted cuttings--should you wait for them to get established before fertilizing?

Fertilize when the plants first start blooming. If it is needed, you can also feed in the fall after a killing frost. In many cases, a good mulch, well maintained, may provide all the plants' nutritional needs. Remember, too much fertilizer is worse than none.

For cuttings, feed them as soon as they are well rooted using 1/3 strength liquid fertilizer .

   
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What is the best mulch to use?

A coarse mulch such as shredded pine bark or wood chips (Make sure this is composted! Fresh material will rob the soil of nutrients.), pine needles or oak leaves all make good mulches for rhododendrons. Don't use maple leaves as they tend to pack down. The amount to use depends on the material. For loose items such as oak leaves or pine needles, up to 4" is OK; for the smaller chips, only an inch or so is enough. The mulch needs to be loose enough to allow oxygen to pass through. (Caution: don't apply the mulch too deeply, like the mounds often seen around trees in shopping centers and the like.) Also, a small area a few inches in diameter close to the stem should be left clear. Don't use hardwood mulch, as it can cause problems with azaleas and rhododendrons.

   
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I have heard that azaleas and rhododendrons often don't grow well next to the house. Is this true?

No, they will grow well next to the house so long as there is not mortar or other alkaline material mixed in with the fill around the house. They should also be planted far enough away from the house so they have room to grow normally and the roof overhang doesn't prevent them from getting the moisture necessary for growth.

   
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Is there any way to keep the deer from eating my azaleas and rhododendrons?

Most chemicals will deter deer to some extent. However, when the population is high (as it is now due to the mild winters recently), a dog or a fence may be the only cure. Rhododendrons with a heavy indumentum (fuzz on the underside of the leaves) are not popular with deer and are usually not bothered. Some of the cultivars known as "Yaks" have this indumentum. Among the most popular plants are 'Mist Maiden' and 'Ken Janek.' There are some links on our links page that will refer you to more information.

   
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Is there any way to lessen the impact of drought?

A well-maintained mulch will help to keep up the soil moisture. When water is needed, water the plants deeply so that the entire root ball and the surrounding soil gets the moisture. Shade will also help cut down moisture loss through the leaves.

   
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What exactly does it mean when a plant is said to be hardy to zero?

Hardiness ratings usually refer to flower buds. A plant said to be hardy to 0 F would be expected to flower after a low temperature of 0 F. The plant itself will likely survive a colder temperature. It should be noted that the roots are more sensitive to the cold than the top growth, so plants grown in containers need to be protected in the winter.

   
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Is there any standard meaning to the terms "slow-growing" or "fast-growing?"

No, these are just general descriptive terms.

   
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What are some of the best plants to grow alongside rhododendrons and azaleas?

There are number of effective companion plants to use in the landscape with rhododendrons and azaleas, both to extend the flowering season and provide contrasts with either the flowers or foliage of the rhododendrons and azaleas. Some of the best trees and shrubs are dogwood (Cornus), mountain laurel (Kalmia), and holly (Ilex), along with many of the conifers. Bulbs such as snowdrops (Galanthus), daffodils (Narcissus), and lilies (Lilium) work well in these situations. Well-established plantings of snowdrops may start to flower in early January and the lilies may extend the flowering season through much of the summer. Perennials such as hostas (Hosta), foxgloves (Digitalis), and bloodroot (Sanguinaria canadensis), as well as a number of others, also grow well with rhododendrons.

   
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Can you prune rhododendrons? How long before they start to grow back?

Yes, rhododendrons may be pruned if required. Most azaleas and rhododendrons usually look best in their natural state, but pruning may be required for various reasons. Bushier plants can be encouraged by pinching out the terminal buds. (If the growth buds are all about the same size, branching will probably occur naturally. Be careful here and don't pinch out the flower buds, normally much larger than the growth buds.)

For plants needing major attention, either a 2-3 year program or an all at once effort can be undertaken. If new shoots are appearing below the pruning point, cutting back all at once should allow the plant to recover within 3-4 years. If these new shoots are not there, a 2-3 year program allows light in to encourage the growth of new shoots.

Naturally, dead, damaged, or diseased branches should be cut off when the problem is first noticed.

   

BLOOMING

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How much sun does the average rhododendron need to bloom well?

Rhododendrons should have bright light for at least half the day.

   
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When do the flower buds for the next year form?

The flower buds form during the late summer. On most rhododendrons, the flower buds are obviously larger and fuller than the leaf buds.

   
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How long can you make the blooming season last with the right plants?

In a normal year, it is possible to have plants in bloom from March until June in the mid-Atlantic area. In recent years, some fall blooming cultivars have been developed.

   
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About how long does it take for azaleas and rhododendrons to bloom from seed?

First of all, only self-pollinated species and hybridizers' crosses should be grown from seed. Hybrids will not come true from seed. That said, Rhododendron yakushimanum may bloom in only 2-3 years, slower ones may take up to 7 years.

   
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What is deadheading?

Deadheading is the removal of dead flowers. This is important with the large leaved rhododendrons as it encourages strength to go toward more bud production and improves the following year's bloom. Otherwise this effort would go into seed production. Azaleas and elipidotes should flower well without deadheading.

   

DISEASES: MAYBE, . . . OR MAYBE NOT

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What are the most troublesome diseases for rhododendrons in the mid-Atlantic area?

The most troublesome diseases are fungus related: die back, root rot, and petal blight.

   
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One of my rhododendrons is wilting even though the soil is moist. What is wrong?

The cause is most likely Phytophthora root rot (caused by Phytophthora cinnamoni) or some other type of root damage. Root rot is a fungus disease that causes the entire plant to wilt and die. It usually affects smaller plants that have recently been transplanted. There is no cure. The entire plant should be removed and disposed of in the garbage.

   
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I have heard of something called Sudden Oak Death that is causing numerous problems on the Pacific Coast. I live in the mid-Atlantic region. Should I be concerned about this?

This is something that we should all be aware of. Sudden Oak Death (also known as Phytopthora canker disease) is a fungus-like disease caused by Phytopthora ramorum and was first identified in Europe in 1993 and in California shortly thereafter. The actual geographic origin of P. ramorum is not known, and it is thought that it arrived in Europe and the United States separately. It affects as many as 60 species of plants, including oaks (where it does the most damage), rhododendrons, camellias, mountain laurel, pieris, viburnum, lilac, and a number of other common landscape plants.

Phytopthora ramorum causes two types of problems: cankers on the trunk (the most serious infection) and various types of lesions on leaves. The disease seems to be deadly only in certain species of oaks, when their trunks are girdled by the cankers. The species with foliar infections are normally not killed but play a key role in Sudden Oak Death, acting as hosts to aid in the spread of the spores. [It is a foliar disease on rhododendrons, causing brown lesions with fuzzy edges on the leaves. Stems can be attacked. Unfortunately, symptoms are similar to leaf damage caused by many other fungal problems and can only be diagnosed in the lab.] The situation is compounded when other diseases attack the weakened plants. Pytophthora ramorum appears to do best in cooler, wetter climates.

As of this date, the disease has been found in at least 18 states, including Maryland, Virginia, and Pennsylvania, but in nature only in parts of California and a small area in Oregon. And that is where the great danger lies: that Phytophora ramorum could be spread throughout the United States by infected nursery stock. At least 39 states have received potentially infected plants. A number of states have blocked any shipments from nurseries in the affected areas of California, and the Federal government is also restricting the movement of plants. Gardeners must still be on the alert for signs of the disease in recently purchased host plants. This is easier said than done, however, as the symptoms vary from species to species. Positive identification must be done through laboratory testing.

The situation regarding Sudden Oak Death is very fluid, with new information appearing constantly. For more, including a list of affected plants, and the proper action to take if you suspect you may have a contaminated plant, see the following web sites: California Oak Mortality Task Force, University of California Cooperative Extension in Marin County, California Department of Food and Agriculture, the USDA's APHIS (Animal and Plant Health Inspection Service) site, and US Forest Service. Several Maryland sites have valuable information from both a local and national perspective: Maryland Home and Garden Information Center, Maryland Department of Agriculture, and Maryland Department of Natural Resources. [Note that much of the material on all these SOD sites is in .pdf files, so you will need Adobe Reader to view them.] Using a search engine such as Google will also help keep you up-to-date on this ever-changing topic.

   
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The leaves on a couple of my plants are very pale with little veins visible. Is there a problem with them?

This is probably an iron or magnesium deficiency, most likely due to the pH of the soil being too high. Use iron sulfate or an acidic fertilizer to lower the pH. Get your soil tested to be sure. (Maryland Cooperative Extension no longer provides soil tests, but their site does have information on using a soil testing lab and a list of regional soil test labs.)

   
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The undersides of the leaves on some of my plants are sort of fuzzy. Is this OK?

This fuzz is called indumentum and is a normal part of the leaf structure on certain varieties of rhododendrons.

   
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Some of them have new leaves that have some gray stuff on. Is this mildew?

New leaves on R. yakushimanum and some other species have indumentum on both the top and bottom of the leaf. That on the top usually wears off as the leaf hardens.

   
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When it is cold in the winter, the leaves on many of my rhododendrons curl up almost like cigars. Is there something wrong with them?

Leaf curling in cold weather is a natural defense against moisture loss through the leaf surface.

   
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Some of my rhododendrons' lower leaves are turning yellow and dropping off. What is the problem?

It is perfectly normal for rhododendrons to drop the oldest leaves each year in late summer or earlier during drought conditions .

   

These answers were largely provided by Ed Reiley, the past President of the American Rhododendron Society and a charter member of the Mason-Dixon Chapter. For more details on these and many other subjects, see Ed's books, Success with Rhododendrons and Azaleas (Timber Press, Portland, Oregon, revised edition, 2004) and Ortho's All About Azaleas, Camellias, and Rhododendrons (Ortho Books, 2001). He is also the author, along with Carroll L. Shry, of Introductory Horticulture (Delmar Learning, 2000).

Copyright Mason-Dixon Chapter, ARS, 1999-2004.

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Last modified: June 22, 2004