Elisabeth C. Miller Library

Gardening Answers Knowledgebase

Search Results for: Rhododendron | Search the catalog for: Rhododendron

Recommended Websites

Meerkerk Rhododendron Gardens

Rhododendron Species Foundation & Botanical Garden

American Rhododendron Society

Azalea Society of America

Seattle Rhododendron Society

More websites

Plant Answer Line Question

Keywords: Pruning shrubs, Rhododendron

I need to know what to do with a rhododendron that has grown too big. I want to keep it, since it is a bookend to another plant. Can I cut it back, and if so, how far and when? Will it be okay and continue to bloom if I cut it back? Could you suggest something and also suggest a really good book on care, etc., for rhodies?


For general care, quick information is available at

1) The American Rhododendron Society, click on "Need Help Growing Rhododendrons?"

2) The Seattle Rhododendron Society has a Rhododendron Care page

For more extensive information, there are scores of great books. A good one that includes specifics about rhodies is Cass Turnbull's Guide to Pruning (Cass Turnbull, 2004). Plant Amnesty, founded by Turnbull, also has information on pruning an overgrown rhododendron.

You can also select one by thumbing through the paperbacks available at (almost) all nurseries. Sunset Publishing and American Horticultural Society are reliable publishers.

As an aside, there were some ancient, neglected, potentially beautiful rhodies at my home. They were pruned slowly over 3 to 4 years and look great now. So, don't be shy!

Date 2018-04-21
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Transplanting, Rhododendron

Some friends of mine just bought an old house with a huge rhododendron up against the house. It is at least 8 feet high and probably 10 feet wide. I did not dig around and there may be multiple shrubs growing next to each other. What are the chances of moving the rhody successfully? Should it be cut way back before hand? Any particular time of year for moving it?


Fortunately, rhododendrons are very likely to succeed in being transplanted. Most experts recommend fall as the best time to transplant. Spring or late winter is second best.

The real challenge is getting a large enough rootball. A five-to-six foot plant requires a rootball of about 3 feet in diameter.

Step 1- dig a 12-18 inches deep trench around the rootball.
Step 2 - under cut the rootball to sever the roots from the underlying soil. The most important roots are the small feeder roots, not the big old ones. You can use a steel cable with a tractor or you can use a shovel and digging iron and a lot of hard work. The rootball will probably be about 8 - 12 inches deep and 3 feet in diameter.
Step 3 - tilt it on its side and slide a piece of 1/2-inch plywood under the rootball and set the plant upright. Use the plywood to move the plant to its new location. (A tarp works, too, if you can get it underneath the rootball.)
Step 4 - dig a new hole 4 feet in diameter and deep enough so that the rootball is 1 inch higher than the depth of the hole. (Slightly above grade)
Step 5 - water well and mulch around the perimeter of the plant BUT keep the mulch at least 2 inch away from the trunk of the plant.

Newly transplanted plants need some tender care and especially need to be watered regularly, but not over watered.

There were no recommendations to cut the foliage back. But it is always ok to prune out dead, dying, diseased or deranged stems. This also means you can prune out twiggy growth.

This information comes from Success with Rhododendrons and Azaleas by H. Edward Reiley (1992).

Date 2018-12-28
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Frost, Hardy plants, Rhododendron

A friend in Illinois has sent a photo this spring of a very healthy looking rhododendron - leaf buds fully elongated and beginning to unfurl, while the green, blunt flower buds remain unopened. The flower buds don't look brown, diseased, frozen or injured, but they remain tightly closed, foliage bud growth preceding blooming. He says he has 6 plants doing the same this month. Possible reason?


Though we can't diagnose plant problems by phone/email, early autumn frosts can inhibit flowering and not all buds are equally affected.

"Autumn frosts: These can lead to damage...if they either occur in early autumn or immediately after a late season warm spell. Continental climates with extremes of heat and cold are more likely to suffer sudden temperature changes than those with maritime climates...A sudden temperature drop will catch a plant before it has had a chance to reach maximum hardiness and it may suffer accordingly, even if normally perfectly able to withstand such a temperature in mid-winter...Speed of ripening varies considerably...There is also a variation in the hardiness of flower buds compared to foliage and growth buds. Commonly, flower buds may be as much as 10 F. less hardy than foliage..."
(Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 119-120)

Date 2018-12-28
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Pruning shrubs, Rhododendron

A friend was told that pinching out growth buds before they begin to elongate as a means of shaping young rhododendrons would only stimulate buds further down the stems that were less than 4 years old - older than that and the growth buds would no longer be viable. I cannot find any information to suggest 4 years viability of dormant buds to be true, or untrue. Can you help?


Though pinching encourages multiple branching lower down the stem, I find no reference to it being done at a particular age.

"This practice (pinching) is recommended for most larger rhododendrons until they reach flowering size..."
(Source: A Plantsman's Guide to Rhododendrons, by K. Cox, 1989, p. 101)

That statement indicates a younger plant, but the author then mentions several exceptions.

Here is some how-to information about pruning online:
7 Solutions to the Too-Big Rhododendron.

Date 2018-04-05
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Rhododendrons--Washington, Rhododendron

My two rhododendrons did not produce any blooms this year- they are healthy otherwise. Why?


I had the same problem with one of my rhodies this spring (all the others were fine), as did many other people in the Pacific Northwest. Following are the most likely causes:

NO FLOWERS, BUDS DO NOT OPEN. This is most likely to be caused by frost, either in mid-winter by the hardest frosts of the year, or in spring when the buds are swelling and about to open. Certain varieties have very frost-vulnerable swelling buds, while many species have buds which are easily destroyed even by quite mild winter frosts.

NO FLOWERS, NO FLOWER BUDS. There are several possibilities why rhododendrons may not flower freely:

  • Too much shade. This is very common in North America where, in order to regulate sun and soil temperature, plants are placed in deep shade. This allows healthy, if straggly growth, but can inhibit flowering. The more light you can give a plant, the more likely it is to flower, so there is a trade-off between the need for shade and the need for light.
  • The variety takes many years to flower (it does not sound like this is your situation).
  • Kindness. Rhododendrons flower in order to reproduce. A contented, well-fed, well-watered well-shaded plant may not feel any need to reproduce, as it perceives no threat to its survival. Do not feed after mid-summer, as this encourages growth at the expense of flowers. Nurserymen cut down watering in late summer to stress plants into flowering the following year.

(Source: Rhododendrons: A Care Manual, by K. Cox, 1998, p. 73).

The above is corroborated in other sources, e.g. Success with Rhododendrons and Azaleas, by H.E. Reiley, 1992, p. 132-133.

Date 2019-02-23
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Failure to flower, Frost, Rhododendron, Master gardeners

This is the second year in a row that my rhododendron Blue Peter has flower buds but they are dry and somewhat dark and have no flowers at all. These buds are easy to deadhead. Can you help me salvage this rhododendron, which is very old, and beautiful when it blooms?


In order to get an accurate diagnosis you will need to take a sample of your plant (including both healthy and affected parts if possible) to a Master Gardener clinic. If they do not know what it is, ask them to send the sample to the pathology laboratory in Puyallup. It is best to go through Master Gardeners first so you will not be charged. If you send the sample yourself there will be a fee.

Meanwhile, several sources mention frost, drought, and "bud-blast" (unlikely in the Pacific Northwest) as potential causes of bud failure. Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought reports from various places give mixed results from applying magnesium (usually as Epsom salts)... By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently loose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.

Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244.

Date 2018-04-11
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Rhododendrons--Diseases and pests, Rhododendron, Master gardeners

My rhodies have black spot, rust. Is there a plant medicine I can put in the soil so it will get absorbed by the entire plant rather than spraying every other leaf.


I am sorry to hear about your sick Rhododendrons. You should take a take a leaf sample into a Master Gardener clinic for (free) diagnosis. I have linked a list of clinics in Snohomish County below. Their volunteers are trained in identifying plant diseases and suggesting solutions.

If you cannot get into a clinic try the HortSense webpage from Washington State University Cooperative Extension, and search the common ailments affecting rhododendrons (in the Ornamental Shrubs section).

The reason why it is vital to get an accurate diagnosis is because some fungal diseases do not have treatments that really work, such as rust, while others "leaf spot problems" are not caused by fungus at all, therefore spraying with fungicides or applying a systemic to the soil would only be a waste of time and money!

Try contacting the Snohomish County Master Gardener Clinics to see if you can bring in samples.

Date 2018-07-13
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Rosaceae (Rose Family), Rhododendron, Fertilizers

Is it okay to fertilize my rhodies, azaleas, and roses in September? I missed doing it in August.


Generally speaking, it is best not to fertilize your shrubs after mid-summer. The tender new growth that results is susceptible to frost, disease, and insects just at the time of year when the plant is beginning to shut down. This is also true of roses, which are even more tender and susceptible than rhododendrons and azaleas.

An article by Terri Richmond (British Columbia) on the American Rhododendron Society website, entitled Fertilizing Rhododendrons the Organic Way supports the practice of fertilizing in spring. (Keep in mind that azaleas are in the same genus as rhododendrons.)

Oregon State University Extension suggests that budbreak in spring is a good time to fertilize roses, just as new growth is beginning. Stop fertilizing in late summer. Oregon State University also weighs in on fertilizing rhododendrons (if needed, in spring shortly after flowering, and preferably with organic fertilizer).

Date 2018-06-22
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Transplanting, Rhododendron

Can I move my rhododendrons now, in late winter to early spring?


According to A Plantsman's Guide to Rhododendrons by Kenneth Cox (Ward Lock Ltd., 1989), "rhododendrons are generally quite easily moved, most even in full flower and at considerable age. [...] Size is really no problem, provided you have the means to do the digging and the moving. Obviously, the more rootball you can take with the plant the better, but usually you can reduce it considerably without too much harm being done. If you end up with a disproportionately small rootball, you can reduce the size of the top somewhat to compensate. The roots of a rhododendron generally extend to about 50% of the plant's foliage diameter [...] it can be far more or much less. The roots are usually less than 18 inches deep, even on a very large plant. To move a large plant, start digging [...] quite far out from the stem, and continue towards it until you meet roots. Then dig all round underneath the rootball [...] gently rocking the plant to ease the rootball from the soil. Watch out when lifting a plant by its main stem; it may not be strong enough to carry the weight of the rootball. The root can best be reduced by prising soil from it with a fork. [...] A rhododendron can remain out of the ground for considerable periods if you keep frost and sun from the roots, and ensure that it receives regular watering. Heeling it into the ground, or covering the roots [...] usually gives adequate protection. Although rhododendrons can be moved during the growing season, they will require extra watering after transplanting."

In addition, you may find the Royal Horticultural Society's directions on moving a mature tree or shrub helpful.

Date 2018-03-01
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Rhododendron, Poisonous plants, Pets

Are azaleas poisonous to cats?


Azaleas are indeed a problem for cats and other pets. See this link from Purdue University's Veterinary Program.
"These ornamental shrubs aren't commonly nibbled on but they can cause fatal heart problems in dogs, cats, and pet birds. Signs to watch for are similar to that of the yews and include weakness, fainting, salivation, difficulty breathing, vomiting, and diarrhea."

According to the Handbook of Poisonous and Injurious Plants, 2nd ed., by Lewis Nelson et al. (New York Botanical Garden/Springer, 2007), all Rhododendron species, including Azaleas, contain grayanotoxins in their leaves. Honey made from the flower nectar would also be toxic.

Date 2019-01-11
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Shrubs--Care and maintenance, Rhododendron

Should you remove old blooms from rhododendrons and, if you should, which is best: to prune or snap them off?


Here is what the American Rhododendron Society says on the subject:

"It is desirable, with the large flowered rhododendrons, to remove the withered flower clusters after the blooming season. This is fairly easily done as the central axis of the cluster, usually called a truss, will break free from the plant with a quick snap of the thumb pushing on the side, or can be cut off with a hand pruner. With the smaller flowered rhododendrons and azaleas, dead-heading is labor intensive and and generally is not required.

Dead-heading is usually done to make the bush look more attractive, to reduce the prevalence of fungus and to prevent a heavy set of seed. If it is not possible to remove the old flowers, it is usually not too detrimental, but flowering the next year may be reduced."

I have several mature rhododendrons in my own garden, and I deadhead the parts of the shrubs which are easily reachable, leaving the other areas to their own devices. For me, it's an aesthetic choice, and I would probably do them all if I could reach and if I didn't get very tired of the task. (It's hard to do well with gloves since you can't easily feel the right place to snap off the flower head, but it's sticky work without the gloves.) I've never tried pruning them off, because it seems less precise (leaves a bit of a stub), but if rhododendron experts approve (as indicated above), I may just try it this year.

Date 2018-03-14
Link to this record only (permalink)

Garden Tip

Keywords: Species Rhododendrons, Rhododendron

You need only take a walk through Washington Park Arboretum or peer into your own backyard to notice that May is the season of the rhododendron. It would seem that rhododendrons are native to the Northwest, the superb way they thrive both in the cultivated garden and wild forest floor. But those rhododendrons, which have become such a mainstay in Pacific Northwest flora, are relative newcomers to these parts and have been plucked by enchanted plant hunters from China and the Himalayas.

Jane Brown tells the dramatic and long history of the rhodies global travels in her recent book, Tales of the Rose Tree: Ravishing Rhododendrons and Their Travels Around the World, (Harper Collins, $36.75). In this accessible historical account, Ms. Brown tells of the legend & lore, as well as the botanical significance, of the rhododendron. She includes many fine illustrations and color plates of many notable representations of the rhododendron. In addition, she lists many of the best places to find rhododendrons, mainly in the UK, where she resides. Travel to the Royal Horticultural Society's Wisley Garden or the Edinburgh Royal Botanic Garden for grand displays of this woody and lovely plant.

Or remain closer to home and visit The Rhododendron Species Foundation and Botanical Garden in Federal Way. Twenty-two acres encompass nearly 10,000 rhododendrons in all shapes, sizes, colors, and scents. From March through May, the Garden is open from 10:00 - 4:00 six days a week (closed on Thursdays). June through February, the Garden is open 11:00 - 4:00 five days a week (closed Thursdays and Fridays). Admission is $3.50 for adults and $2.50 for seniors and students. For additional information and directions, call: 253-927-6960.

Date: 2007-04-03
Link to this record (permalink)

Garden Tip

Keywords: Sarcococca, Plant cuttings, Salvia, Lavandula, Propagation, Rhododendron, Gaultheria shallon, Penstemon, Holly, Cistus, Ceanothus

Make new plants by taking softwood cuttings. Cuttings Through the Year, a booklet published by the Arboretum Foundation(available for sale at the Washington Park Arboretum gift shop) suggests which plants to propagate month by month and how to do it. A few September plants include: Rock Rose, Salal, Lavender, Holly, Penstemon, evergreen azaleas, Sweet box, Salvia, California Lilac and many others.

For a tutorial on taking softwood cuttings go online to a Fine Gardening article complete with clear color photos: www.finegardening.com/propagate-your-shrubs-softwood-cuttings

Date: 2006-10-23
Link to this record (permalink)

Reviewed by: Brian Thompson on 2014-10-01

Back in the Garden with Dulcy cover

A book that took me totally by surprise is “Back in the Garden with Dulcy”, a selection of articles by Dulcy Mahar, who for 22 years wrote a gardening column for The Oregonian in Portland. Clearly, I haven’t been paying enough attention to gardening south of the Columbia!

Sadly, Dulcy died in 2011 after a long battle with cancer, but she continued writing up to a few weeks before her death. Fortunately her husband, Ted Mahar, has edited and published a selection of her writings. I am completely charmed by the results.

While Ted is understandably also a fan, I heartedly agree with him when he describes her columns as “…filled with solid advice, warnings, lists, ideas and experiments worth trying, the latest trends, yearnings for a change of season, and more. Whatever the subject, Dulcy’s wit glowed through. Pick a week, and you’d likely find a quotable quote.”

I would add that she had a knack of reaching out to young or inexperienced gardeners, putting them at ease, urging them not to be afraid to just go for it. She also had a love of animals, especially her cats (although one lucky dog, Hector, gets a lot of press, too). One of her Wagnerian felines is posed with her on the front cover, “helping” in the garden.

An example of her advice: “Make a list so that you can get exactly what you need when you hit the nurseries and plant sales. Oh, I could hardly say that with a straight face. I am practically rolling on the floor, and the cats and Hector the dog are looking askance. Of course, it is excellent advice. But can I follow that advice? Hardly.”

Excerpted from the Fall 2014 Arboretum Bulletin.

Link to this review (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again: