hobbitmomFebruary 21, 2007

hi Im currently fertilizing many plants right now, and am wondering if you folks fert rhodys and azaleas now, or just after bloom. If I were to fert now, would it make any difference for this spring's blooms?? thanks

Thank you for reporting this comment. Undo
morz8(Washington Coast Z8b)

Compared to most shrubs, rhododendrons and azaleas have low nutritional requirements and often don't need fertilizers when grown where soil is the correct ph (like PNW). If they are showing symptoms of needing supplemental fertilizer, you could address those symptoms.

Mine are top dressed with compost each Spring - none have ever been fertilized.

    Bookmark   February 22, 2007 at 10:56AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

I agree totally with morz8. I fertilize about once every 5 or 10 years. Then I fertilize in the spring before blooming and at half the rate on the Hollytone package. In addition to a little Hollytone, I must add superphosphate also since my soil is very deficient in phosphorus. I also add a little extra sulfur for acidity. But that is only once every 5 or 10 years.

    Bookmark   February 23, 2007 at 12:30PM
Thank you for reporting this comment. Undo
Embothrium(USDA 8 Sunset 5 WA)

Key words "my soil". Sample your soil and have it analyzed before fertilizing. If fertilizer indicated, do it in fall - unless a severe deficiency is discovered. In that case fertilize right away.

Spring is less suitable for fertilizing hardy plants than fall.

    Bookmark   March 4, 2007 at 4:45AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

Do not fertilize in fall. Rhododendrons should go dormant in fall and fertilizing in fall can delay dormancy leading to frost killing new growth and buds. This is more of a problem on some varieties than others. Do not fertilize in fall.

The correct time is to fertilize before spring bloom and then only use half the recommended rate on the package of good fertilizers like Hollytone. I personally only fertilize about once every 5 to 10 years. I have to adjust acidity and increase phosphorus levels in my soil. Your soil may not need any help.

    Bookmark   March 4, 2007 at 10:48AM
Thank you for reporting this comment. Undo
Embothrium(USDA 8 Sunset 5 WA)

"The best time to fertilize most existing woody landscape plants is in the fall. Consider the following contrast of conditions:

In the spring the air is getting warmer with the longer days but the soils are cold, generally wet and poorly aerated. With the spring flush of growth, root activity proceeds slowly since the bulk of the energy from the leaves goes to the new top growth. On the other hand, in the fall the soils are warm while the air is generally better aerated and when combined with greater energy levels in the stems and roots, root absorption of nutrients proceeds rapidly....

A great deal has been written regarding fall fertilization, growth continuing late in the season and increased winter injury. This is mostly false. With temperate zone plants the decreasing air temperatures and shorter days discourages new growth even with high levels of nitrogen. There are exceptions, however. For example, crapemyrtle (Lagerstroemia indica) is not photoperiod (length of day) sensitive with respect to vegetative growth and if sufficient nitrogen and moisture are available and the temperatures remain warm, late fall growth may continue and winter injury may be greater than if less nitrogen were available...."

--Carl E. Whitcomb, Establishment and Maintenance of Landscape Plants (Lacebark Inc., 1987 (1991))

Here is a link that might be useful: Lacebark Inc.

    Bookmark   March 4, 2007 at 3:41PM
Thank you for reporting this comment. Undo

There is a bit of an issue with what constitutes "dormancy" in evergreen plants. A viable argument can be made that these never enter a fully dormant state and that while growth may slow considerably during colder months, they never shut down or suspend the biological process to the same extent that herbaceous or deciduous plants do. It is exremely common here in the NW to see rhodies that have been cut back hard to flush shoots and new growth throughout the winter. At any rate, all types of plants will continue root growth as long as soil temperatures remain above 40F and will continue to absorb and metabolize nutrients during this period. And in the milder zones - like the OP's - where soils never freeze, it is not difficult for soils in protected locations to remain at this temperature throughout much of the winter.

As long as the formulation doesn't contain high concentrations of nitrogen, the cautions against late season fertilizing promoting top growth vulnerable to winter damage are pretty much groundless. Bboy's comments are sound and based on documented evidence.

    Bookmark   March 5, 2007 at 8:59AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

Most people that fertilize in the fall are using too much fertilizer. Rhododendrons have fine roots near the surface and are very susceptible to fertilizer burn. They are not your run-of-the-mill bush.

Here in Pennsylvania, the ground freezes and the fall fertilizer either burns the roots or just sits on the ground and does nothing until spring except maybe wash away. Fertilizing in the spring is best for rhododendrons and azaleas here because the plants can use the nutrients while forming the new buds for the next years growth and flowers.

One common misconception is that fertilizer is plant food. It is not. The food for plants is water and CO2. The N, P, K and the micro nutrients are nutrients that are needed in small quantities like vitamins to humans. So fertilizer is vitamins, not food. Many successful rhododendron and azalea gardeners use no fertilizer.

I don't profess to be an expert on rhododendrons, but I know rhododendron experts and I listen to their advice.

Howard Greer, internationally known rhododendron expert, advocates the use of fertilizer to promote flowering, but he warns "Avoid feeding in fall, just before onset of winter as this encourages new growth at a time when the plants are going dormant, thereby stressing the plant."

Ed Reilley, a retired educator with a B.S. in horticulture, an active member of the American Rhododendron Society since 1969, and past ARS president who operates a small nursery specializing in field-grown rhododendrons and azaleas and authored the book "Success with Rhododendrons and Azaleas" says "Don't fertilize after June 1st."

Clemson University points out that fertilizing rhododendrons and azaleas too late in the spring is dangerous because plants become stressed by drought during the summer months. Without water, plants are unable to absorb nutrients. It is best not to fertilize if water is unavailable.

The American Rhododendron Society recommends "a complete fertilizer designed for acid-loving plants may be applied in late winter or early spring."

In the ARS Journal article below, Terry Richmond recommends fertilizing around the end of March.

Here is a link that might be useful: Journal of the American Rhododendron Society

    Bookmark   March 5, 2007 at 12:50PM
Thank you for reporting this comment. Undo
Embothrium(USDA 8 Sunset 5 WA)

Over-fertilizing is over-fertilizing regardless of timing. The problems due to over-fertilizing are because it is over-fertilizing and not because of when it was done. Most of the second half of your post merely serves to illustrate Whitcomb's observation that "A great deal has been written regarding fall fertilization, growth continuing late in the season and increased winter injury" and does not address the concepts involved.

    Bookmark   March 5, 2007 at 10:35PM
Thank you for reporting this comment. Undo

rhodyman, I certainly don't wish to argue with you - you have considerable experience with these plants and I respect that - but there are points made that could use some clarification.

Fully understanding how plants access nutrients from the soil and through the process of photosnythesis, convert these nutrients into the proteins, sugars and starches that constitute their "food", I am curious as to why you think rhododendrons would metabolize these available nutrients differently from any other woody plant. Is there something unique to their genetic make up that would prevent them from accessing nutrients at specific times of the year? Do they not experience the same active root growth in the fall that other plants go through at this time, which facilitates the absorption of soluble nutrients? There are a good many other types of plants that share a similar shallow and fiberous root system that have no problems with this method - why would rhodies be so distinct?

The comments made regarding drought stress affects most other plants as well - no plants should be fertilized when under stress and if using a water soluble chemical fertilizer, it is pretty much a waste of time and money to apply it when there is insufficient soil moisture present to put the elements it contains into solution. But in most areas drought tends to be a summer issue and the recommendation here is for a fall application - a time of year that usually welcomes cooler temperatures and resumption of rains and drought relief. Regardless, deep irrigation after fertilizing can suffice even if rains are not present. And I'm suprised Clemson University is not advocating regular irrigation during these periods of drought to relieve whatever stress there may be regardless of any attempts at fertilization.

Most chemical fertilizers, unless treated, provide nutrients that are immediately available to the plant roots. It seems a bit pessimistic (or perhaps the result of monumentally bad timing) that the ground would freeze solid immediately after a fall fertilizer application, rendering the product useless. Surely in PA you don't go directly from summer to winter without passing GO and period of mellow, cool and generally moist weather?

If one opts for an organic formulation as opposed to a chemical-based fertilizer, there is virtually no chance of fertilizer "burn" regardless of when applied. Organic products just don't work that way and seldom contain nutrients in sufficiently high concentrations to cause any kind of "burn".

While I accept that the gentlemen you quote are perhaps experts with these plants, nothing they have stated provides any real substance to negate the benefits of a fall fertilizer application, should any supplemental fertilizer be necessary in the first place. It seems to me that they are simply restating what horticulturists believed for many years until scientific studies, field trials and the resulting evidence convinced somewhat more forward-thinking individuals otherwise: a fall fertilizer application with a modest concentration of nitrogen does not encourage lush, succulent late season growth more vulnerable to cold damage and is most often more easily accessed and metabolized by an active root system than a similar application in the early part of the year.

    Bookmark   March 5, 2007 at 10:45PM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

gardengal wrote: I am curious as to why you think rhododendrons would metabolize these available nutrients differently from any other woody plant.

It is because rhododendrons and most azaleas are evergreen woody plants. Deciduous azaleas and other deciduous plants loose their leaves and have a definite hardening process. Many rhododendrons will delay hardening. They are opportunistic. That is why they are so popular in the florist trade, they are easy to trick and force to bloom and grow most any time of the year. I have many rhododendrons that will be tricked into opening their buds in the fall and that is without any fertilizer.

I want to apologize for trying to change Harold Greer's name. He is an expert on rhododendrons. He has been perhaps the leading grower of rhododendrons for 40 years besides having done much rhododendron research, been president of the ARS, and been very successful at what he does. I would believe him any day over a guy that writes books about woody plants.

I have a great deal of respect for Carl Whitcomb, but he is in Oklahoma. He writes about generic plants. He seldom mentions a genus. His goal is to get rapid growth of transplants. For example he wrote:
"Fertilize the plant, on the soil surface,
immediately after planting (spring or fall) or use
slow release fertilzier in the planting hole. Fertilize
at the rate of about 1 Ib. of nitrogen/1000 sq. ft.
of soil surface area. Spread the fertilizer over the
area which has been kept clean of weeds and
grass and mulched. Repeat fertilization procedure
2-3 times during the growing season but especially
in the fall when soils are warm and roots are
especially active"

Well in the case of rhododendrons, it has been found that fertilizing at the time of planting can harm the tender roots and usually nurseries have already placed slow release pellets on the plant after it was transplanted in the nursery. So rhododendron nurserymen say to delay fertilizing a new rhododendron until the following spring.

Whitcomb was the leader in spreading the gospel that when transplanting, to not add soil other than what is in the original hole. However I am sure that he agrees with using a raised bed for rhododendrons and azaleas with all new soil, soil that is not from the location. He just doesn't write about rhododendrons and azaleas. He is planting oak trees. His advice must be put in context. He specialized in deciduous trees. His profession is talking, not doing.

You can learn a lot talking to people who have done research on a single genus their entire life. A professor that experiments with oak trees doesn't hold a candle to their decades of developing techniques to improve a single genus. We must listen to the results of his experiments but we must not throw away our real world knowledge. His experiments don't negate our experience. They should open our eyes to better ways of doing things, but his generalizations may not be applicable to things he has never done.

Here is a link that might be useful: Reducing Stress and Accelerating Growth

    Bookmark   March 6, 2007 at 11:02AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)


Rhododendron and azalea growers call "bud blast" (not the disease) the fall partial blooming of a rhododendron or azalea. They have done much research on it. They have found a number of contributers.

Fall fertilizing

The Encore Azalea people have perfected it. But their product is not for cold climates. Rhododendrons and azaleas in cold climates do better if they do not waste buds by partially opening them in the fall. Then they freeze and are lost. Rhododendron people have worked for decades developing practices to prevent bud blast.

    Bookmark   March 6, 2007 at 11:09AM
Thank you for reporting this comment. Undo

Thanks for the response, but I don't think you've answered my questions. Rhodies and azaleas may be broadleaved evergreen shrubs, but they are only a couple of species of zillions of broadleaved evergreens, including many more in the Ericaceae. I am still wondering what there is about them that should require such unique treatment.

FWIW, Whitcomb is certainly not the only horticulturist to promote fall fertilizing of woody plants - it is a pretty widely held convention.

And that's a very interesting comment about the florist's trade - in all my years in this industry I've seen florist's (well, greenhouses) force just about anything, but I've yet to see a forced rhododendron flower or plant. Azaleas, yes - rhodies, no. And I'm not sure what the relevance might be anyway - forcing is just a matter of manipulating light, temperature, nutrients and growth hormones and all manner of flowering woody plants can respond to this treatment.

    Bookmark   March 7, 2007 at 4:45AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

All plants are unique. Why shouldn't they be. Some are more unique. Why would anyone think it is advisable to lump all plants and think they react the same. They don't.

Why would anyone in their right mind lump evergreen woody plants with deciduous woody plants. That is one very big stretch. There is even a big difference between deciduous azaleas and evergreen azaleas; a huge difference. Propagation is different. Culture is different. Deciduous azaleas have a tendency to stay dormant while rhododendrons are sometimes difficult to make go dormant.

If you want to see forced rhododendrons go to the Philadelphia Flower Show. It is full of them. The ARS has their own booth and forces rhododendrons for the show every year. The major exhibitors also force many rhododendrons. Also, they force them at the Chelsea Flower Show (which started in 1862), the London version of the Philadelphia Flower Show (started in 1829).

What is significant about forcing is the use of fertilizer in forcing. Some times fertilizer is used to keep a plant from going dormant. Obviously day length doesn't control dormancy in rhododendrons when in a warm spell in late fall or winter they open their flower buds.

It is not a trivial matter to make a rhododendron go dormant.

    Bookmark   March 7, 2007 at 4:03PM
Thank you for reporting this comment. Undo
Embothrium(USDA 8 Sunset 5 WA)

"Fertilizing Landscape Plants: Three Examples....

.....Research has shown little or no correlation between fall fertilization, even at fairly high rates with winter injury....This is in direct contrast to the old and widely distributed recommendation that plants should not be fertilized in the fall. This recommendation probably resulted from studies done many years ago when sodium nitrate was the primary source of nitrogen. Sodium nitrate has a very high salt index....

The same study(s) probably was the basis for the similarly erroneous recommendation that newly planted trees and shrubs not be fertilized the first growing season. By fertilizing in the fall, roots of woody plants are at their peak of activity since the energy level in the plant is at an all-time high for the year....The nutrients absorbed during the fall and winter will provide a greater support for the spring flush of new growth, since in spring, soils are cold and nutrient absorption is very low.

Example #2) Azaleas, rhododendrons and other species sensitive to fertilizer salts and either just planted or just established in the landscape. This is where natural organic and/or slow-release fertilizers are easily justifiable. Choosing between the natural organic and slow-release becomes a matter of personal preference, cost and duration of nutrient release at the specific site. Natural organics decompose and release nutrients much more rapidly in warmer climates. The slow-release fertilizers are also affected by the warmer conditions, however, in most cases, the effect is less than with natural organic fertilizers. Since azaleas and rhododendrons are very sensitive to micronutrient availability (especially iron, manganese and copper) the acid-forming and micronutrient fertilizers may provide a slight advantage. Dry chemical fertilizers may be used on these plants, but rates must be low and application frequent in order to maintain good plant growth without fertilizer burn. Mulches greatly assist these species since the effects of salts from any source (fertilizer, water, mulch) is greatly reduced when moisture availability is made more uniform. Since mulches also moderate temperatures somewhat, they extend the duration of release of both natural organic and slow-release fertilizers."

--Establishment and Maintenance of Landscape Plants

    Bookmark   March 8, 2007 at 12:04AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

There are those that talk and those that do. Those that talk deal in generalities. Those that do deal in specifics.

There is no one size fits all in botany. Any one who claims there is doesn't get their hands dirty.

You have your choice, believe a professor in Oklahoma or believe nurserymen in Oregon, Washington, Pennsylvania, Virginia and South Carolina.

The azaleas and rhododendrons grown in Oklahoma are the iron clads which are difficult to trick into fall blooming. Those grown in more temperate climates are more tender varieties which are easily tricked into fall blooming.

The American Rhododendron Society convention will be in Oklahoma next year. Perhaps we will hear from your Professor. It could be interesting.

    Bookmark   March 8, 2007 at 9:11AM
Thank you for reporting this comment. Undo

I guess it's just hard to accept that occasionally new horticultural practices are discovered/developed that contradict old theories. You may be interested to know that not all of your fellow rhododendron growers/experts/enthusiasts necessarily share a blind adherence to older methods. The Canadian Rhododendron Society supports fall fertilizing:
and the American Rhododendron Society is funding grants to study the effects/benefits of fall fertilizing:

And both the National Arboretum in Washington D.C. and the Holden Arboretum on Ohio - both arguably considered cold winter climate areas - advocate fall fertilizing as well.


    Bookmark   March 8, 2007 at 12:00PM
Thank you for reporting this comment. Undo
Embothrium(USDA 8 Sunset 5 WA)

Rhodyman, your characterizations really do not fit Carl Whitcomb and his work very well. If anything, he is the opposite of your portrayal, a trailblazer (and inventor) who has tested the assumptions and the textbooks with field trials.

College libraries with hort. programs have his books, if you care to look.

    Bookmark   March 8, 2007 at 1:38PM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

I did do a literature search. He never mentions any varieties of Rhododendrons or azaleas that he has tested his hypothesis on over a long period in areas prone to fall blooming so he could evaluate fall bloom and winter damage. I may have missed this data, but it didn't show up in my literature search and he apparently never published in the Journal of the American Rhododendron Society. He really hasn't shown much interest in the genus. But that is normal in Oklahoma.

Many other people have done this research and it is in the literature. One that comes to mind is Dr. Sandra McDonald, Plant Geneticist. She has observed that for some reason, fall blooming of azaleas is more common in Georgia. Plants which are not sufficiently hardened off or are exposed to unseasonable warm spells can start bloom prematurely. These blooms are seldom satisfactory and many times get frozen before opening fully. In any case, the seasonal bloom is lost. The recommendation is to avoid application of nitrogen after late spring, but phosphorus and potassium may be applied throughout the growing season. Application of nitrogen too late in the season can hinder hardening off in the fall.

    Bookmark   March 8, 2007 at 3:36PM
Thank you for reporting this comment. Undo
railroadrabbit(7b - Atlanta)

Rhodyman, If you say Never, somebody will always disagree with you. Instead tell them, "I don't because . . . . " That makes it more difficult for them to disagree, but they probably will anyway.

They are in z8 where it rarely freezes until late. They may have never experienced what you discussed. I've seen azaleas blooming in November and they were not forced in a florist's greenhouse. They were in my neighbors' yards.

Dad had a nation-wide lawn service that sprayed the same fertilizer under all of his trees and shrubs. His azaleas bloomed in the late Fall and looked pitiful in the Spring. But a woody plant is a woody plant.

I've propagated deciduous azaleas and evergreen azaleas by cuttings. You are correct. Both are woody plants, yet there is indeed a huge difference. Some techniques that work with evergreens just won't work with the deciduous azaleas.

The West Coast deciduous azalea varieties are generally difficult to grow on the east coast for extended periods of time. They frequently decline and die! Why? Do they just not like to travel? Or could it be that there is something different between here and there? Even experienced nurserymen specializing in deciduous azaleas experience this problem. Nurseries in my area won't sell the West Coast native azaleas. But the East Coast varieties will grow there. Go figure.

There is even a difference between some subspecies. For example, the Swampy Azalea likes wet soil while others die in it. That's the roots taking up what the plant needs to live. The Cumberland Azalea thrives in the high altitudes of the mountains. The Plumleaf Azalea thrives in cool shade while the Admiral Semmes hybrid will do well in heat and sun. Some people even experience differences between some cultivars of the same subspecies.

Thanks for doing your part by pointing out that the two plant types addressed in the question do have special concerns that all woody plants do not. If some live in a part of the country where those concerns do not apply, then do what works where you are. But if you are having problems with your plants, this may be something to think about.

When you read an answer on this forum you need to remember that this is a HUGE country with a vast variety of climates, soils, altitudes, etc. What works for one person where they live might not be the same everywhere else. But everyone has the opportunity to say what works for them. It may not apply to you, but it may help someone else.

Last year I read the Ohio extension service web site on tomatoes. Their fertilizer recommendations were different from the recommendations of the experts at the extension service where I live. Who is wrong? Probably nobody. The Ohio grown tomatoes are probably just as good as mine. But they don't live here so they may need to do things differently.

    Bookmark   March 19, 2007 at 5:44AM
Thank you for reporting this comment. Undo
rhodyman(SE PA, USDA Z6)

One problem with West Coast deciduous azaleas on the East Coast is if they are hybridized with R. occidentale, the native West Coast azalea. Many people have tried to raise it on the East Coast and none I am aware of have succeeded. Even Fred Galle, the world's late foremost azalea expert tried and failed. Most occidentale crosses are difficult to grow here in the East. A lot of work is being done in this area because R. occidentale is such a beautiful plant. It grows in the wild on rocky ledges besides streams. It could be the constant moisture and the uniformly cool temperatures or something else; something that we on the East don't have.

Also, evergreen azaleas that do well on the West Coast frequently are not hardy enough to take the winters in the Northeast or the summers in the Southeast. I am originally from Western Oregon and didn't realize what a wonderful climate we had until I moved east.

    Bookmark   March 19, 2007 at 3:23PM
Sign Up to comment
More Discussions
rhododendron macabeanum
I am having a friend send me one of these over from...
Rhody leaves getting smaller every year
Hi! I have a rhododendron (I think it's a loreley)....
Galloping gardeners strike again
Yikes! My two wonderful dogs have a completely different...
cyn427 (zone 7)
Azalea ID
This is an azalea left over from some that my great...
© 2015 Houzz Inc. Houzz® The new way to design your home™