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Rhodies: how mulch w/o raising soil level to to where won't bloom

Posted by wynswrld98 z7 WA (My Page) on
Mon, Feb 1, 10 at 20:19

I have several rhodies on my 1 acre property here in a suburb of Seattle/Tacoma. Some existed when I purchased the home a few years ago, some I planted (heights from 3' up to 15'). I put them all on a drip system when I purchased the home and I do twice weekly deep waterings when dry weather (e.g., late spring through summer into fall). Most of the plants look pretty healthy but some don't bloom at all, some have very few flowers. They're in a mix of conditions from partial sun to partial shade a few full sun but I see no pattern to lighting conditions which are/aren't blooming and suspect soil conditions.

I've read in an Ed Hume book if rhodies are not blooming 85% chance that soil level too high (he states shouldn't be above rootball level) so I'm questioning comments I read on this forum about mulching rhodies with 3-4" of mulch -- wouldn't this raise the soil level too high and cause it to not bloom even if healthy plant? How close to the trunk are people suggesting mulching rhodies??
The soil level was higher than the root ball on SOME of my rhodies, I removed the extra soil but I also suspect the rhodies aren't planted in great soil nor have they ever been fertilized/mulched based on what I know of previous owner who left landscaping an unmaintained/unirrigated weedy jungle.

I would like to improve the soil by beginning composting and/or fertilizing on a periodic basis, but am concerned about raising soil level to where they won't bloom.

Ed Hume suggests poking 8-10" deep holes in soil to put fertilize down into roots which makes sense to me vs. just throwing it ontop of soil. But I'm particularly wondering how to add compost to soil without raising soil to where rhodies won't bloom. Any advice on this plus timing of when to fertilize/add compost greatly encouraged!

Thanks in advance!


Follow-Up Postings:

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RE: Rhodies: how mulch w/o raising soil level to to where won't b

P.S. I have same issue with Camellias as I do with rhodies, very little if any blooming


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by jean001 z8aPortland, OR (My Page) on
    Mon, Feb 1, 10 at 23:00

Few flowers in woodies is often due to insufficient light and/or pruning excessively or at the wrong time of year. Another possibility is that they were too dry while setting buds last summer.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

pruning is a non-issue, they haven't been pruned in many years (I haven't pruned them in 3 years I've had them and know previous owner hadn't touched them).

So possible lack of fertility in soil could have nothing to do with rhodies/camellias not blooming????

regarding dryness, I was very specific about stating they get drip system watering (deep watering) 2x/week when lack of rain


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Mon, Feb 1, 10 at 23:21

Failure of mature established rhododendrons to bloom can usually be attributed to too much shade, over fertilization with high nitrogen fertilizers, failure to dead head leading to blooming every other year (this decreases as the plant ages), and drought in late summer when flower buds are being set.

Planted too deeply originally and they would be reluctant to bloom, but chances are they would look overall unhealthy too, sometimes dying slowly as those surface roots are unable to function.

You are not adding to the soil depth by mulching unless you are mulching with soil :) In the wild, rhododendrons are constantly 'mulched' naturally with falling leaves, twigs, needles, other organic material. The result is a loose, moist, acidic growing condition that is a perfect home for the fibrous dense surface roots.

Any organic mulch or top dressing will contribute to the health, fertility and aeration of your soil. My own shrub and perennial beds are top dressed with compost each Spring, sometimes adding more in Fall to keep a continuous supply in place...I buy it to supplement what I make.

I wouldn't be drilling holes to fertilize myself, but then I've never fertilized mine...the compost seems to contribute to any nutrition they might need, these aren't heavy feeding shrubs. If you suspect your soil is lacking something, a soil test would be in order first.

Your description makes it sound like your light is adequate, that none are in dense shade. Have you checked with shovel after watering to see what kind of depth is being covered, particularly in August and September?


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

morz8: thank you so much for your post, I think you're getting to the bottom of some things...

Where some of the rhodies are the soil has a lot of fir roots and layers and layers of fir needles over the years -- I do water as said but perhaps there is more fir needles than good soil for some of these rhodies from what I've seen when digging, particularly the ones that aren't 100% healthy so irrigation may not stay near rhodie roots because "soil" is too loose due to such a deep layer of fir needles -- this is along the lines of why I was asking about starting a composting program to improve soil conditions. So bagged Cedar Grove Compost can be placed a few inches thick ontop of the rhodie rootball with no ill effects -- should it go right up to trunk or be kept away a bit?

Regarding dead heading I have NOT done any deadheading. Perhaps adding this to my annual regimen will help matters. I have many rhodies and it will take a lot of time (especially to try and use extension loppers to get to the 15' ones that have alot of flowers at the very tip top), as long as I get this done June/July timeframe that should be fast enough so it's done before the Aug/Sept timeframe you mention the rhodies are setting their buds for the next spring?

Regarding fertilizer, I was planning on using some fertilizer marked Rhododendron Fertilizer I see in stores. Perhaps this will be a short-term thing and once the compost starts establishing the soil with more nutrients it won't be needed.

Thanks again, want to get myself on a regimen to get nicely blooming rhodies.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Tue, Feb 2, 10 at 0:46

Most people I know don't deadhead the tops of those 15 footers, but rather choose to snap off with fingers, not cut off, those that are easier to reach. If you did want to make the effort, I'd suggest a ladder.

The sooner after blooming you are able to remove the spent trusses, the better. Seed production takes an enormous amount of energy from the plant - energy that would otherwise be channeled into setting flower buds for the coming year...thus the reason for heaviest blooming every other year when seed has been allowed to set.

I normally keep compost top dressing, mulch, away from the trunks but just by a couple of inches. The mulch/top dressing of compost will stay loose and aerated so does not count toward burying the shrub too deeply.

If you are talking about using a granular type fertilizer, apply it with a light hand, at less the rate than suggested on the package. Fertilizer companies are in the business of selling product, the more you use the happier they are. Your plants may well be happier with less and you won't risk burning those fine surface roots.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

how many inches of compost do you apply spring/fall?


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Wed, Feb 3, 10 at 0:11

2-4" in Spring. In Fall, I'll sometimes mix in leaves that have been run over with the lawn mower a couple of times as part of the 'organic content' and may go deeper then - will break down quickly with our mild temps and Fall rains.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

In addition to drip, I manually water during the worst of the summer because the drip irrigation is not very appropriate to use with plants that have shallow roots in the top 4-6".


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Being anything but an expert,however I am a learning student and I had several I put in heavy shade and they did quite poorly after several years I moved them which was quite easy to do and put them in considerably more sun and in a month I could see a difference even in this cold winter they look better so anxious to see them thrive this summer.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Morz8 gave you lots of good advice. I would be reluctant to fertilize with granular fertilizer. It tends to destroy the mycorrhizal fungi that make phosphorus accessible to your plants. Phosphorus, light, and adequate moisture are all necessary to get good bud formation. An all organic fertilizer such as Hollytone is good if used at half the rate on the package and only once in the spring. A light feeding of super phosphate is OK.

The fir roots are extremely pertinent. Get a grubbing hoe and dig down around the drip line of the plants to cut fir roots that are intruding into your rhododendron and azalea beds.

Too much shade is definitely a problem. Rhododendron and azaleas like "high shade". Prune the lower branches off any nearby trees.

Any mulch that will suffocate the roots is extremely bad. Most people I know have the roots coming up into the mulch they use. Since mulch is organic, when it rots, it basically goes to nothing (water, methane and CO2)and doesn't raise the soil level.

Here is a link that might be useful: How to care for rhododendrons and azaleas.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by botann z8 SEof Seattle (My Page) on
    Thu, Feb 11, 10 at 14:33

I think Luis hit on something when he said that drip irrigation is not very appropriate for Rhododendrons. It doesn't wet the whole mulch down and keep it wet in our dry summers, consequently the roots suffer because a lot of them are dry. I would think a good soaking of the mulch by hand or soaker hose would be in order. Those Fir roots and needles are the culprit.
Any chance of moving the rhododendrons to a better place for them? They move quite well this time of year. I'm moving a bunch myself. (for other reasons)


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

I have seen a lot of mist irrigation in the Pacific Northwest where a summer drought is the norm. Of course large fields have sprinkler irrigation. In the East, only wholesale nurseries have irrigation systems. It is very rare other than on "Scott's fertilizer" type lawns.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

The rhodies are in a great place where some color is desperately needed. I was asking about fertilizer and composting as I'd like to keep them where they are but improve the soil which I'm willing to be patient and keep composting until soil does improve.

I guess one drastic thing I could do is dig them up, mix compost with the soil then replant them but I'm not sure if the shock it would give them is worth it vs. just adding compost periodically and being patient that it will take awhile until the soil conditions improve?

I think when they're in the budding phase (August/Sept) I will use a garden hose for periodic deep soaking (in addition to drip system which agreeably doesn't soak entire root ball). This does pose the question though how these bloom in nature considering summers, especially August, are often very dry.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by botann z8 SEof Seattle (My Page) on
    Thu, Feb 11, 10 at 23:02

Most of the Rhododendrons originate in S.E. Asia. They are genetically set up for summer monsoon rains and cool, dry winters. Our summers are dry. Our winters, cool and wet.

Can you change the drip irrigation heads to mist heads?
They would use more water,(longer time, more flow) but not a significant amount.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Fri, Feb 12, 10 at 16:43

Slightly OT, but the mention of Hollytone here has me again wondering if it's available to us in the PNW as I've never seen it. I admit to not making a completely focused search, looking out of curiosity and not need, but I do know I've checked in more than one larger Seattle nursery, one of Olympia's longest established nurseries, and walked around in the building with all the boxes and bottles at Bainbridge Gardens one day where most products are organic in nature the last few years. I haven't seen Hollytone.

Another other PNW gardeners know? Never having seen it, I've never recommended it to anyone gardening in W Washington.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Fri, Feb 12, 10 at 18:30

Hollytone is for different soils than here. Speaking of soils here, as elsewhere these are seldom deficient in P except when heavily cropped for years or leached heavily by outer coastal rains. Mulching should always be done with the stems and trunk not being buried by the mulch, regardless of kind of plant. Apart from drought, unsuitable soil or aspect (usually too hot and sunny, rather than the opposite), honey fungus and water molds (causing root rot and stem cankers) the common bane of rhododendrons here these days is foliage mildew. Like bamboo mites on bamboos this is now highly pervasive, so much so that I've long since scaled back on planting of rhododendrons (and bamboos are getting another hard look).

Older plantings in this area are full of rhododendrons that have reduced leaf size and density, and often markedly reduced flower production due presumably to the chronic annual reduction of their foliage area by mildew. Infested leaves drop off prematurely, surely reducing the amount of food the plants make each year. The most susceptible varieties are defoliated severely and die. As I remember it a lot of rhododendron mildew really started to show about the time of the killer 1990 winter or soon thereafter, so that many seemed to mistake the subsequent gauntness of their previously normal-looking specimens for cold injury.

It takes a real tar pit of shade to make an otherwise more or less healthy, established and sizable (in other words old enough to bloom) hybrid rhododendron not bloom at all, with not even a few trusses.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Fri, Feb 12, 10 at 23:47

Thanks BBoy, as I suspected with the Hollytone.

I still haven't seen what I recognize to be mildew widespread here but never say never - I did find my first heavy infestation of lace bug here last summer. Not precisely here, up the hill at my SILs.

I was noticing some kind of what I assume to be a spotty fungal issue on a recently planted Yak (my own) that has extremely linear foliage (name isn't coming to me right now of course :)) When it stops raining I'll see if any more has developed since I pulled off affected leaves early Winter, get some photos, make it a separate thread.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

I don't know why Holly-Tone would be considered for "different soils". It is a standard, organic (or nearly so) ARC fertilizer - it just has an east coast distribution (or by mail order), just as many of our organic ferts available here have a limited west coast distribution. Local substitutes for Holly-Tone (or any other Espoma product) would be Dr. Earth, Whitney Farms or EB Stone - all provide an ARC or acid lovers formulation.

Mildew is selective - some hybrids/cultivars are very susceptible, others far less so. The assortment of long ago planted and otherwise neglected rhodies in my new garden have remarkably unblemished foliage - no signs of mildew, leaf spot, lacebugs or even root weevil damage. This will be my first spring in residence and I'm interested in seeing how they bloom - some are in quite heavy shade but look incredibly robust.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Holly-tone contains sulfur which would not be appropriate in areas of the PNW where the soil is already too acidic. In some areas of the PNW they apply dolomitic limestone in order to grow rhododendrons which would be considered heresy in the East.


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hay mulch

I have some decomposing hay I would like to use as mulch. When is best time to apply? One reference says avoid using til weather warms up so already damp soil has chance to dry a bit.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Too acidic? I think not :-) Most NW soils are only mildly acidic, much less so than many parts of the east coast. In fact, rhodies and other acid loving shrubs here often suffer from chlorosis as the soil is not acidic enough to make sufficient iron availble. ARC fertilizers here all contain acidifiers to help maintain adequate soil acidity - sometime sulfur but often cottonseed meal.

I have never heard of anyone here applying lime to rhodies.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by botann z8 SEof Seattle (My Page) on
    Mon, Feb 15, 10 at 6:44

I have applied lime to rhodies. The soil was peat based and highly acidic. So much so , that if handled without gloves, it would crack my skin.
Lime was an ingredient in the rhododendron potting soil at the nursery I worked at. We even sprinkled the rhododendron growing fields with it.
What about lime applied on a lawn that leaches down slope into a bed containing rhododendrons? Anyone see any damage? Some people put a lot of lime on their lawn. I would expect some negative influence in the lower adjoining beds, but I never have.
Lime is routinely put on a compost pile and that compost is used as a top dressing on rhododendrons with no ill effects.
I guess is what I'm trying to say is that a little lime applied infrequently will do no harm to rhododendrons in our acid based soil.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Mike, it's unusual for NW soils to be that acidic. Typically most will be in the 6.0 to 6.8 range, or slightly acidic. I'm sure there are more highly acidic peat based soils around (I am aware of some in Bellevue) but they are probably the exception rather than the rule. None of the local growers I am familiar with lime their rhody growing soils:-) And as a former nursery buyer, I am pretty familiar with most of the local sources :-)

I have also seen liming a lawn create problems for other plants, specifically the acid lovers. I doubt the lime migrates or leaches much but is due more to a case of carelessness in application. And seeing rhodies and other acid lovers suffer when planted too close to house foundations or other concrete structures seems to confirm their general sensitivity to lime in our soils for the most part.

Not saying it is never needed here, just not often :-)


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Mon, Feb 15, 10 at 14:04

Fireweed, I hope you meant straw, not hay. Hay contains too many seeds, would need to be completely composted, not decomposing.

I personally don't like the looks of a straw mulch, other than possibly a winter cover for new unplanted beds or vegetable beds as a means of preventing winter rains from compacting soils. I do have one elderly neighbor who uses it but her direction came from an article written by a Z5 gardener, suggestion was to apply after hard frost but before soil had frozen to great depth. A little hard to time where soil does not freeze.

Once delivered, straw is easy for her to apply. Unshredded, it presents a very untidy look and if early damage to her emerging dahlias, hostas, delphs is an indication, I think makes a great winter hidey place for slugs and their eggs.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Diane Pertson came up with a cure for chlorosis that includes lime since a pH that is too low also causes chlorosis. Here is what she wrote:

Diane Pertson, Otter Point, Vancouver Island, wrote:

"I have found the following foolproof formula for chlorotic leaves or a rhododendron that isn't looking healthy:

Purchase a bag of Epsom Salts crystals (magnesium sulfate) (available here in bulk at farm-and-feed outlets), about $4.00 for a 5 lb. bag - and a bottle of FULLY Chelated Iron & Zinc (this is a very concentrated liquid - the chelation means it is in a form that can be readily absorbed by the plant), about $7.00 for 1 quart; In a one gallon watering can, put in 2 Tbsp. of Epsom Salts crystals and 2 Tbsp. of Iron and Zinc liquid - fill with warm water and stir to dissolve; Sprinkle this over the rhododendron - by that I mean drench the leaves with the solution and pour the remainder around the drip line of the root ball.

In 1-2 weeks, the leaves should be nice and green. You could repeat the process at this time if the leaves aren't fully green.

This works even better if, a month before, you have sweetened the soil by sprinkling a little Dolomite Lime on the roots. Very acidic soil can prevent the roots from taking up nutrients. As many of my rhododendrons are planted in very acidic soil under a canopy of giant cedar trees, I find an application of Dolomite and a light topdressing of mushroom manure in late spring is all they need.

If soil is too acid, the symptoms can be the same. Very acidic soil can prevent the roots from taking up nutrients. In the western USA where many rhododendrons are planted in very acidic forest soil, an application of Dolomite and a light topdressing of mushroom manure in late spring is all they need. Sprinkle the lime on in late winter, very early spring. Don't overdo it - just a light sprinkle. If it is mid-spring, get the lime on right away so the rhododendron roots will be able to take up the soil nutrients in time for new growth. If you don't have rain, water it in well."


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by botann z8 SEof Seattle (My Page) on
    Thu, Feb 18, 10 at 9:35

FWIW, I've never put lime in my own garden. I have never felt the need for it. I also don't grow plants that require a high ph.

When I was working for someone else as a landscaper we used soil from a peat bog, mixed with sand, aged bark from a millpond, and chicken manure. A little lime was called for then, especially on the lawn. We had customers complain that their lawn was growing too fast!


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

Very informative and interesting. My question is if you have a lot of leaves (not oak) can you apply it as mulch on rhododendrons. I already put it on other beds but since I have a lot I would like to use it there as well. My concern is how will it affect the acidity. All we have around where I live is a lot of maples. Many of my neighbours are very keen on cutting branches off their various evergreens, which I collect and lay them over rhodo beds, which is not sufficient, I think. Mulch is very loose when first applied and eventually goes down in volume through decomposition. I need to know how I can utilize what's already on hand without harm.


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RE: Rhodies: how mulch w/o raising soil level to to where won't b

  • Posted by morz8 Z8 Wa coast (My Page) on
    Tue, Mar 9, 10 at 14:42

Leaves (including oak) and other plant remains release organic acids as they decompose - wherever accumulation is faster than decomposition, an acid condition follows.

If you want to use leaves as mulch before allowing them to begin to break down into compost, you may want to shred them first...run over them a couple of times with your lawnmower.


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