Mulching Rhodos

ptwonlineOctober 1, 2012

Gardening newbie and just planted my first Rhododendrons. I was wondering about mulch. Currently I've been using compost as mulch (whatever I can find, but around here it's mostly decomposed sheep or cow manure) for all my plants, but since Rhodos like more acidic soil I've been reading suggestions to use conifer needles or bark.

Well, that is not always easy to find. Plus I like the uniform look of my garden beds with the nice black compost on top. So is it ok to keep using compost as my mulch? Or should I really be switching to a more acidic mulch?


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Pine materials work best as mulch for rhodies. Go to the box store and buy a bag of "mini" pine bark nuggets. Your rhodie will thank you as will your wallet!

    Bookmark   October 1, 2012 at 6:18PM
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rhodyman(SE PA, USDA Z6)

What you have been reading is correct. Conifer needles or bark are the preferred material. Pine Bark comes in many sizes. I like the nuggets, but many use the mulch (small nuggets) since they compost faster. The mulch provides nutrients when it composes, but it serves to conserve moisture and keep the soil temperature stable protecting the roots from cold winter winds and and hot summer sun. Your plants will thank you.

I like composted peat but it is a good soil amendment, not a mulch. The roots will grow into the compost, so it doesn't serve as a mulch.

    Bookmark   October 1, 2012 at 7:01PM
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Thanks! I have a boxwood it safe to use pine nuggets for that too, or will that make the soil too acidic? I am trying to keep each stretch of garden bed with consistent mulch type for more uniform appearance. Thanks again.

    Bookmark   October 1, 2012 at 8:13PM
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rhodyman(SE PA, USDA Z6)

Yes, the boxwood will do well with a pine nugget mulch. The conifer mulches only have a slight effect. The object is to not use a mulch that will have the wrong effect. We use pine bark nuggets on all of our ornamental beds. Besides rhododendrons and azaleas we have many different shrubs including boxwood as well as perennials, and annuals. If you need to modify the pH, use sulfur powder to increase acidity. It is very slow acting but is long lasting.

    Bookmark   October 2, 2012 at 9:07AM
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Yikes! Pine nugget mulch here is about double the price of cedar mulches--about $5 per cubic foot vs $2.50 per cubic foot (and usually cheaper since the cedar ones are often on sale). Is the extra price justified? I used shredded cedar before and it seemed to work well but I was not happy with the look.

    Bookmark   October 2, 2012 at 11:30AM
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rhodyman(SE PA, USDA Z6)

Any conifer mulch is OK and cedar is a conifer.

    Bookmark   October 2, 2012 at 1:28PM
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There is a difference between conifer bark mulches and shredded pine,cedar, etc. sap wood mulches. The former lasts much longer and will not cause any nitogen problems due to rapid decomposition. Bagged material tends to be very expensive. See if you can find someplace nearby that sells real bark mulch by the yard. Such places can deliver and many allow you to load your own into bags, 5 gallon buckets, etc.

    Bookmark   October 3, 2012 at 5:38AM
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rhodyman(SE PA, USDA Z6)

Pine and cedar are conifers and their bark is a conifer bark mulch. You are correct that conifer bark mulches are preferred to conifer sawdust or shredded conifer wood. But reports of nitrogen deficiency are exaggerated. It is easily remediated with a little organic nitrogen fertilizer in the spring.

A friend who is an avid hybridizer has been using maple sawdust for many years on his rhododendron seedlings with no problems.

The Rhododendron Species Foundation used conifer sawdust to mulch it's plants for years with no problems. They did eventually get a problem when the oldest sawdust turned into a slime that held moisture and created disease problems. They had to replant everything. By this time the plants were quite large. It was a major project.

    Bookmark   October 4, 2012 at 9:53PM
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A couple of points need some clarification here. Conifer bark or needles (or other acidic plant parts) used as a mulch have little impact on soil pH. There may be some minor leaching of the acidic properties onto the soil surface but nothing of significance and most soils tend to buffer against any overall pH change. And as the acidic organic matter decomposes, it approaches a relatively neutral pH anyway.

Wood products - bark, sawdust, wood chips, etc. - used as a mulch have a minimal impact on nitrogen availability as well. Nitrogen tie-up tends to be a concern only when these products are incorporated into the soil.

Both of these concepts are rather common horticultural myths that are often still expounded by nurseries and even some extension services but have been determined by soil scientists to be inaccurate assumptions.

    Bookmark   October 14, 2012 at 5:25PM
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rhodyman(SE PA, USDA Z6)

What is unique is that the roots of rhododendrons and azaleas many times prefer the mulch layer to the soil layer, so it is important that this layer be acidic.

Their roots are very shallow and in these cases are more into the mulch than into the soil. I have seen transplanted rhododendrons and azaleas that were just picked up and the roots took the mulch with them. That is why it is important to replenish the mulch layer as it decomposes.

A few years back, the Rhododendron Species Foundation had to transplant many of their very large plants, some over 10 feet tall. It was because the saw dust they were planted in and mulched with had turned into slime that was harming the plants. They replanted just about every plant. I saw them doing it and the root layer was just a couple inches thick layer sandwiched between two shipping pallets when they were being moved by a front end loader. It really brought home how shallow this root layer is. It also pointed out why bark chips are a much better mulch than sawdust.

    Bookmark   October 15, 2012 at 12:38PM
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In regard to gardengal48's comment about wood products used as mulch versus incorporation into the soil itself: earthworms and other soil fauna are remarkably diligent in mixing the mulch with the underlying soil. The idea that mulch remains largely seperate from the soil beneath is just not true. While the chances of nitrogen depletion are not great, it very definitely can happen. As rhodyman points out, rhododendrons are extremely shallow rooted. This means that the interface between mulch and soil is precisely where depletion can take place and have a detrimental effect on rhododendron growth.

    Bookmark   October 16, 2012 at 5:26AM
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rhodyman(SE PA, USDA Z6)

Nitrogen depletion sounds fine in theory, but in practice, I have never seen it. A friend who is a hybridizer and grows great many seedlings and cuttings mulches them with maple sawdust and chips from a wooden spoon factory. He never has signs of nitrogen deficiency on these small plants.

I stopped using hardwood chips after I got a bad infestation of artillery fungus which shoots little "tar balls", actually spore cases, onto anything, especially thins that are white.

All in all, pine bark chips are a lot more trouble free and are much better in the long hall. But, I don't place nitrogen deficiency as a major concern with wood chips.

    Bookmark   October 16, 2012 at 9:45AM
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I don't think nitrogen deficiency happens very frequently, but it is very easy to confuse with other things, such as pH imblance, chlorosis, etc. so it's not easy to say for sure.

If a soil contains sufficient nitrogen on its own, the explosion of fauna that can result from an abundance of easily digested organic matter will not lead to nitrogen deficiency because there is no competition between plant needs and the soil fauna. This may well explain why the use of maple chips and sawdust doesn't create any problems.

There's a very interesting study at which shows that the carbon/nitrogen ratio of mulching materials is the crucial factor and that nitogen deficiency does indeed show up in rhododendrons and other plants when that ratio is exceeded.

    Bookmark   October 16, 2012 at 11:30AM
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botann(z8 SEof Seattle)

I use woodchips as a mulch. It has a lot of varying size pieces that break down at different speeds. Sawdust, being a lot finer, breaks down relatively fast. Hence, goo. It needs body. You don't get the gradual breakdown necessary for good root development. Bark nuggets break down slowly, but don't do a good job of weed control unless applied rather thickly. (costs more) Here in the Pacific Northwest, we use a lot of fine ground Western Hemlock bark. It works pretty good, but can form an impermeable layer over time when continually applied, if conditions are right. I look at mulches as a temporary groundcover until the live ones take over. Then it's topdress time.
I too believe the nitrogen deficiency is over exaggerated and easily overcome if it's an issue.

Short story. I was in a bar in Montana a number of years ago and noticed they had rock salt in shakers on the bar instead of ordinary table salt. Some people like to add salt to their beer. More popular then, than now. I asked why they used rock salt. The bartender said table salt is like flash powder. You use it and the beer bubbles vigorously and you end up with salty, flat beer. Rock salt just slowly dissolves and doesn't leave your beer flat right away.
I hope that's relevant.

    Bookmark   October 27, 2012 at 10:05AM
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rhodyman(SE PA, USDA Z6)

Hi Mike,

Given your location, I would guess that your wood chips are from conifers (softwood).

Here in the east we have what is called triple shredded hardwood mulch. There is no guarantee that our hardwood mulch doesn't have walnut in it. All parts of walnut trees are toxic to rhododendrons and azaleas.

If free of walnut, both can be used, but I would guess that your chips are a better choice for rhododendrons and azaleas than our hardwood mulch.

    Bookmark   October 29, 2012 at 10:21AM
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botann(z8 SEof Seattle)

I get a lot of Acer microphyllum and Alnus rubra. In the winter the chips come with no leaves. I like to use them for the trails.
Yo're right though, in that most of the wood chips I get are from our large growing conifers, Doug Fir and Western Hemlock.
Those are the ones most frequently removed because of their size.
So far I've never had a Walnut load of chips delivered. I told my tree trimmer friend not to deliver any.
At least I don't get ground up pallets and scrap wood.

    Bookmark   October 29, 2012 at 2:05PM
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jimbobfeeny(5a IN)

I planted a Rhododendron maximum this spring, and the roots have spread 6 inches in all directions - This was a small, quart pot! I had mulched it with ground up bark from an Eastern hemlock that died last summer. Looks great, for around here!

    Bookmark   October 30, 2012 at 2:49PM
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Karolina11(6b Central PA)

Is there a variety that you enjoy? I have no semi-shade climbers but some that I have read good reviews on in semi-shade are David Austin's - Graham Thomas, Gertrude Jekyll, Benjamin Britten, and one that I have ordered for spring myself is Golden Celebration.

Here is a link that might be useful: Semi-shade David Austin roses

    Bookmark   November 20, 2012 at 5:00PM
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Karolina11(6b Central PA)

The above is why you don't have two windows open at a time. You end up posting in the wrong forum.

    Bookmark   November 20, 2012 at 5:01PM
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Birdsong72(7/Northshore NJ)

While the poster is from Canada with ready availability of conifer bark/nuggets and/or pine needles, to those who chimed in, you can't get much better than oak leaves should you have them. While we got hammered here with Sandy (at the Jersey shore) and with many of the leaves not falling into the gardens rather than being windblown away from them, I've taken to grabbing whatever leaves I can (from my neighbors curbside - whom I know do not treat their lawns - and at this late date, it's mostly just a blow off and still providing me with enough inventory to finish the job of mulching this winter). My neighbors think I'm a bit 'off', but I've found that I have nowhere near the amount of oak litter that I've had in prior years (though losing an 80' red oak and a 50' black oak the past 4 years hasn't helped - along with another large white oak during Sandy). I'll continue to dragging a 4-6 loads each day and supplementing the existing beds - especially since the most recent loss will have increased sun/heat exposure come July/August which is where I'm concentrating much of my augmentation (it is here that I have Rh. Barmstedt, Rh. Shamrock, Rh. Delp Orange, Rh. Calsap and Clethra Barbinervis. By the way, yesterday's nor'easter which had winds topping out at 74 mph down here did not displace hardly any of the leaves in my beds (either newly top dressed or existing). In closing, they're the right price as well. A Happy New Year to all of the gardeners who play in black earth and pray for it to all live again.

    Bookmark   December 27, 2012 at 1:51PM
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rhodyman(SE PA, USDA Z6)

The trick is to get oak leaves without any walnut parts. Most rhododendrons and azaleas are killed by juglone, the toxic agent in black walnut. I know that a couple rhododendrons and azaleas are not killed. They include:

Rhododendron periclymenoides
Azalea 'Gibraltar'
Azalea 'Balzac'

Fortunately walnut leaves aren't nearly as big a problem as the roots and hulls.

Conversely, some rhododendrons will kill plants planted near them. That is one thing that is so insidious about R. ponticum in England. The ponticum runs rampant and kills nearby plants. This property of one plant to secrete a chemical that affects other plants to help it gain an edge in plant competition is called "allelopathy".

Here is a link that might be useful: allelopathy

    Bookmark   December 30, 2012 at 11:14AM
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Birdsong72(7/Northshore NJ)

Thanks RhodyMan. The only black walnut that I know of in the neighborhood is 5 houses away. I only use what I know to be either oak or hickory (never maple).


    Bookmark   January 16, 2013 at 12:19PM
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