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Chlorosis in R. arborescens

Posted by ejr2005 Eastern MA (My Page) on
Thu, Aug 23, 12 at 22:07

I planted a small R. arborescens in the late spring in a bed newly prepared by our landscaper. It had been lawn - he took the grass off and added top soil and I think some Biotone. I had him add extra peat moss.

Our soil is normally about ph 6. Pretty soon after the arborescens was planted the new leaves showed what I thought were classic signs of chlorosis - yellowing leaves and green veins. In July I added some Hollytone around the plant hoping that would help. It didn't seem to make a difference, but the plant hasn't put out new leaves since.

Today I noticed that the leaves were looking even more yellow so I'm worried. I have some "Color Me Blue" (to change Hydrangeas blue) which says it is a soil acidifier with "90% free sulfur from elemental sulfur." The inert ingredients are 8% Bentonite Clay and 2% Proprietary Wetting Agent. I'm wondering if I should add that to the soil now. There are three other Rhodies in that area that have leaves that look like they getting more yellow - though not with green veins.

I haven't had chlorosis before so not sure what to do.


Follow-Up Postings:

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RE: Chlorosis in R. arborescens

It would be best to test the soil pH before doing anything - inexpensive test kits are available at garden centers, hardware stores, etc. If a test indicates that the pH is too high, the 'Color Me Blue' sounds ok since it's mostly elemental sulfur. This takes a while to work, so adding it now should be ok.

Hollytone is an excellent fertilizer but it will not acidify soil. Rather, it provides nutrients in a form that makes them available in acid soils. The addition of peat moss to alkaline soil will not change the pH very much, either, unless it is used in very large quantities and then you're confronted with the problem of it drying out and resisting re-wetting.


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RE: Chlorosis in R. arborescens

I should have included this in the prior reply:

There are several conditions, including excessive moisture, dryness, lack of air in the root zone, that closely mimic the appearance of chlorosis.

Rhododendrons need a planting medium that contains at least 25 to 50% coarse organic matter that allows for water retention but also provides ample oxygen at the roots. "Topsoil" is often of poor quality and very unsuitable for rhododendrons because it contains little organic matter and packs down so much that air is excluded. The addition of the ultra fine powdery peat moss available in bales does little or nothing to improve things.

There are many knowledgable landscapers, but there are also many who are surprisingly clueless about plants and their needs. You may be better off adding suitable "soil" on top of the topsoil bed rather than trying to improve what's there.


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RE: Chlorosis in R. arborescens

Thanks mainegrower - lots of great info. We did test the soil pH and it's 6.5-6.6.

I'll have to check with the landscaper to see what he used. I've had really poor "topsoil" that I bought at a big box soil. It seemed to be mostly sand. His stuff was in bags but seemed more like the good stuff - dark brown, not too sandy, etc.

I have some "Premium" topsoil from Mahoneys that I've used in other places - says it has lots of organic material, aged manure, etc. I also had Coast of Maine Compost and Peat which is what I usually use for my plantings. I have a feeling that I was in a hurry when I planted this bed (between vacations), and thought that his soil was good, so I may not have done my usual amendments.

So are you saying that I should take out all the plants, add some better soil on top (will one of these two things do?), replant, and then retest the pH and amend if needed??? That's a whole lot of work that I'm willing to do if necessary, but of course would prefer not to have to do. This bed has mostly rhodies, azaleas, hostas and a dogwood. The hostas and dogwood seem okay, and three out of the six rhodies/azaleas look fine. There are also two large leaf rhodies in an older attached bed that are looking a bit yellow - though not the classic chlorosis symptoms. I'm wondering if this wacky weather has something to do with it. Also, even our normal 6 pH is I know borderline for them.

The other rhodies on our property seem to be doing well. Last year we planted a whole "rhodie" hill in back. The same landscaper prepared it. I did probably take more time amending in the planting holes, and we also shredded lots of oak leaves as mulch. We did just do a pH test there and it was the same - 6.5-6.6.


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RE: Chlorosis in R. arborescens

Anything above pH 6.0 is not really suitable for rhododendrons and other ericacea, but to my mind you have two issues. One is the soil pH. It should not be terribly hard to lower what you have to a more suitable 5.5 or so. A natural pH of 6.0 is pretty unusual for New England, so this may be an ongoing process due to underlying limestone or some other factor which accounts for the relatively high pH.

The other issue is the physical condition of the soil. Even premium quality topsoil generally does not contain sufficient organic matter for rhododendrons. Such topsoil may be fine for lawns, vegetable gardens, and many woody plants, but most would fall well below the 25% organic matter content rhododendrons need.

Amending the entire bed is a pretty daunting task, but in the long run may be best. Partially decomposed softwood bark is my first choice for an amendment, but you have to be careful. Much of the stuff sold in bags at the big box stores is ground up dyed debris from construction and demolition. Real bark is available from places that sell mulch, but even there you have to ask to be sure of what you're getting. Evergreen needles, rotted wood, oak leaves (but they break down fairly quickly) are also excellent. Avoid the powdery peat moss. Many years ago the person I first learned about rhododendron culture from said you ought to be able to scoop out a planting hole for a rhododendron with your hands; if you need a shovel, the soil is not right. A slight exaggeration, but not by much.

The one thing giving me pause here is the rhododendrons doing well with a 6.5 pH and those doing well in the new bed. You might want to make absolutely sure you're dealing with chlorosis. Many things can cause yellowish leaves.


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RE: Chlorosis in R. arborescens

Sorry I've been MIA - dealing with some other problems in the garden. Also have been on the hunt for partially decomposed pine bark fines.

Mainegrower - thanks for your statement about "one thing giving me pause." I was so sure it was classic signs of chlorosis, but I started rethinking what was going wrong. I took out two of the rhodies - one good and the other looking a bit yellow for other reasons (near plant with foliar nematodes but that is a whole other story). The soil around the good azalea was fine - a little moist. The soil around the rhodie that looked a little yellow was much drier. Then I took out the arborescens and the soil was bone dry!

I have an irrigation system - drip in the beds and non-drip on the lawn. When the weather is dry in the past I usually supplemented the drip with occasional manual watering. But this year in part because I have a new crop of rhodies and kept reading about the problems of overwatering them, and partly because I've been very distracted by the foliar nem problem, I did very little extra watering. In the driest times I did make the drip run for a longer time. So it turns out the good azalea was near the edge of the bed and was getting some water from the lawn system, the yellowish rhodie was near a drip line and may have been getting a bit from the lawn, but the arborescens was a little further from the drip line and far from the lawn.

So I'm thinking that the plants were suffering from drought. I put all three (they're small) in pots with some soil acidifier ( about 1/3 of what you're supposed to use in the ground). I'm keeping a closer watch on them as far as water and health goes. They all look about the same now, but not worse. Not sure what to do about them in the future, but I will post about that separately.

I did finally find aged pine bark fines - not an easy task. I'll use that to amend the bed. I also have aged hemlock mulch that I've used to mulch the beds. I'm thinking I could just mix that in. I could also add the soil acidifier. Not quite ready to do that until I deal with the foliar nem problem.

I'm still a little unsure of our pH situation. Whenever we've tested it it's between 6.0 and 6.6. A well known garden center (Weston's) also tested it and I think it was 6.4. Our other rhodies seem to be doing well, the lawn is turning into moss, etc.

Rhodyman - thanks for your input. I think I'm pretty good about planting - opening up the rootball, etc. - though it's always good to review it. I think the roots looked pretty good when I took the plants out. I'll check again when I replant them. I just threw them into pots with Miracle Grow soil - probably also a mistake but I'll do another thread on that. The aged pine bark I bought is called a soil conditioner - Fafard.

Have to run now but will have more questions about rhodies in containers and foliar nematodes.


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RE: Chlorosis in R. arborescens

Sorry I've been MIA - dealing with some other problems in the garden. Also have been on the hunt for partially decomposed pine bark fines.

Mainegrower - thanks for your statement about "one thing giving me pause." I was so sure it was classic signs of chlorosis, but I started rethinking what was going wrong. I took out two of the rhodies - one good and the other looking a bit yellow for other reasons (near plant with foliar nematodes but that is a whole other story). The soil around the good azalea was fine - a little moist. The soil around the rhodie that looked a little yellow was much drier. Then I took out the arborescens and the soil was bone dry!

I have an irrigation system - drip in the beds and non-drip on the lawn. When the weather is dry in the past I usually supplemented the drip with occasional manual watering. But this year in part because I have a new crop of rhodies and kept reading about the problems of overwatering them, and partly because I've been very distracted by the foliar nem problem, I did very little extra watering. In the driest times I did make the drip run for a longer time. So it turns out the good azalea was near the edge of the bed and was getting some water from the lawn system, the yellowish rhodie was near a drip line and may have been getting a bit from the lawn, but the arborescens was a little further from the drip line and far from the lawn.

So I'm thinking that the plants were suffering from drought. I put all three (they're small) in pots with some soil acidifier ( about 1/3 of what you're supposed to use in the ground). I'm keeping a closer watch on them as far as water and health goes. They all look about the same now, but not worse. Not sure what to do about them in the future, but I will post about that separately.

I did finally find aged pine bark fines - not an easy task. I'll use that to amend the bed. I also have aged hemlock mulch that I've used to mulch the beds. I'm thinking I could just mix that in. I could also add the soil acidifier. Not quite ready to do that until I deal with the foliar nem problem.

I'm still a little unsure of our pH situation. Whenever we've tested it it's between 6.0 and 6.6. A well known garden center (Weston's) also tested it and I think it was 6.4. Our other rhodies seem to be doing well, the lawn is turning into moss, etc.

Rhodyman - thanks for your input. I think I'm pretty good about planting - opening up the rootball, etc. - though it's always good to review it. I think the roots looked pretty good when I took the plants out. I'll check again when I replant them. I just threw them into pots with Miracle Grow soil - probably also a mistake but I'll do another thread on that. The aged pine bark I bought is called a soil conditioner - Fafard.

Have to run now but will have more questions about rhodies in containers and foliar nematodes.


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RE: Chlorosis in R. arborescens

The only Fafard Soil Conditioner I can find contains limestone which is not a good idea for azaleas. Azaleas need an acidic mix. Pine bark and gypsum are great choices, but not limestone. Some areas of the coastal pacific northwest do have very acidic soil and actually do use some limestone, but that is unusual.

Fafard Organic Soil Conditioner - For gardeners who want to add organic matter back into their existing topsoil, Fafard Organic Soil Conditioner is formulated to hold moisture, break up hard, dense soils, and stimulate plant roots. Our formula contains processed pine bark, limestone, and gypsum. It's excellent for conditioning vegetable gardens, raised beds, and flower beds, and planting trees and shrubs.


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RE: Chlorosis in R. arborescens

Hi rhodyman -

You're amazing - thanks so much for the heads up. I checked the bag and nowhere did it state the ingredients, though it had the ingredients for a bunch of other products. So I called Fafard and they did confirm that it was 100% pine bark. They said there is a retail product that does have limestone, but the white bag (mine) is just pine bark.

I think this is a product that is normally just sold wholesale. I bought it from the back reaches of a nursery - they don't normally sell it. After searching high and low for aged pine bark (tough to find in the burbs) I snapped it up. It was really good to verify that it's just the bark - I never would have done that w/o your heads up.


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RE: Chlorosis in R. arborescens

Another mystery - I have a few other new Rhodies/azaleas that had problems with dryness this summer. I was aware of it because their leaves would droop. One was a large leaf (Scintillation) and the others were azaleas.

The arborescens and other yellowish plants in this area however did not have droopy leaves. I'm wondering why not. Could dryness affect the uptake of nutrients but not the turgidity of cells? I would think that turgidity would be the first thing that would be affected.


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RE: Chlorosis in R. arborescens

Even if planted properly with attention to loosening the roots and soaking the root ball if necessary, rhododendrons and azaleas pot grown - as most are today - in a peat based soilless mix can easily dry out. Once that peat loses most of its moisture, it can be very difficult to re-wet. Running a hose with just a trickle of water for an hour or more at the base of the plant is the only way I've found to rewet the root ball short of digging it out and soaking in a container. This may need to be done even after the plant has been in the ground for more than a year or even two years. The greater the incompatability between the container mix and what you're planting into, the more likely it is that the rootball will dry out.

Since you're in MA, check out the website for New England Organics. There is a list of places where you can find NEO's Nutrimulch, a mixture of bark and compost. I've been using this product as a soil amendment and mulch for some time with excellent results. I believe dealers produce Nutrimulch individully under license from NEO, so there may be some variation from place to place. It's a bulk, not bagged, product, but even with delivery charges it's far cheaper than anything bagged. Some places will also let you load your own.


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RE: Chlorosis in R. arborescens

Even if planted properly with attention to loosening the roots and soaking the root ball if necessary, rhododendrons and azaleas pot grown - as most are today - in a peat based soilless mix can easily dry out. Once that peat loses most of its moisture, it can be very difficult to re-wet. Running a hose with just a trickle of water for an hour or more at the base of the plant is the only way I've found to rewet the root ball short of digging it out and soaking in a container. This may need to be done even after the plant has been in the ground for more than a year or even two years. The greater the incompatability between the container mix and what you're planting into, the more likely it is that the rootball will dry out.

Since you're in MA, check out the website for New England Organics. There is a list of places where you can find NEO's Nutrimulch, a mixture of bark and compost. I've been using this product as a soil amendment and mulch for some time with excellent results. I believe dealers produce Nutrimulch individully under license from NEO, so there may be some variation from place to place. It's a bulk, not bagged, product, but even with delivery charges it's far cheaper than anything bagged. Some places will also let you load your own.


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RE: Chlorosis in R. arborescens

Thanks again mainegrower - I did find Nutrimulch nearby and will probably get some in the spring.

Do you put this on top as a mulch or do you mix it in with the soil?


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RE: Chlorosis in R. arborescens

ejr2005 - I've used the nutrimulch both ways - soil amendment and mulch. The Coastal Maine Botanic Garden - where I first heard about the product - uses over 500 yards of it per year pretty much exclusively as a mulch.
New England Organics, btw, has a new name and, I assume, ownership. Can't recall what it is at the moment.


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RE: Chlorosis in R. arborescens

Good to know...The new name is Casella Organics.

We just visited the Coastal Maine Botanical Gardens - what a treat. We lost William Cullina to it - didn't know why until I went there!


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RE: Chlorosis in R. arborescens

I'm curious. If it is Chlorosis would a product like Ironite help? My neighbor has a Rhododendron which has leaves that are yellowing out. Not like the yellow before leaves are going to drop but a sickly greenish yellow where you can see the outline of the leaf ribs. She has it planted in shade but it is near the foundation of the house so wondering if the concrete foundation could be affecting it? I use Ironite (just a sprinkle) every few years on all my plants (not just Rhodies) and all are doing wonderfully well. Wondering if I sprinkled a little on her plant if it would help. Any advice appreciated. Thanks.


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RE: Chlorosis in R. arborescens

If the foundation is relatively new, enough free lime can leach into the soil to cause the symptoms you describe. A more likely culprit, however, is the soil itself as the previous posts discuss. Products like Ironite can green up leaves, but they really don't do anything to address the underlying conditions. Only a soil test and attention to the physical properties of the soil can do that.


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