# Removing yellowing leaves



## abax (Jan 4, 2016)

Some time ago someone here mentioned that he/she left old
yellowing leaves on Paphs. and Phrags. until all the energy
was absorbed back into the plant. This seems counter-
intuitive to me. My take is that leaving dying leaves on
a plant puts additional stress on the plant because the plant
is trying to maintain an old leaf that is either diseased or
not needed anymore. So no yellow leaves are allowed in
my collection. Is there any scientific information available
to support either view?


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## gonewild (Jan 4, 2016)

Naturally aborting old leaves are very different from diseased and dieing leaves.

Sometimes you must rely on logic and experience rather than science.
Diseased or damaged leaves are likely a burden to the plant as it will spend resources in an attempt to repair and heal and in my opinion are best removed if there are enough healthy leaves to support the plant. 

Old leaves that fall off naturally can be removed any time you like without impact to the plant. There really is no need to allow the plant to relocate nutrients from an old leaf. No one really knows how much that actually happens in plants like orchids anyway. And there is so little nutrients stored in the leaf it is very little effort for the plant to replace the nutrients from the fertilizer you supply. That's my opinion.


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## Paphluvr (Jan 4, 2016)

abax said:


> Some time ago someone here mentioned that he/she left old
> yellowing leaves on Paphs. and Phrags. until all the energy
> was absorbed back into the plant. This seems counter-
> intuitive to me. My take is that leaving dying leaves on
> ...



It would seem to me that removing yellowing leaves which are not dead, either by cutting or tearing, would open a wound in the living tissue where disease could enter. My personal preference is to wait for the lower leaves to die (completely brown and dry) and then remove them only if they come away with a light tug. Granted the plant doesn't look as good in the interim.


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## monocotman (Jan 4, 2016)

*Removing old leaves*

I am definitely in the ' leave them on until brown' camp.
All plants have a very clear recycling policy of all possible nutrients
From a leaf once it's useful life is over.
There are lots of scientific papers about the 'senescence' process and its control around, although maybe not on orchids.
It is a very carefully controlled process involving many enzymes when it is happening naturally and not due to disease.
However I doubt it is much different in this group of plants and here it may be even more important given the nutrient poor environment most epiphytes occupy.
To deprive a plant of these nutrients seems perverse when you just need to wait a bit.
However in the artificial environment with plenty of feed where we grow our orchids it may not make much difference in the long run,
David


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## NYEric (Jan 4, 2016)

My advice is to always look for the cause of the yellowing; and if it's natural let the leaf die off.


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## Linus_Cello (Jan 4, 2016)

I pull off when loose; unfortunately bugs may be hiding which may suggest pulling the leaves off sooner.


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## troy (Jan 4, 2016)

When leaves die that means the plant is dead, I drench plant in litre fluid & gun powder then throw a match, problem solved


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## naoki (Jan 4, 2016)

I think David has good explanations. Orchids (and epiphytes) are known to have amazingly high nutrient recycling capacity (Arditti's book has reviews of the topic).

Here is a somewhat related paper about Paph insigne leaf shedding phenomenon:
https://www.jstage.jst.go.jp/article/ecb1963/18/4/18_4_95/_article
Generally, they shed leaves in autumn and winter (regardless of their growing temperature). It doesn't appear to be just the age of the leaves, and they seem to control the number of leaves more actively. This somewhat makes sense because they would rather relocate the resource toward the roots when leaves are not so useful. And roots are probably safer place to store energy in the winter. The root biomass is not measured in the study, but hopefully my speculation is a reasonable interpretation. So they seem to have the capacity of adjusting the root:shoot ratio seasonally. At the same time, many Paphs don't seem to have the leaf abscission layer at the base of leaves. In other orchids like Phalaenopsis, Vanda etc, they have a planned area called abscission layer at the base of the leaves, where the leaf cleanly break off when they decide to drop. Abscission layer is the place where leaves break off in deciduous trees, too. Maybe there is a reason why Paphs don't have this planned removal of leaves (lack of abscission layer). One speculation may be that the dead leaves stay around the plant, so that they ensure the decomposition occurs right next to their roots. But this may be far-fetched. I let them decompose in the pot (if I'm not taking photos of the plant).


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## troy (Jan 4, 2016)

Doesn't leaf, bud, flower decay release ethylene gas?


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## naoki (Jan 4, 2016)

Decaying is not the cause of releasing ethylene. Ethylene is the signal to initiate some action (e.g. leaf shedding, ripening of fruits, bending behavior of stems etc). In plants with abscission layers, interaction of two hormones (ethylene and auxins) determines the timing of leaf shedding. Auxins reduce the sensitivity of the abscission layers to ethylene (i.e., the cell walls doesn't break-down). I'm not sure if this is applicable for Paphs without abscission layers.


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## 17andgrowing (Jan 4, 2016)

Very informative, thanks.


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## Justin (Jan 4, 2016)

I leave them on until brown then cut or gently peel.


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## gonewild (Jan 4, 2016)

naoki said:


> Decaying is not the cause of releasing ethylene.



Rethink that.....Decaying vegetation does in fact release ethylene gas.


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## Happypaphy7 (Jan 4, 2016)

Actually if you think about it, leaving them cannot add stress. Otherwise, plants would have evolved such that they would just cut their old leaves off instead of having them around until they brown and dry. 
This is just how it is. 

The only time to cut the leaves will be to stop the spread of diseases. 
I know some people cut off yellowing leaves for better appearance.
I never cut anything off unless they brown and go crisp dry, then I just pick them off with hands. 




abax said:


> Some time ago someone here mentioned that he/she left old
> yellowing leaves on Paphs. and Phrags. until all the energy
> was absorbed back into the plant. This seems counter-
> intuitive to me. My take is that leaving dying leaves on
> ...


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## NYEric (Jan 4, 2016)

Justin said:


> I leave them on until brown then cut or gently peel.



No cutting.


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## Stone (Jan 4, 2016)

gonewild said:


> Rethink that.....Decaying vegetation does in fact release ethylene gas.



That's what I understood as well.

As for leaving or removing, I leave until the are ready to fall off. 
Strangely with brachys, Iv'e noticed that the point of attachment to the stem can be completely black and dead before the old leaf shows much difference in colour. Perhaps a bit of yellowing.


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## abax (Jan 4, 2016)

Interesting comments. Perhaps the rules for trees, annuals
and perennials are quite different from my experience with
orchids. Almost anytime I've seen critters or decay on
Paphs. in particular, they're on yellowing leaves and I see no
necessity for the plant to deal with the problem. I do seal
cuts with torched pruners and often a little squirt of
Cleary's. I'm specifically referring to old bottom leaves.


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## polyantha (Jan 5, 2016)

Cutting or not cutting... It seems that's a matter of taste. Cutting yellow leaves will not harm the plant after my experience, as long as they don't have chlorophyll anymore (I let them dry out completely tough). This molecule is quite hard to produce by the plant and is generally absorbed before a plant lets tissue die off (the reason why leaves of trees have nice colours in autumn. They store chlorophyll in the wood over winter and the leaves show the colour underneath).


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## gonewild (Jan 5, 2016)

abax said:


> Interesting comments. Perhaps the rules for trees, annuals
> and perennials are quite different from my experience with
> orchids.


The need (benefit) for cultivated orchids to translocate nutrients from old leaves before they are shed is eliminated by providing fertilizer and an elevated nutrient supply that is not found in nature. 



> Almost anytime I've seen critters or decay on
> Paphs. in particular, they're on yellowing leaves and I see no
> necessity for the plant to deal with the problem.



Removing dead leaves has always been considered an important part of greenhouse management, and actually often required by agriculture inspectors in licensed nurseries. There are plenty of published documents that address the reasons why removing decaying and dead leaves is beneficial to plant health but I have not seen any yet that prove or justify a benefit to leave old leaves on orchid plants.


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## monocotman (Jan 5, 2016)

*abscission zones*

Naoki,
Interesting paper.
I wonder why abscission zones did not evolve in paph leaves?
Maybe the dead leaves play a part in acting as a barrier on the ground and encouraging humus and detritus to accumulate near to the plant, ultimately providing food.
They may also shade and protect the roots from the sun for some time after their death.
Regards,
David


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## gego (Jan 5, 2016)

I believe there are reasons why older leaves die, for leaves 4-6 leaves down from top, it could be a natural thing for the plant but for 2-3 leaves from the top, something is wrong. I wish I can just keep all the leaves intact just like some in the wild I've seen.


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## gonewild (Jan 5, 2016)

gego said:


> I believe there are reasons why older leaves die, for leaves 4-6 leaves down from top, it could be a natural thing for the plant but for 2-3 leaves from the top, something is wrong. I wish I can just keep all the leaves intact just like some in the wild I've seen.



If you have trouble keeping older leaves and the new leaves and roots are healthy it may be a result of low humidity.


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## naoki (Jan 5, 2016)

gonewild said:


> Rethink that.....Decaying vegetation does in fact release ethylene gas.



That's true, and I didn't think about it. But it it is probably a negligible amount. Not many bacteria emit ethylene, but some fungi appear to produce ethylene as byproduct. In the following study, they had to enclose the plant with soil in small bottles to see the effect (maybe abstract only):
http://aem.asm.org/content/54/11/2728.short
It will be even smaller amount for epiphytic orchids.



gonewild said:


> The need (benefit) for cultivated orchids to translocate nutrients from old leaves before they are shed is eliminated by providing fertilizer and an elevated nutrient supply that is not found in nature.



What is the main nutrient in plants? It is true that nitrogen is the most studied element for the relocation/remobilization during the leaf death, but there are several studies showing the carbon reallocation (in forms of lipid, amino acids, and sugar). Carbon assimilation is a costly operation for plants. But setting my nitpicky comment aside, I agree that nutrient recycling from old leaves probably have a relatively small impact under cultivation.



gonewild said:


> Removing dead leaves has always been considered an important part of greenhouse management, and actually often required by agriculture inspectors in licensed nurseries. There are plenty of published documents that address the reasons why removing decaying and dead leaves is beneficial to plant health but I have not seen any yet that prove or justify a benefit to leave old leaves on orchid plants.



Can you possibly tell us a couple publications (I'm not doubting you; if you already know about them, it would save me time)? In greenhouse, where you need to spray pesticides, old leaves could be problems because it provides places where pests can hide. So it is a common practice, but I was just curious if someone really quantified the magnitude of benefits.



monocotman said:


> Naoki,
> Interesting paper.
> I wonder why abscission zones did not evolve in paph leaves?
> Maybe the dead leaves play a part in acting as a barrier on the ground and encouraging humus and detritus to accumulate near to the plant, ultimately providing food.
> ...



That's an interesting idea. Sometime we get into the trap of thinking that everything has some adaptive significance, but the trapping mechanism might give some advantage.


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## Stone (Jan 5, 2016)

Removing any green material (or reducing the mass) from any plant will reduce growth however little that may be. Pruning just one branch from a shrub will reduce root growth. It does not necessarily mean much in most cases but it certainly does nothing to help the plant so why do it if you don't need to?


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## gonewild (Jan 5, 2016)

naoki said:


> That's true, and I didn't think about it. But it it is probably a negligible amount. Not many bacteria emit ethylene, but some fungi appear to produce ethylene as byproduct. In the following study, they had to enclose the plant with soil in small bottles to see the effect (maybe abstract only):
> http://aem.asm.org/content/54/11/2728.short
> It will be even smaller amount for epiphytic orchids.



The amount from a single leaf is not much but consider the amount from a greenhouse full of plants with old leaves as being a potential problem. And add too that the decaying organic matter in the potting media (bark, peat moss, ect)



> What is the main nutrient in plants? It is true that nitrogen is the most studied element for the relocation/remobilization during the leaf death, but there are several studies showing the carbon reallocation (in forms of lipid, amino acids, and sugar). Carbon assimilation is a costly operation for plants. But setting my nitpicky comment aside, I agree that nutrient recycling from old leaves probably have a relatively small impact under cultivation.



The leaves on a Paph got there in the first place because the plant had enough nutrients from fertilizer to build all the leaves... without the aid from relocating nutrients from old leaves. So the loss of un-relocated nutrients by removing an old leaf is easily offset by nutrients from the fertilizer supplied.... That is why we apply so much fertilizer in cultivation! 



> Can you possibly tell us a couple publications (I'm not doubting you; if you already know about them, it would save me time)? In greenhouse, where you need to spray pesticides, old leaves could be problems because it provides places where pests can hide. So it is a common practice, but I was just curious if someone really quantified the magnitude of benefits.



 It's your turn to find studies that show leaving old leaves in a greenhouse is a good idea! I'm interested to see what you come up with.

Here are 4 from a google search page. Granted they are not science peer reviewed papers but they are horticulture peer known facts based on science.

http://www.oardc.ohio-state.edu/joneslab/images/ethylene_extension.pdf

http://content.ces.ncsu.edu/ethylene-sources-symptoms-and-prevention-for-greenhouse-crops

http://www.agf.gov.bc.ca/ornamentals/floriculture/ethylene.htm

https://u.osu.edu/greenhouse/2014/0...he-production-greenhouse-sources-of-ethylene/


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## gego (Jan 5, 2016)

Stone said:


> Removing any green material (or reducing the mass) from any plant will reduce growth however little that may be. Pruning just one branch from a shrub will reduce root growth. It does not necessarily mean much in most cases but it certainly does nothing to help the plant so why do it if you don't need to?



Agree on this. If the culture is right and adequate nutrients are provided, those leaves should not turn yellow or yellowish.


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## troy (Jan 5, 2016)

Read post 7


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## gonewild (Jan 5, 2016)

Here is some info that tells the levels of ethylene gas that cause damage.... 100ppb

But how many old leaves does it take to make 100ppb in the growing area?
Maybe no one knows so they just remove the old leaves.


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## abax (Jan 5, 2016)

Stone, I'm a nursery person and used to have a garden
center for retail of all types of plants. I base my opinions
on years of experience. The health and beauty of most
nursery grown plants depends on appropriate pruning
techniques applied at the proper time to avoid decaying
plant material from causing bacterial/fungus infection on
old growth. When one prunes a shrub or tree, one forces
the plant to develop more and deeper roots and facilitates branching. I think the same applies in good orchid greenhouse management. Dispose of dead or
dying leaves and seal the cut. Keeping the growing environment clean is of utmost importance.


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## Happypaphy7 (Jan 6, 2016)

I believe the practice of cleaning up the dead leaves are literally for cleaning purpose, at least that is the main purpose. It just makes the growing area look better. 
Certain disease organisms and even insects might stay on dead plant material and getting rid of these makes sense.
However, I believe fungi responsible for breaking down of dead plant material are different than disease-causing ones that live on live plant materials.
All in all, getting rid of dead material is good. 

Pruning does stimulate certain plants to grow or flower better ( roses for example), but I guess different plant require different care to do best.

For orchids, I do not think it is needed to prune unless they turn yellow/brown and die on its own, except for the diseased leaves which should be removed to prevent spreading further.


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## gonewild (Jan 6, 2016)

It's too hot to go outside so I've been reading about old leaves.
There is a lot written about the benefit of removing and clearing away old leaves but I have yet to find any published reason to leave them on the plant...any kind of plant.

A few places have eluded to the idea that plants may use old leaves as a dump for excess salts and toxic compounds. The suggestion is dropping them off is like throwing out the garbage.

The leaf is also abscissed to regulate the flow of auxin, ethylene and other hormones. dropping the leaf keeps auxins produced in the old eaves from moving to new forming leaves. 

Here are a few interesting links related to old leaves:

http://agridr.in/tnauEAgri/eagri50/PPHY261/lec24.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC541608/


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## gonewild (Jan 6, 2016)

This one completely outlines the ethylene gas problem. every wonder why a bud blasted for no reason? Many other maladies without obvious answers all have symptoms of ethleyne damage.

Known Fact: yellowing and decaying leaves produce excessive amounts of ethylene gas.

Unknown fact: What is the volume of ethylene that is produced by a few dead/dieing leaves?

This is a huge support for the need to remove old leaves from the plant and growing area.
I cant find a supported reason for leaving the old leaves on the plant.

This article is easy to understand and is related to plants and flowers:

http://www.greenhousemag.com/article/gm1112-crops-preventing-ethylene-damage


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## C. Rothschild (Jan 6, 2016)

Some plants are easy to repot and don't mind it so you can just remove the whole leaf. Others hate it and it damages the roots so just cutting it off is okay for a little while.


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## Stone (Jan 6, 2016)

abax said:


> > Stone, I'm a nursery person and used to have a garden center for retail of all types of plants. I base my opinions
> > on years of experience.
> 
> 
> ...


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## Stone (Jan 6, 2016)

Happypaphy7 said:


> I believe the practice of cleaning up the dead leaves are literally for cleaning purpose, at least that is the main purpose. It just makes the growing area look better.
> Certain disease organisms and even insects might stay on dead plant material and getting rid of these makes sense.
> However, I believe fungi responsible for breaking down of dead plant material are different than disease-causing ones that live on live plant materials.
> All in all, getting rid of dead material is good.
> ...



You stole my words. Or I stole yours. I read this after I wrote my reply.


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## abax (Jan 6, 2016)

mea culpa! May we agree to disagree? ;>)


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## cnycharles (Jan 7, 2016)

The possibility of leaves giving off ethylene is another reason why encouraging air movement is a plant healthy practice


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## naoki (Jan 7, 2016)

Thanks for a couple interesting links. Here is the nutrient recycling rate in orchids:
https://www.researchgate.net/public...ves_of_vascular_epiphytes_from_lowland_Panama

It include only NPKCaMg, and the major part (Carbon resorption) is not considered. It is true that if there is plenty of mineral nutrients in cultivation, only the limiting elements may be recycled. But most people aren't fertilizing with C. There are other studies (especially in non-orchids), but I'm a bit tight in time now.

With regard to Angela's point of pruning, there are interesting studies in plant herbivory (bugs, zebras etc eating plant leaves). Pruned plants (mimicking herbivory) created a larger above ground biomass (meaning weight of leaves and stems). This is partly because it removes apical dominance (promoting the branching). Also some plants appear to over-compensate for the loss of the leaves. So some people thought that herbivory could "help" plants (which is a weird idea). With longer term studies, the plants pruned weren't as big as control. So as Mike said, they probably sacrifice roots and reallocate the energy temporarily to make more above ground leaves after pruning. I learned about this long time ago (around 1990's), so the details may be a little off, but I think the general story is right.


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## gonewild (Jan 7, 2016)

naoki said:


> But most people aren't fertilizing with C.



Thanks for the recycling link.

Yeah, the problem with C recycling is that the atmosphere is deficient now. oke:

Back to the point of the thread... There seems to be no valid reason to leave yellowing, dieing or dead leaves on the plant.


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## tnyr5 (Jan 7, 2016)

I yank em off. My plants exist for my aesthetic amusement. If they don't look nice, there's no point in having them.


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## troy (Jan 7, 2016)

Lol... straight to the point tony, I agree dead and dying vegetation to me creates and inhibits the behavior


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## NYEric (Jan 8, 2016)

tnyr5 said:


> I yank em off. My plants exist for my aesthetic amusement. If they don't look nice, there's no point in having them.


 Sadiste! oke:


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## C. Rothschild (Jan 8, 2016)

Although I guess in the wild they'd probably just decompose with leaf clutter. Leaving them in a pot would probably attract some kind of bugs I would think.


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## Happypaphy7 (Jan 9, 2016)

True, and makes sense, too. 

As mentioned, except for certain cases like modern hybrid roses, pruning is not a must nor needed for plants, rather a horticultural practice. 
After all, domesticated plants depend on human for care. 
That, and aesthetic aspect of course, to make, say, shrubs and trees look certain way or size.




naoki said:


> Thanks for a couple interesting links. Here is the nutrient recycling rate in orchids:
> https://www.researchgate.net/public...ves_of_vascular_epiphytes_from_lowland_Panama
> 
> It include only NPKCaMg, and the major part (Carbon resorption) is not considered. It is true that if there is plenty of mineral nutrients in cultivation, only the limiting elements may be recycled. But most people aren't fertilizing with C. There are other studies (especially in non-orchids), but I'm a bit tight in time now.
> ...


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