Removing yellowing leaves

[abax]

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?

 

[gonewild]

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.

 

[Paphluvr]

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?

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.

 

[monocotman]

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

 

[NYEric]

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

 

[Linus_Cello]

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

 

[troy]

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

 

[naoki]

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).

 

[troy]

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

 

[naoki]

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.

 

[17andgrowing]

Very informative, thanks.

 

[Justin]

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

 

[gonewild]

Decaying is not the cause of releasing ethylene.

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

 

[Happypaphy7]

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.

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?

 

[NYEric]

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

No cutting.

 

[Stone]

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.

 

[abax]

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.

 

[polyantha]

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).

 

[gonewild]

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.

 

[monocotman]

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