The science behind defoliation?

[paulg]

I’ve done it, even schawazzed. I can’t definitively say it improvedmy grow or not. Some folks swear by it others swear at it! Just wondering if there’s any science behind it.

 

[BII]

There seem to be 2 basic views of schwazzing and defoliation. It is counter-productive, with it crazy to remove perfectly healthy solar panel leaves, with this also the "natural" way. Or aggressive leaf removal is seen as inducing stress leading to improved bud quality and/or leaf removal is often portrayed as 'channeling' plant energy/activity into forming better buds.

With schwazzing surely done and argued about by growers for decades and yet there is no strong consensus for it, I presume it doesn't add much. [Until proven otherwise, from knowledge of basic plant function, I presume not schazzing is the best option].

I don't expect needed studies have been done yet. Studies would need have a sizable enough number of plants schwazzed or defoliated vs. not, and even then this would likely grow just a single or at best very few strains (with each potentially responding differently), and how do you standardize growing and measure improvements (in what) or not? Who would fund and perform such studies on a practice limited (can't be automated, scaled-up) to individual pot growers willing to do a lot of manual labor?

So is there any 'science" out there? What does defoliation actually do to cannabis plants relevant to our interests (quality, potency, yield, appearance, etc.)?

 

[elcoloan]

this has some referneces that might be useful: https://www.sciencedirectt.com/science/article/pii/S0926669025014645

"
Concurrently, growers often prune, or mechanically damage, their plants (Danziger and Bernstein, 2021a). Mechanical damage to plant tissues has been shown to induce wounding responses akin to the herbivory response wherein the synthesis of secondary metabolites is increased (Ali et al., 2024). This can alter source-sink relationships, particularly the partitioning of carbohydrates (Verdonk et al., 2023), which can influence biomass allocation (Nie et al., 2021, Nie et al., 2023). When applied to C. sativa, pruning can take many forms (e.g., topping, lollipopping, defoliation etc.). While definitions do not appear to be uniform between studies, all approaches aim to increase flower yield and/or cannabinoid concentrations by affecting biomass partitioning and stimulating secondary metabolite synthesis, respectively.
Topping, lollipopping, and defoliation have distinct modes of action affecting cannabis growth and yield. By removing the shoot apical meristem, ‘topping’ encourages the growth of lateral branches due to relaxed apical dominance, thus achieving greater flower mass across the canopy (Folina et al., 2020, Crispim Massuela et al., 2022). This method can be paired with the ‘Screen of Green’ cultivation method, whereby fewer plants with highly branched canopies are maintained (Arora and Yun, 2023). ‘Lollipopping’ involves removing non-productive growth low in the canopy that is thought to have low flower mass or low cannabinoid yield, thereby directing carbon allocation towards floral production (Crispim Massuela et al., 2022). Indeed, it has been shown that the concentrations of most cannabinoids is higher in inflorescences closer to the apical meristem, and self-shaded branches lower in the canopy produce less floral biomass and have reduced cannabinoid content (Bernstein et al., 2019, Crispim Massuela et al., 2022, Stack et al., 2023). Finally, defoliation involves removing fan leaves (large palmately compound leaves that grow directly from the main stem and side branches, often made up of five to nine leaflets) from both the main stem and side branches to achieve an open canopy structure that is thought to allow better airflow (which helps mitigate disease risk) and light penetration to flowering sites. Light intensity (LI) correlates with floral biomass in cannabis (Holweg et al., 2024), so increasing the available photosynthetically active radiation at all flowering sites can theoretically increase dry floral biomass.
Defoliation is perhaps the most labour- and time-intensive of the described pruning methods as it involves carefully removing fan leaves across the entire plant canopy. Danziger and Berstein (2021a,b) show that defoliation increases the concentration and uniformity of certain cannabinoids across the canopy. However, cannabinoid yield was not increased by defoliation for all cultivars because dry flower weight sometimes decreased (Danziger and Bernstein, 2021a, Danziger and Bernstein, 2021b). The relatively low harvest indexes reported for each of their treatments may have come about due to low canopy LI. Further, Sandoval et al. (2024) show that damage to field-grown hemp with a flail (which they term ‘defoliation’) increases CBD, cannabidiolic acid (CBDA), cannabidivarinic acid (CBDVA), cannabigerol (CBG), Δ9-THC), tetrahydrocannabinolic acid (THCA), tetrahydrocannabivarin (THCV), and tetrahydrocannabivarinic acid (THCVA) concentration if performed during the early flowering stage. Toth et al. (2021), on the other hand, found that mechanical wounding of field-grown hemp did not cause significant changes to the cannabinoid potency of the flowers. Beyond this, there is a lack of research into the effects of proper defoliation on C. sativa growth and yield in indoor settings."

 

[BII]

Yes with their physical stress and damage "Topping, lollipopping, and defoliation have distinct modes of action affecting cannabis growth and yield." But are the results any better/desirable vs. if fully functional leaves/biomass were not removed, if nutrients and fuels/sugars/ATP formed and stored in the leaves were allowed to be resorbed naturally by the plant and support bud growth? Does stress beat out avoiding stress, such as leaf tucking with very selective pruning where tucking doesn't work? If defoliation stress is a positive for growing, how much is needed, does it really have to often be so extreme?

I don't consider topping to be a form of defoliation - it's too precise. and is a well proven method. More branches/apical buds generally result in more and often better buds.

 

[Son of Hobbes]

I’ve done it, even schawazzed. I can’t definitively say it improvedmy grow or not. Some folks swear by it others swear at it! Just wondering if there’s any science behind it.

There are numerous proven, scientific reasons to defoliate cannabis, but there are some scientific considerations (you can't just "Edward Scissorhands" everything and hope for the best.)

On the brief on defoliation and photosynthesis, a few key points:

• Leaves make sugars.
  
  
• Cannabis has both sun and shade leaves, which are anatomically and physiologically distinct:
  
  • Sun leaves maintain higher light-saturated photosynthesis
    
    
  • Shade leaves saturate at lower PPFD and are optimized for low light
    
• In a dense canopy, lower leaves often operate far below their potential.

(The science, the source: PubMed )

The surface of the canopy attenuates (reduces) light exponentially with leaf area (which is known as Beer–Lambert behavior), so reducing effective leaf area index (or removing shading surfaces) -raises PPFD- deeper in the canopy—if (and this is a key point)—you don’t overshoot and cripple total leaf area.

The Beer–Lambert law is a physics principle that describes how light weakens as it passes through a medium. In plants, that “medium” is the canopy, or the layers of leaves stacked above one another.

When sunlight or light from your grow light hits the top of the canopy, the first leaves absorb or scatter much of it. By the time light filters down to the lower leaves, the intensity has dropped off sharply. This explains why the top of a cannabis plant gets the most photosynthetically useful light, while the shaded inner and bottom leaves generally contribute less to overall growth.

(The science, the source: Science Direct)

This matters because:

• Dense canopies can shade out bud sites and increase humidity (which can raise disease/mold risk).
• Training and defoliation practices change how light is distributed, sometimes making the lower canopy more efficient.
• Light penetration limits mean that adding more intensity at the top doesn’t always help if most of it gets absorbed before reaching the lower layers.

How defoliation comes into play is that methodical, selective leaf removal can improve light penetration, airflow, and uniformity without starving the plant, but so long as you do it at the right time, in moderation, and alongside good environment control.

Even with techniques like schwazzing, it's only suggested to do at VERY SPECIFIC times of growth, with a certain amount of specific growth removed, and that's it or you risk high stress to the plant. Even with proven defoliation methods, you risk stressing the plant (which is why "how much and how often" is important.)

The science behind this is because of the altering of the physiological state of the plant tissues and metabolism, as there are changes in "sink-to-source" ratios as well as hormonal changes induced by the removal of apical dominance and carbon sinks.

Here's the gist:

A plants "sources" are leaves that photosynthesize, which provides carbohydrates to its "sinks" (growing areas like roots, new leaves, and flowers).

Source-sink regulation is the relationship between tissues that produce resources (sources) and tissues that utilize or store resources (sinks.)

Schwazzing (or other types of defoliation) reduces the number of sources, which can create a stronger demand from the remaining sinks. This seems like a bad idea, right? But, the remaining leaves respond to this increased demand by ramping up their photosynthetic activity, which is what is called "compensatory photosynthesis."

Compensatory photosynthesis is an increase in the photosynthetic rate of remaining foliage on a plant after tissue damage (in this case, defoliation.) This is a physiological response that allows the plant to partially recover carbon gain lost from the damaged or removed parts, leading to higher photosynthetic rates per unit area of undamaged leaves compared to those on an undefoliated plant.

The finesse is "how much" and "how often," because too extreme of defoliation negates any benefits of compensatory photosynthesis (plus there are a number of other things that happens when you pluck or defoliate leaves that you probably don't even think about; like new growth being generally more palatable to certain pests (far easier to digest for most insects compared to older leaves,) or that wounding the plant changes the plant volatiles that it produces (plant volatiles are VOC's that plants produce that act as like a communication "switch," which can attract or repel certain types of insects for example.)

When sinks are strong and leaf carbohydrates don’t accumulate, remaining leaves can up-regulate photosynthesis per unit area.

From what I've researched, reviews and experiments across different species suggest (and document) increased leaf-level Pn (net photosynthetic rate) after defoliation or when sink demand rises.

The catch 22 (and take-away) is that canopy-level net carbon gain depends on how much source (leaves) you removed and their timing.

(The science, the source(s): Oxford Academic, BES Journals, PMC, Frontiers )

 

[paulg]

Well, that answers my question! Thanks for all the informative responses.

 

[BII]

Well, that answers my question! Thanks for all the informative responses.

Doesn't answer much relevant to our growing interests (bud quality, potency, yield).

So removing leaves results in the remaining leaves ramping up their photosynthesis rate. Seems expected, straightforward adaptation to stress. But considering the whole plant, does having fewer leaves but with increased photosynthesis rate result in more total photosynthesis/ATP/sugars generation (or better buds) vs. not removing the leaves? Would you collectively get more sugars stored by leaving the leaves? Presuming good light penetration (which current lighting can provide) and air flow (such as fans), what does radical defoliation, schwazzing, etc. contribute?

 

[Son of Hobbes]

Doesn't answer much relevant to our growing interests (bud quality, potency, yield).

So removing leaves results in the remaining leaves ramping up their photosynthesis rate. Seems expected, straightforward adaptation to stress. But considering the whole plant, does having fewer leaves but with increased photosynthesis rate result in more total photosynthesis/ATP/sugars generation (or better buds) vs. not removing the leaves? Would you collectively get more sugars stored by leaving the leaves? Presuming good light penetration (which current lighting can provide) and air flow (such as fans), what does radical defoliation, schwazzing, etc. contribute?

Sure it does.

Defoliation doesn’t magically add more sugars, cannabinoids, or terpenes.

It changes where the plant puts its resources, how evenly light and airflow reach bud sites, and how well the canopy avoids stress (but within context.) These are indirect effects that can improve quality and consistency, but the plant’s total carbon gain usually depends more on environment and genetics than on leaf-plucking alone.

Leaf Removal = Fewer Sources

Even though remaining leaves can ramp up photosynthesis from the compensatory response, they usually don’t replace all the sugar-making capacity of what was lost. The takeaway is that per-leaf efficiency goes up, but whole-plant capacity may go down if you take too much (hence the emphasis on how much and how often.)

Context matters. In dense, humid canopies (or under weak light), selective defoliation can improve yield/quality indirectly by:

• Preventing shading, which tends to lead to underdeveloped buds (larf)
• Reducing mold risk (by reducing the density of the foliage)
• Improving spray or beneficial insect coverage (less leaf area = less areas for pests and pathogens to establish in)
• Allowing lower bud sites to reach closer to their genetic potential (again, less larf, increased cannabinoid production in those lower bud sites, etc)

In these cases, selective defoliation can preserve more of what you’d otherwise lose.

To your comment on good light penetration, if your light already penetrates evenly and climate is stable, there’s likely less to gain from leaf removal. Since grower setups vary vastly (including commercial setups,) sometimes circumstance and situation often dictate necessity.

The old adage "Can you? Of course. Should you? Maybe" comes into play.

Light, selective defoliation is a tool to balance canopy structure—not a guaranteed way to make better buds. You -usually- won’t get more total sugars from a heavily defoliated plant compared to a healthy, fully-leafed plant in a well-managed environment, but again this is a very broad statement that doesn't take into account the type of defoliation, to what extent, when it happens, etc.

Think of defoliation less as “increasing power” and more as “redirecting energy efficiently” when your canopy or climate would otherwise work against you.

 

[BII]

Sure it does.

Defoliation doesn’t magically add more sugars, cannabinoids, or terpenes.

It changes where the plant puts its resources, how evenly light and airflow reach bud sites, and how well the canopy avoids stress (but within context.) These are indirect effects that can improve quality and consistency, but the plant’s total carbon gain usually depends more on environment and genetics than on leaf-plucking alone.

Leaf Removal = Fewer Sources

Even though remaining leaves can ramp up photosynthesis from the compensatory response, they usually don’t replace all the sugar-making capacity of what was lost. The takeaway is that per-leaf efficiency goes up, but whole-plant capacity may go down if you take too much (hence the emphasis on how much and how often.)

Context matters. In dense, humid canopies (or under weak light), selective defoliation can improve yield/quality indirectly by:

• Preventing shading, which tends to lead to underdeveloped buds (larf)
• Reducing mold risk (by reducing the density of the foliage)
• Improving spray or beneficial insect coverage (less leaf area = less areas for pests and pathogens to establish in)
• Allowing lower bud sites to reach closer to their genetic potential (again, less larf, increased cannabinoid production in those lower bud sites, etc)

In these cases, selective defoliation can preserve more of what you’d otherwise lose.

To your comment on good light penetration, if your light already penetrates evenly and climate is stable, there’s likely less to gain from leaf removal. Since grower setups vary vastly (including commercial setups,) sometimes circumstance and situation often dictate necessity.

The old adage "Can you? Of course. Should you? Maybe" comes into play.

Light, selective defoliation is a tool to balance canopy structure—not a guaranteed way to make better buds. You -usually- won’t get more total sugars from a heavily defoliated plant compared to a healthy, fully-leafed plant in a well-managed environment, but again this is a very broad statement that doesn't take into account the type of defoliation, to what extent, when it happens, etc.

Think of defoliation less as “increasing power” and more as “redirecting energy efficiently” when your canopy or climate would otherwise work against you.

So significant defoliation seems to be well, if not best, suited for already stressed or endangered (e.g., by mold) plants, particularly plants with internal and lower leaves not getting enough ventilation and/or light.

Despite being done by growers for decades (or centuries?), there still is no clear consensus that defoliation results in better outcomes or when, how or why to do it (such as do it to relieve or strategically induce stress?). Significant defoliation seems risky with no predictable outcome and no real need if the plant's internal and lower biomass is getting sufficient air, light and healthy). In these contexts, with fairly good lighting and ventilation, I will continue to do only very selective defoliation, mostly just removing fan leaves that resist leaf tucking or stem repositioning/LST. The "science" and also grower community broad consensual support for defoliation as a general practice just isn't there yet.

[Note, to me defoliation doesn't include keeping say the bottom 1/5th or more of the plant bare, allowing no new branching, buds or leaves, with this established practice and not leaf removal].

 

[Son of Hobbes]

So it seems that significant defoliation is best suited for already stressed or endangered (e.g., by mold) plants, particularly plants with internal and lower leaves not getting enough ventilation and/or light. Significant defoliation seems risky with no predictable outcome and no real need if the plant's internal and lower biomass is getting sufficient air and light and healthy.

So removing less light-exposed leaves ramps-up the metabolism of the remaining leaves/biomass, but can also decrease the total sugars/energy available. Despite being done by growers for decades (or centuries?), there still is no clear consensus that defoliation results in better outcomes or when, how or why to do it (such as do it to relieve or induce stress?). In these contexts, with fairly good lighting and ventilation, I will continue to do only very selective defoliation.

No, you typically would -not- want to defoliate an already stressed plant (because you're adding a potential stressor on top of already existing stress.)

A lush canopy isn't a sign of stress, it's a sign of growth.

Whether you've managed that canopy or not, that's where you'd want to make the decision on defoliating. I don't think anyone is making a case that you "have to" or "don't have to," but more about "does this make sense for me to do based on my setup with my plants and how they've grown."

In general, selective and timely defoliation is a canopy-management tool, not a magic yield button that makes your plants more potent or increase weight inherently.

It mainly improves light distribution, airflow, and uniformity (this is the basic core concept of defoliation for cannabis.)

If you looking for causation or correlation on its effects on total yield/potency, that is context-dependent. It doesn't "just happen," but can create more favorable conditions for it to happen.

If we are talking specifically about schwazzing, that is a specific and timed, very heavy defoliation. And not suited for every plant and every situation.

[Note, to me defoliation doesn't include keeping say the bottom 1/5th or more of the plant bare, with no new branching, buds or leaves, with this established practice and not leaf removal].

This is actually called "skirting," is considered a specific type of defoliation. It's more than just removing foliage, but also branching from the bottom X amount of the plant (there is no specific ratio or percentage to my knowledge, it's simply the act of removing bottom foliage/branching to promote airflow and to remove bud sites that simply aren't going to produce anything (again, redirecting energy elsewhere.)