It may surprise you to hear, that there is a small but very dedicated group of cannabis farmers who firmly believe they can increasing their crop production by removing the big leaves – even during the later parts of the flower cycle. The technique has come to be known as “Schwazzing” or more commonly, “Big Leafing.”
Proponents of the technique insist that they’ve done it forever and they absolutely KNOW it works, and hundreds of their buddies do it too, but I’ve asked quite a few to show me some reason for why they believe in this, and so far none of those I’ve spoken with can produce any studies or data of any kind – even of their own.
I’ve always disagreed with this technique, but guess what…
I can’t show my data either because this study has never been done!
Here’s what I think I know about flowering Cannabis…
#1) The simplest fact that I think we can all agree on, is that leaves are the solar panels that gather energy from light and convert that energy into food, which is then used to grow the body of the plant. Nobody seems to argue about that part.
#2) When you flip the lights to 12/12, the race is on and the clock starts counting down on a fixed timeline. After that point, creating a stress event is the equivalent of tripping a runner in the middle of a race. That runner can still get up and continue on to the finish, but it will either be later, or your flowers will be smaller. Longer time, or smaller flowers both equal money lost.
This is where the disagreement begins…
My thinking is… If you take the leaves off, you reduce the energy gathering capacity of the plant, and since you are on a fixed time schedule, you can’t just pause the clock to recover, so you’ve diminished the size of the end product relative to the level of stress you created.
Their thinking is… if you take the leaves off, you stimulate the plant to grow, and it will not only grow back but it will recover to produce even more end product than it would have done without this.
I still don’t see any mechanism that might allow this to be possible. The defoliation myth requires us to believe that a SEVERELY defoliated, super stressed plant, can not only recover, but then come back and EXCEED what it would have done naturally without that stress.
Unless I’m missing something, this part simply defies physics. Everybody generally knows that you can’t get something from nothing, but few cannabis farmers come from the horticulture science world where students spend years studying the intricate chemical processes that take place in every part of the plant. Lacking that deep depth allows for a lot of speculation based on the premise of what “seems” like it should be right. Plants are amazing creatures and all cannabis farmers certainly appreciate the beauty of what they are, but the mystery surrounding how plants grow and how they do what they do makes it cloudy and creates room for a lot of speculation about the details.
One Grower had this argument…
We feed the plants so hard they don’t need the leaves any more because they get all their food from us!
I can certainly see how someone might think like this, but the missing part in that story is that Photosynthesis creates sugars, and that sugar provides the energy the plant needs to combine all the elements in that fertilizer in order to build the plant. Fertilizer alone is not what plants live on, it’s more like the fertilizer provides the building blocks, but the plant has to supply it’s own energy to put all those blocks together and make itself bigger.
If you think photosynthesis isn’t important, just try turning the lights off for a week and see what happens…. You can feed your plant like crazy in the dark, but I hope we can all agree that won’t go well.
I believe defoliation is a misguided “production technique” that is costing the cannabis industry millions of dollars each year in lost profits, yet nobody has ever done any sort of study on it.
You might think that any technique claimed to make a significant difference in the production of Cannabis flowers would be important enough that it would be tested and proven with some real data, but I’ve searched all over and so far I’ve found nothing!
Apparently, there are no scientific studies specifically looking at late term defoliation of Cannabis
I’ve started to outline such a Defoliation Study and you can look or even participate in the formation of the study on that page.
Until a real Cannabis study happens, here’s what we can learn from studies on other plants…
Papaya
75% defoliation reduced new fruit set by 60%, compared to the controls during the 6 weeks after defoliation. Continual defoliation reduced new fruit to less than one-third of controls in the first 8 weeks of defoliation and less than one-fourth of the controls in the 168 day experimental period. Continual defoliation significantly reduced ripe fruit Total Soluble Solids (-15%) and fruit weight (-23%). Flower abortion increased nine fold and new fruit set was reduced 30% in plants subjected to 75% defoliation from the oldest to youngest leaves.
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Kiwi
Removing 50% of the leaves resulted in a small, but significant, reduction in fruit weight compared to the control vines. More severe defoliation (75%) reduced fruit weight by 13 g in the RCZ (replacement cane zone) and 7 g in the FZ (fruiting zone). The defoliation treatment reduced the concentrations of starch and total soluble carbohydrates in the shoots and starch in trunk bark compared to those in control vines. The treatments caused a large reduction in return bloom, expressed as number of flowers per winter bud, by about 25% and 53% in vines with 50% and 75% defoliation, respectively.
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Strawberries
Plants were grown in a greenhouse in Tunja, Colombia, in hydroponics with an aerated nutrient solution. As leaves emerged, one or two leaflets were removed from each compound leaf, to attain 38 % and 67 % reduction of leaf area. Control plants grew intact, without defoliation. Fruit yield and fruit quality characteristics were evaluated for each plant. The pH, total soluble solids, ratio of total soluble solids to fruit juice acidity, fruit yield, fruit mass and size, and the ratio of leaf area/fruit yield were reduced drastically in defoliated plants. Leaf area reduction adversely affected physical and chemical characteristics of strawberry fruit. Removal in excess of 38 % created fruit which did not meet marketing criteria.
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Tomatoes
Artificial defoliation of tomato plants by removing more than half of each leaflet, detachment of whole leaves from the lower three-quarters of the plant or of two leaves between each truss, caused up to 40% yield loss.
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Sugar Beets
The results revealed a relationship between yield loss, level of defoliation, and growth stage when defoliation took place. The most critical growth stage for the effect of defoliation on yield was between 1700 and 1800 degree-days, when 100% defoliation produced a 42% yield loss. The higher the level of defoliation, the higher the yield loss. This study produced a set of regression equations that can be used to assess final yield loss caused by defoliation of sugar beet by such different agents as hail storms, wind, insects, and other mechanical damage.
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Cowpea
The finding from the study revealed that removal of leaves from vegetable cowpea affects the vegetative and developmental characters and yield and yield parameters of cowpea. The performance of the crop was poor for defoliation imposed at vegetative and flowering stages while 75 and 100% defoliation was detrimental to cowpea growth and development.
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Soybeans
Defoliation of soybeans by phytophagous insects was simulated with band defoliation at various percentages and stages of growth of soybean plants. Significant differences in yield and seed weight were obtained following single defoliations of 33, 67, and 100% at several stages of growth. Continuous and progressive defoliations at all percentages drastically reduced yields and seed weight at all stages of growth tested.
Comparison of data obtained in 1969 and 1970 indicated that early season single defoliation (up to full bloom) of 1/3 or 2/3 was more detrimental to yield at higher yield levels than at lower yield levels.
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There were a few common themes in all of these studies.
- Late stage defoliation did NOT increase ANY of the variables measured.
- Defoliation has a negative impact on all measurements of production.
- All studies found the concept of a Tipping Point in every plant they tested.
- Leaf removal less than the tipping point cause little effect on production.
- Leaf removal beyond the tipping point causes a rapid drop in production.
The Tipping Point
The basic idea seen over and over is that healthy plants maintain a leaf area that is somewhat in excess of what is needed for normal fruit/flower production. If that leaf mass is removed gradually, there is some tipping point that is different in each species and cultivar, where any further removal of leaf area beyond this point causes a reduction in fruit/flower production which gets worse as more leaves are removed.
The tipping point is different in each plant but even the more sensitive species can seem to tolerate the loss of roughly 25% (especially in the lower leaves) before significant adverse effects start to show up as a measurable production loss.
This is good news for Cannabis farmers that feel the need to thin the canopy before flowering.
Of all the variables tested in these studies, every type of leaf removal caused a negative effect.
Not a single plant was found to benefit from having leaves removed.
I have not found any studies that showed defoliation to create any benefit in weight based measures of production. There are some studies that show that stress will create high quality fruit, but always at a severe loss of production quantity.
This holds true in Cannabis where the most stressed plants that flower out on the brink of death, will often produce a tiny handful of outrageously good buds.
Why is there a “tipping point”?
What this suggest is that plants store up resources throughout the season. These stores are kept in the lower leaves, branches and roots. These nutrients are moved up and used in the production of flowers and fruits in the late season.
The presence of this tipping point concept suggests that plants are not capable of storing up the complete needs of the flower/fruit production. If that were true you could remove all the leaves in late season and the fruits/flowers would produce as normal, but that is not the case.
Storage is clearly only sufficient to assist the leaves, but the leaves are still doing the vast majority of the production right up to the end of the fruit production cycle.
This evidence suggests that most plants have evolved the ability to store enough nutrition to allow for a certain amount of defoliation by herbivores, spider mites, grasshoppers, etc, particularly on the lower leaves.
A 25% reduction of the effective leaf area, can safely be handled by most plants and up to 50% in some species.
In Summary:
If cannabis holds true to the pattern demonstrated by the studies above, then YES you can safely prune or defoliate in small percentages (under 25%) to cut down on mold issues, without much damage to production.
If you remove much more than 25% you will reach a point at which the crop producing ability starts to diminish rapidly.
Another Study
Review: PLANT RESPONSES TO DEFOLIATION: A PHYSIOLOGICAL, MORPHOLOGICAL AND DEMOGRAPHIC EVALUATION
https://agrilifecdn.tamu.edu/briske/files/2014/03/Briske-Richards-SRM-CHAPTER95.pdf
This 107 page review of available literature on the topic of Defoliation came to the following conclusions…
Summary statements:
Steady-state plant growth is immediately disrupted by defoliation in response to a substrate limitation imposed by a reduction in photosynthetic area. A reduction in whole-plant photosynthesis and preferential carbon allocation to active shoot sinks reduces root growth and nutrient absorption as root carbohydrates are depleted.
Plants rarely respond to defoliation as isolated individuals in the field. Competition from undefoliated plants has been demonstrated to constrain growth of defoliated plants to as great an extent as the direct effects of defoliation. Defoliation reduces the competitive ability of plants by decreasing resource acquisition from the environment.
Studies of numerous grazing-tolerant C3 and C4 forage grasses growing with high nutrient availability in controlled-environment or greenhouse conditions have demonstrated that root elongation essentially ceases within 24 hours after removal of approximately 50% or more of the shoot system and root mortality and decomposition may begin within 36-48 hours.
Root respiration and nutrient acquisition are also reduced following defoliation, but to a lesser extent than root growth.
Along with the reduction in root respiration following defoliation is a rapid reduction in nutrient absorption. Experiments conducted with perennial ryegrass growing in nutrient solution demonstrated that the rate of nitrate (NO3-) absorption began to decline within 30 minutes following removal of 70% of shoot biomass. NO3- absorption decreased to less than 40% of the pre-defoliation rate within 2 hours following defoliation (Clement et al. 1978). In these experiments, NO3- absorption continued to decline over the next 4-12 hours until it became negligible for 2 or 7 days before recovery began.
NO3- absorption did not resume until a positive daily carbon balance was reestablished within the plant (Clement et al. 1978). Rapid reductions in root respiration and nutrient absorption following plant defoliation are proportional to the intensity of defoliation (Davidson and Milthorpe 1966a, Thorgeirsson 1988).
PLANT RESPONSES TO DEFOLIATION: A PHYSIOLOGICAL, MORPHOLOGICAL AND DEMOGRAPHIC EVALUATION
David D. Briske and James H. Richards Department of Rangeland Ecology and Management, Texas A&M University, and Department of Land, Air and Water Resources, University of California, Davis.
Another Study
Plant Architecture Manipulation Increases Cannabinoid Standardization…
Nadav Danziger and Nirit Bernstein at the Hebrew University of Jerusalem, Israel https://doi.org/10.1016/j.indcrop.2021.113528
This study looked at ways to crop and train cannabis to see if there was any change in production or in the distribution of cannabinoids from top to bottom along the plant.
Bottom line….
They found no increase in production caused by defoliation.
They also looked at whether there was an increase in bud quality in the lower canopy, due to defoliation, and of course there is! A little at least. This should be no surprise to a grower as it is well known that direct contact from the light increases the quality of the buds. Does this indicate that you should grow a tall jungle then defoliate it to let the light hit the buds all the way down the stalk?
No!
Even though there was a small increase in quality of lower buds, there was no overall increase in finished product.A
What do you think??
Does this data apply to all plants??
If you think cannabis is different, please tell me WHY?
Interested in taking part in a Cannabis Defoliation study?
We’re designing the study right now…. Join in the discussion here.
1 thought on “Should You Defoliate Cannabis?”
Finally.! Now i can show my friendes some more scientific studies that confirm what i have been saying for 15years. Although its just logic if one knows anything about plants.
Everything happens in the leafs. Every growth is a result from photosynthesis. If you remove the leafs. How is it suppose to produce higher yield? From what? With what?
An ordinary gardener would never think of removing the leafs since.
I think many new cannabis growers, often search for information on forums where in many cases the posts are 100% wrong and foolish. I once read a post “tip how to get more females” he had birthcontrol pills for woman in the wather. 🙂
(The only way you can affect the ratio fe/ ma is by lower temp, high N and more blue lights in 3weeks from 3e pair of leafs. study made by Dutch passion seedbank prior the introduction of feminized seeds)
So thank you for spreading some more reliable information.
Today i learned about cuttings….
Best regards
Joakim