The issue of leaf removal for cannabis through the process of defoliation has troubled growers for a long time and will probably continue to do so regardless of any evidence. Here i will be discussing what defoliating is, the most popular theories and the science of each.

Every method that removes material from a plant is a form of pruning, which is to control the shape, structure and function of a plant and improve its overall health, performance or appearance.

Defoliation describes the process of removal or loss of leaves. It does not detail the methods or purposes it may be used for. That requires extended terminology whether that is current or new. The method of defoliating in agriculture is not a practice used for improving product yield, but for other practical purposes such as harvesting efficiency, forced dormancy, disease control etc. Cannabis horticulture is the only field where defoliating is used to try improve yield. Its probably not known where it started but cannabis growers are always trying to find ways of improving their grow and as such will inevitably be a target for unproven practices. There are various methods on how defoliation is recommended to be performed, but typically it will involve the selective or non selective removal of leaves, of a varying amount at varying times during a plants grow. This is dependent on the method provided by the grower and their reasons or beleifs which define the process.

Air & Light On Leaf Photosynthesis:

As a leaf ages, its ability to photosynthesize follows almost like a bell curve, with a peak around 20-60 days. Here it can be seen how leaf age is very impacting on its photosynthetic capacity. As the leaf gets older, aging or senescence brings about deactivation of enzymes and degeneration of chlorophyll. This is why older leaves start dying and fall off on the lower end of the plant. 

It has been suggested that older leaves may act like sinks when they can no longer continue to efficiently convert light into food. But this is not what happens. In the early stages of leaf development, leaves act like sinks. In just a few days, leaves begin the Sink To Source transition where they start supporting themselves and begin exporting photosynthetic resources, as a source. Its at this point and for the remainder of its life until cessation that it stays as a source. Removing leaves does not provide more photosynthate for flowers, as leaves are providing energy, not requiring it. They are sources. The plant also utilizes the nutrients and energy leftover from the leaf, prior to abscission during senescence. This is what happens during the late stages of flowering when the plant is searching for nutrients.

One could argue that reducing foliage will help prevent leaves from the top, from blocking light and reaching lower into the canopy, however this is only going to end up with the same 'total' photosynthetic yield. All that is changed is where the photons will land. Any light which is not absorbed from higher leaves will pass through to lower leaves. This is called the transmittance effect. The idea that top leaves block lower leaves and waste usable energy, is incorrect for this very reason. As light is captured efficiently. Infact exposing shade leaves could be harmful because of the shade vs sun leaves problem. Shade leaves have a different anatomical structure compared to sun leaves, as such its capacity to process light is limited and is negatively affected through factors such as photoprotection and photoinhibition. It takes time for leaves to adapt to new lighting environments. Removing sun leaves and exposing shade leaves, could slow growth and add time to the overall schedule. This is the reason why plants have problems when being introduced into new lighting environments.

With airflow and transpiration, the process is completely passive. But we can prevent problems such as water vapor barriers from static air, sufficiently with active air movement. By removing leaves, you ultimately limit the plants ability to move water and nutrients through transpiration, as the leaves that are removed contribute to this through stomatal movement. By removing leaves you prevent the plant from breathing so to speak, as such the plant tries to recover and attempts to generate more leaves as a result.

Floral Morphogenesis:

It has been suggested that a flowers interaction with light and wind could cause such reactions that enables the flower of cannabis to develop, bulk or swell more as a result.

Morphogenesis is the biological process that causes an organism to develop its shape. A plants morphological response to environmental factors has been studied for several decades. Examples of environmental stimulus are light, air, wind, temperature, touch, damage etc. Although we may not fully understand the physiology or biochemistry of plant sensory chemicals. We do however understand the fundamentals of their functions, which have been wonderfully displayed through many experiments over the decades. Its through the hard work of plant science that we can understand these processes and use them to our advantage.

Plants not only use light for photosynthesis, but also for mediated responses where light is used as signals or information to generate growth patterns determined by specific wavelengths of light. These are mediated by photoreceptor proteins such as phytochromes, cryprochromes, phototropins. I like to think of it as photosynthesis manufactures energy, where as morphogenesis says what is done with some of that energy. Examples of photomorphogenesis are photoperiodism, phototropism, shade leaf avoidance, stem elongation, chloroplast movement, stomatal opening.

Plant science has not observed any photoreceptor proteins in flowers, or that there are any properties of flower morphology to light. Phototropism for example has been shown to only be found or function in the apical meristem of plants. Photoreceptors responsible for photoperiodism are only found in leaves, as such exposing light to other parts of plants has no effect on flowering.

Its also been showed that light itself has very little impact on the characteristics of flower development, suggesting that during flowering. Light factors such as different wavelengths, has very little to no impact on floral morphology.

Hans Mohr 2012 · Science of photomorphogensis:

"The characteristics of the flowers (including stamens, pistils, corolla and calyx) are hardly modified by light. A potato plant will form very similar flowers under all circumstances which allow flower formation at all."

Thigmomorphogenesis is the morphological response by plants to mechanical sensation. Examples of this are plants response to wind. Outdoors where wind sways branches, this creates chemical messages that tells the plant to strengthen stems and leaves. This is observed well with greenhouse plants where air movement is minimal. Branches are spindley and weak with leaves having very thin cuticle layers.

There has been no evidence or indication of floral morphology from mechanical stimulation such as wind or airflow. As such it is hard to conceive the idea that defoliating improves growth through thigmomorphogensis.

Stress & Phloem Translocation:

Studies on defoliation have showed that a majority of species of plants, show an ability or buffer to compensate for some type of structural damage during its life. Its almost expected by plants that some event will happen and that it be prepared. What defoliation studies have in common, is that plants are not negatively impacted by the loss of leaves up to a certain point (around 25%). It seems that a plant will typically have more leaves than it requires, almost as if history has taught it to anticipate the loss of leaves through enviromental factors. This was some of the conclusions by people conducting these studies.

Now a plant holds the ability to store nutrients (photosynthate) in its roots, branches, leaves and fruit. This is its major storage sink during growth and vegetation. However in the event of stress, injury or sickness, a plant may be able to redirect this stored energy through a process called phloem translocation. Some plants have enough stored resources that they can revegetate, if all their top growth is removed. Cannabis does not hold this ability but it may be possible that all plants have some degree of stored energy to respond to stressful events. In the partial defoliation studies, some report the event where the variables grew taller than the controls. Suggesting that the plants response to reduced leaf numbers, caused them to branch out to create new leaves to sustain its level of photosynthetic capacity. This response could be aided by stored resources through phloem translocation. This also brings up the interesting reason of why supercropping (i like to call it scropping) may appear to increase growth and vigor.

However, to all extent there has been no evidence to support this. The problem here is that any resources that is moved because of a stress response, is prioritizing it for the repair of damage or the development of new leaves. This has been observed by many, as a delay in flowering due to the stress induced by defoliation.

Evidence And Conclusion:

I looked all over for any and all controlled grows, comparing defoliated plants to controls. Unfortunately i found very few. The ones i did find showed clear indications of no improvement but infact a decline in yeild. You would think that of all the people who have grown, they would have done such a simple experiment. However it seems people keep insisting on doing the method without controls and make subjective opinions on the matter. The most surprising one is where people defoliate and somehow are surprised that a few days later there is more growth. Then conclude the success of their experiment. I cannot enough make it clear how important, controlled side by side comparisons are. This is to remove any and all variables that you 'may' or 'may not' expect, regardless of what you have experienced in the past. Visial bias alone is enough to convince somone of something that is not true. Not everything is as it appears.

I have been through many forums and threads where people claim once and for all to answer this issue and would conduct a controlled grow. Only to almost always fall through and never return, countless threads encouraging people to post their results. Nothing!, ever at all comes to light. Only the select few that seriously go through with it, end up showing the failure of defoliation. Most who are educated in botany or horticulture almost always seem to disagree with the method. Field experts in commercial grows do not employ techniques even though there are ways of doing it efficiently. Studies find no connection, no evidence of any kind.

Although i support the idea of providing more light to the lower canopy, this is just for quality rather than quantitiy reasons. Whether this is by removing a select few fan leaves, or by training or pruning the plant to allow more light to get through.

If anyone has any other possible theorys or further information on the subject that may be interesting, please let me know. If there is anything that may show otherwise i am happy to know, however as it stands i am unconvinced. I would love the idea, of the practice of defoliation to increase yields. Another technique for improved yields is always wanted. However i need more than empiricle data. Even if there is no sound science of the reason how it could possibly work, i would need at least some evidence of a control compared to a variable.

The following are testemonies from several published and qualified cannabis botanists and horticulturists. If you trust some of the best botanists/horticulturists in cannabis, follow their work, read their books. Then at least trust them when they also say that traditional defoliation has no bearing on improved plant performance. What does help and is commonly accepted is the traditional work of pruning in horticulture. Controlling the shape, structure and function of a plant and improve its overall health, performance or appearance.

Leafing By Cannabis Botanist, R.C. Clark:
Leafing is one of the most misunderstood techniques of drug Cannabis cultivation. In the mind of the cultivator, several reasons exist for removing leaves. Many feel that large shade leaves draw energy from the flowering plant, and therefore the flowering clusters will be smaller. It is felt that by removing the leaves, surplus energy will be available, and large floral clusters will be formed. Also, some feel that inhibitors of flowering, synthesized in the leaves during the long non inductive days of summer, may be stored in the older leaves that were formed during the non inductive photoperiod. Possibly, if these inhibitor-laden leaves are removed, the plant will proceed to flower, and maturation will be accelerated. Large leaves shade the inner portions of the plant, and small atrophied floral clusters may begin to develop if they receive more light. In actuality, few if any of the theories behind leafing give any indication of validity. Indeed, leafing possibly serves to defeat its original purpose. Large leaves have a definite function in the growth and development of Cannabis. Large leaves serve as photosynthetic factories for the production of sugars and other necessary growth sub stances. They also create shade, but at the same time they are collecting valuable solar energy and producing foods that will be used during the floral development of the plant. Premature removal of leaves may cause stunting, because the potential for photosynthesis is reduced. As these leaves age and lose their ability to carry on photo synthesis they turn chlorotie (yellow) and fall to the ground. In humid areas care is taken to remove the yellow or brown leaves, because they might invite attack by fungus. During chlorosis the plant breaks down substances, such as chlorophylls, and translocates the molecular components to a new growing part of the plant, such as the flowers. Most Cannabis plants begin to lose their larger leaves when they enter the flowering stage, and this trend continues until senescence. It is more efficient for the plant to reuse the energy and various molecular components of existing chlorophyll than to synthesize new chlorophyll at the time of flowering. During flowering this energy is needed to form floral clusters and ripen seeds. Removing large amounts of leaves may interfere with the metabolic balance of the plant. If this metabolic change occurs too late in the season it could interfere with floral development and delay maturation. If any floral inhibitors are removed, the intended effect of accelerating flowering will probably be counteracted by metabolic upset in the plant. Removal of shade leaves does facilitate more light reaching the center of the plant, but if there is not enough food energy produced in the leaves, the small internal floral clusters will probably not grow any larger. Leaf removal may also cause sex reversal resulting from a metabolic change. If leaves must be removed, the petiole is cut so that at least an inch remains attached to the stalk. Weaknesses in the limb axis at the node result if the leaves are pulled off at the abscission layer while they are still green. Care is taken to see that the shriveling petiole does not invite fungus attack. It should be remembered that, regardless of strain or environmental conditions, the plant strives to reproduce, and reproduction is favored by early maturation. This produces a situation where plants are trying to mature and reproduce as fast as possible. Although the purpose of leafing is to speed maturation, disturbing the natural progressive growth of a plant probably interferes with its rapid development. Cannabis grows largest when provided with plentiful nutrients, sunlight, and water and left alone to grow and mature naturally. It must be remembered that any alteration of the natural life cycle of Cannabis will affect productivity. Imaginative combinations and adaptations of propagation techniques exist, based on specific situations of cultivation. Logical choices are made to direct the natural growth cycle of Cannabis to favor the timely maturation of those products sought by the cultivator, without sacrificing seed or clone production.

Pruning By Ed Rosenthal:
Indoors, the canopy absorbs virtually all the light, leaving little in the shadows below. For this reason, the understory below the canopy contributes little energy to the plant. Instead, it costs the plant nutrients, increases humidity, and stops air-flow.

Pruning the lower limbs creates more air-flow under the plants and creates cuttings for cloning. It also forces the plants growth to the top limbs that get the most light, maximizing yield. These lower leaves and branches should be removed to create and open airspace.

Pruning By Mel Frank:
Under sunlight, you can expect to harvest a plant with strong colas on every branch, rather than one large top cola and much smaller branch, if you clip the top growing shoot about two weeks before flowering. Under weaker electric light, removing the top shoot may result in smaller buds and possibly a smaller harvest. Don't use late pruning as a general procedure under artificial lights, except as a last resort when one or two plants are outgrowing the rest of the crop. These fast-growing plants prevent the light from being positioned closer to all the plant tops. Bending and training these tallest plants is the best procedure. When the plants first start to flower, clip off the lowest, under-developed branches. These branches won't be worthwhile buds anyway, and by removing them, the stem and root system feed only the strong branches, which consequently yield larger buds. Removing lower branches also allows reflected light to better illuminate the whole plant and promotes better air circulation.
Once the buds begin to form, remove some of the large fan leaves along the main stem. This allows the light to reach the lower buds and they'll develop more 'fully'. Don't over do it; once a leaf forms it produces more "growth energy" for the rest of the plant. If you strip a plant of its leaves, your lessening its capacity for growth. Removing most of the health leaves also can delay flowering. In an outdoor plot, plants stripped of leaves flowered two weeks later than their sisters that were left intact. Under lights, remove only some upper main stem fan leaves that shade lower branches.

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