The Clone to Flower (CtF) Method for Cannabis Cultivation
Cannabis sativa is one of the plants with which humanity has had the longest and most widespread relationship, spanning millennia and eventually covering the globe. Throughout the 12,000+ year history of cannabis cultivation, methods have slowly expanded from outdoor, seasonal, and naturally irrigated production to highly controlled production that leverages modern technology and scientific knowledge to maximize the plant’s potential. By controlling environments in greenhouses or indoor farms, abiotic stresses (sun, wind, and rain) on the plans can be minimized. By making use of hydroponic and soilless cultivation systems, plant nutrition (fertility, water, and oxygen) can be optimized. Lastly, thanks to IPM (integrated pest management) strategies and good operational practices, biotic stresses (pests and pathogens) can be prevented, controlled or eliminated. All of this goes a long way towards showing us what this crop is capable of as well as increasing our understanding of how it responds to how we treat it.
Sea of Green:
During the prohibition of cannabis, starting early in the 20th century, an increasing portion of all cultivation was driven indoors due to concerns about secrecy/privacy, especially in more developed nations. Cultivators began prioritizing efficiency more than previously, since they had to make the most of the space and resources they devoted to their craft at a smaller scale. With this shift came a technique known as the Sea of Green (SoG), which consists of meticulously pruned and trained plants grown at a high density of ~0.5 to 1 plants/sqf with no significant gaps in between. These plants would most often be pure indicas or indica dominant hybrids with a low BSQ (Bloom Stretch Quotient), as these cooperated better with the method. They had to remain short to accommodate the HID (high intensity discharge) lighting in residential settings with low ceilings. HID lighting produces significant heat output and requires some distance from the crop canopy for optimal light intensity and even distribution. The painstaking preparation involved in the SoG method paid off in a low maintenance bloom period and a dense crop that maximized profit per area. Flower yields were as high as 50 g per sqf of canopy per cycle (g/sqf.cycle), though likely averaging less than 20 g/sqf.cycle due to challenges in environmental control. As a result of this shift, sativas and sativa dominant hybrids increased in demand and value, and these most often had to be imported from warmer climates and developing nations with more predominant outdoor cultivation, such as Mexico, Jamaica, and Thailand.
Screen of Green:
With the advent of cannabis legalization towards the end of the 20th century, licensed operators began growing cannabis commercially inside greenhouses and warehouses, affording them more vertical and horizontal space, more access to resources, and fewer concerns about secrecy/privacy. Now bound by plant count regulations, cultivators began prioritizing yield per plant, and this entailed longer vegetative periods and intensive pruning. With these new priorities and allowances emerged the technique referred to as the Screen of Green (ScroG), which consists of larger plants grown at a lower density of ~0.05 to 0.25 plants/sqf with no significant gaps in between. At this point, the full spectrum of cannabis genetics could be grown indoors, including those with a high BSQ, and this allowed cultivators to take advantage of some of the higher yielding sativa dominant cultivars. Commercial yields of salable product now range from about 20 g to 60 g/sqf.cycle with some exceptions, averaging about 35 g/sqf.cycle across the industry. High quality cannabis flowers are now even exported (on the black market) from places like the USA and Canada to the countries that once exported product to these countries, including Mexico and even Caribbean nations known for their cannabis genetics and outdoor production at scale. One aspect that characterizes the ScroG method is what is termed plant “lollipopping”, meaning the bottom 1/2 to 3/4 of each plant is thoroughly stripped of all leaves, flowers and small branches early in the bloom phase in an effort to improve both airflow and crop consistency. Cultivators experienced in this method will tell you that this reduces the unsalable proportion of the cannabis crop and directs the plants’ energy to the portion experiencing better light coverage and penetration.
Vertical Cultivation:
More recently, we have witnessed a new technological barrier breached, and this is the efficiency of LED luminaries. They have become highly competitive as alternatives to HID lighting for horticulture, both in sole-source (warehouse) and supplementary (greenhouse) applications. This has allowed cultivators to more easily engage in multi-tier or stacked cultivation indoors. With cooler luminaires, tailored spectra, and remote drivers, lighting can be placed in close proximity to the plants, providing more potential to maximize the use of vertical space for plant production. This advancement validates the need for cultivation methodologies that further contribute to maximizing all around operational efficiency in cannabis production facilities.
Clone to Flower:
The CtF (Clone to Flower) method is one with which many cultivators have experimented but that has not been widely commercialized yet, though this is changing rapidly. Simply put, it consists of eliminating or severely limiting the vegetative phase of growth in between propagation and flowering. It requires thorough familiarity with the growth habits of the cultivars being cultivated so that they can be planted at the proper density and afforded the correct amount of vertical space. Cannabis genetics vendors or cultivators themselves may run trials to determine the BSQ for each cultivar under standard conditions as well as what a maximum planting density is before self-thinning occurs.
In preliminary experiments performed on a number of diverse cultivars, it was found that planting densities between 3 (sativa dominant) and 9 (indica dominant) plants/sqf were optimal and provided yields of high quality dry, manicured flowers between 75 and 135 grams/sqf.cycle per tier in flowering periods between 5 and 9 weeks. Temperatures ranged between 75 and 80 F, RHs between 55 and 65%, PPFDs between 700 and 1200 uMol/m2.s, and CO2 concentrations between 400 ppm and 700 ppm. It is likely, therefore, that the yield ranges can be further increased by optimizing cultivation parameters fully. Clones were transplanted after rooting at 6 inches in height and finished at between 8 inches and 24 inches. There was no evidence of cannabinoid or terpenoid content being affected either positively or negatively when compared to conventional methods.
Advantages:
-CtF minimizes or eliminates the space, inputs, and labor needed for vegetative growth.
-CtF eliminates the labor involved in pruning, lollipopping, training, and trellising plants.
-CtF maximizes yields per area, time, and inputs.
-CtF maximizes the potential for vertical growing.
-CtF minimizes the risks of issues thanks to shorter crop cycles.
-CtF expedites ripening and maturity leading to shorter flowering times.
-CtF promotes apical dominance and reduces internode length.
Disadvantages:
-CtF is not incentivized by regulations involving plant count restrictions, as yields per plant are significantly lower than with a vegetative period.
-CtF requires more plant propagation and transplanting labor as well as additional space for mothers and clones (micropropagation reduces this).
-CtF may not allow some indica dominant cultivars the opportunity to stretch sufficiently for an optimal harvest.
Rationale:
Vegetative Growth: Cannabis is typically vegetated for between 2 and 6 weeks in order to increase the size of each plant. This is done to increase coverage area per plant and to increase the number of bud sites, and this involves pruning plants to reduce apical dominance. While certain local regulations seemingly favor this process due to plant count restrictions, it may prove to be altogether unnecessary. Aside from the significant extra space, time, and inputs dedicated to this stage and the labor required for transport and transplant, this method yields relatively uneven canopies and allows more time for potential issues to arise. There is also a common preconception that the plant needs to be fed prior to flower so that it may store up sugars before undertaking the arduous task of reproductive growth. This is not so, as cannabis is not a plant equipped to store significant amounts of sugars or minerals for later use and as such does not contain tubers or other organs for this purpose. Sugars and minerals are simply stored in the leaves where they are produced and used, respectively, and these older leaves are what are commonly removed as part of defoliation practices to improve airflow, light penetration, and reduce disease incidence, meaning their sugars and minerals are not allowed to be reabsorbed by the plant for use at the meristems by normal senescence.
Lollipopping: As a result of the vegetative phase of growth, the common practice of lollipopping is undertaken. It is performed due in part to the empirical knowledge that a lower portion of the plant will not receive the light or airflow it needs to produce flowers equal in quality to the upper portion of the plant, thereby creating a vertical gradient in product quality and yield and rendering post harvest processes inefficient. In addition to the intensive labor required to perform this task as well as the risk of infection, the significant wounding to the plant involved actually creates nutrient sinks that draw energy away from the growing meristems as the plants hurry to heal their wounds, causing a detriment to production outcomes. As a result, cultivators end up with tall, top heavy plants that often require trellising to avoid bending over as buds swell during the flowering phase. The productive upper portion of the plants is then short in comparison to the lower, stripped portion of the plant, which qualifies as wasted vertical space that is incompatible with maximizing volumetric space usage in tiered/stacked growing methods.
Light Penetration: Most cultivators attempt to create dense plant canopies in order to maximize flower production per area. As such, adjacent plants are placed very close to one another and adjacent branches within each plant are either touching or very close to it (leaf’s length away). As a result, light penetration into the canopy is limited, not only because of the shading effect of the upper canopy on the lower canopy but also on the exponential decay of artificial light intensity over distance. Attempts are made to correct this effect with multi-stage defoliation practices and high power lights, but favorable results are limited. A more sensible approach is to limit the depth of the canopy, which can be accomplished painstakingly by training and weaving plants with trellising over time or simply by flowering shorter plants, meaning transplanting into the flowering phase earlier in the life of a rooted cutting.
Water and Nutrient Uptake: When it comes to the transport of water, mineral nutrients and sugars throughout the plants, we can think of the roots as pumps and of the xylem and phloem as pipes. The power of the pump/root system is a function of its surface area (and health), and the resistance to and capacity for flow in the pipes/stalk or branches is a function of diameter and length. The shorter the path from the root crown to the growing meristem is, the less energy is required to transport water and nutrients to it, and the larger the diameter of the xylem, the higher the capacity for transport. Therefore, taller cannabis plants expend more energy to translocate water and nutrients to their growing meristems, and the more branches they have, the lower their capacity to do so due to the reduction of allocated xylem tissue caused by the division of the main stalk into the branches. The path from a root crown on a larger, branched plant to the tip of one of its outside branches is a diagonal line that is longer than a straight vertical line to the same point would be. By planting smaller, more apically dominant plants side by side at densities equaling those of branches on pruned larger plants, the number of root systems and diameter of dedicated xylem is increased, and the distance of water and nutrient transport is decreased, rendering the process more efficient by minimizing wasted energy.
Conclusions:
It has become sufficiently evident that a shift to the CtF method for indoor cannabis cultivation is inevitable, especially where larger companies interested in factory farming are concerned. This is in large part because of its compatibility with vertical farming constraints and the method's ability to help the grower maximize resource usage efficiency and create very lean production and post harvest processes. It follows the gradual shift away from the conventional high bay production method in warehouse environments. By eliminating the vegetative phase of growth, the space, time, energy/inputs, and labor spent on this phase are saved, leading to lowered operating expenditures overall. Further savings are achieved by not trellising or lollipopping plants, and a reduced life cycle duration with fewer cuts means less risk of infection or other issues. In deploying a short canopy of clones straight into the flowering phase, more uniform canopies are achieved, allowing for better use of vertical space and more efficient use of horticultural lighting. As cannabis continues to move into legal legitimacy, champion cultivars will be better characterized and normalized across the board, finally warranting specialized protocols for their specific cultivation. CtF will continue to be the way to get the most out of any given cannabis genetics indoors by being adaptable to specific parameters like plant spacing and canopy height. Innovations like subcanopy forced air circulation systems will help further optimize environments in extremely dense cultivation facilities and the current status quo will be exceeded many times over. While this is the case for indoor cultivation under sole source lighting, the future of optimized greenhouse and outdoor cannabis cultivation involves day neutral/autoflower genetics that will eliminate the need for light deprivation systems while allowing for a faster crop turnover and more homogenous quality. In terms of cultivation systems that are conducive to these changes, they comprise reduced component lists and modular bed formats that eliminate the multitude of containers/blocks and irrigation lines that render current methods more labor intensive and less unsustainable. In summation, the opportunities for increasing overall efficiency and productivity without sacrificing product quality are significant and will continue to be the focus of innovation in this field as it matures and evolves into a bright future.