Automated irrigation systems (fertigation) have been used in large-scale agriculture for decades, though the concept and use of the technology is relatively new to the cannabis industry. Irrigation is one of the main elements of day-to-day operation that can easily be automated, and the crop-improving results can be seen almost immediately.

Let’s be honest, hand-watering thousands of plants is a waste of time, water and resources. Relying on people to provide consistent dosage of a precisely mixed nutrient solution to each plant is highly ineffective.

Problems can arise with any automated system, so routine maintenance and monitoring can prevent and advert any disaster that may arise.

1. Check injection measurements.

Constant monitoring helps ensure you are delivering exactly what your plants need.

2. Monitor total dissolved solids (TDS) or turbidity levels.

Our system recycles 100 percent of our nutrient solution and dehumidification runoff, so we monitor TDS levels of our gray water as well.

3. Clean and calibrate pH and Electrical Conductivity (EC) probes.

This is one of the most common maintenance needs, and it is essential to keeping your system functioning at maximum capacity and consistency.

Plant health, yields and quality of your garden will improve drastically once you learn the system. Increased employee productivity and the employees’ ability to focus on the plants' needs should also be instantaneously noticeable.

Scott Reach is founder and COO of RD Industries, parent company for Rare Dankness.

Whenever cultivators ask my opinion on a problem they are experiencing in their facilities, I always am cautious to answer. I try to make it clear when my feedback is only based on observation of repeated results, and that my opinion may be proven wrong by valid scientific data.

As much as I try to provide factual information, I have, over the past few decades, made a few (unwitting) mistakes in doling out advice to cultivators. There are two perfect examples in my book “Marijuana Horticulture Fundamentals” published in 2015. The first example is about lighting.

Shining a Light on UVB

In my book, I wrote about the potential of ultraviolet B (UVB) lighting to increase THC production, a claim that had gained traction among many legacy growers after early studies seemingly pointed to the correlation:

“Knowledgeable scientists have published many articles pertaining to the benefits of UVB light rays on plants, and cannabis plants in particular,” I wrote. “Many acknowledge that plants grown at high altitudes produce more THC via larger trichome heads. If you took two identical plants and grew them under identical conditions, except one was grown at a [5,000- to 6,000-foot] elevation and the other at sea level, the plant that was grown at high elevation would be superior to the sea level plant in terms of THC content. Some attribute the difference to the drastic temperature differential between day and night at such altitudes;”

I then state:

“I do not believe this to be so. This no doubt has something to do with it, but I believe the increased UVB exposure is the primary cause of the plants increased THC productivity. […]Increased UVB exposure triggers the plants[’] survival responses[,] and it responds by producing more oil and resin, which are then pumped into the glands in an effort to make the trichome heads larger and so act as a form of sunscreen. The bigger and more resin glands there are, the less UVB, UVA and UVC will be exposed to the plants[’] surface. Black lights emit ultraviolet (UV) light but are unacceptable for plant growth, as are UVB enhancers added to existing lighting.”

I do hedge my statement by noting “that there has been no formal compilation of scientific data to confirm this theory regarding UV light as true, and it is only mere speculation at this point. Hopefully in the future we will compile true data on this subject and either prove or disprove the theory once and for all.”

So, how much do I believe in the possibilities of UVB enhancement? Well, I have a patent filed on using UVB light in this exact way (see United States patent #7905052 titled “System of photomorphogenically enhancing plants”). From where did all the focus on UVB and THC production come?

For me it all started with the Research Institute of Pharmaceutical Sciences (RIPS) papers from the University of Mississippi. The institute was established in 1964 to discover and disseminate knowledge of natural products, and the RIPS papers have been published since the early 1980s. I compiled the many volumes of the RIPS papers into a personal book of dozens of studies concluding that there is an increase in THC production when UVB light is applied.

As a whole, the studies compiled in the book represent tens of thousands of dollars and hours of very legitimate research, which boast a very impressive list of scientists who conducted the research and wrote the papers. However, much to the lament of anyone who attempts to re-create the increase of THC production via application of UVB light, the results have proven difficult to repeat outside of lab settings.

That’s not the only lighting belief I’ve held that has since been proven inaccurate. In the January 2008 issue of Skunk magazine, the author of the article titled “Hash Tips: Maximum Resin Production for the Medical Cultivator” claimed (through testing multiple lighting systems) that it is important that lighting fixtures have 660nm of red spectrum and no traces of 680 nanometers (nm) red. “Absorption peaks around 660nm for red and flowering is inhibited at around 680nm red… [The] HPS spectrum contains 680nm red. This inhibits flowering just enough so that resin concentrations are standardized.”

The author continues: “Metal halides are also sometimes used during flowering to promote resin production. This effect is from the UVB light that exists in halides. Halides contain 680 nm red and will not achieve the concentration we’re looking for. Only in high concentrations of 660nm—without the 680nm red—can the cannabis buds reach a point closer to genetic perfection.”

However, according to Université Laval professor Dr. James Eaves, Ph.D., who has studied light spectrum’s influence on cannabis production, “when we consider the impact of particular wavelengths, like R [red] and FR [far red], what’s important are ratios (e.g., R/FR, B [blue]/R, B/G [green]), rather than occurrence of individual wavelengths (e.g., 680 nm). For instance, R causes cell expansion while B reduces it, so it’s B/R that impacts plant development.”

This goes to show that despite how lighting’s impact on cannabis chemistry is one of the most researched topics in the cannabis space today, much more work needs to be done to further investigate how it plays out in the field in order to correct the public record.

Humming on Humic Acid

The second “best practice” that was eventually disproven in my book relates to the use and application of humic acid on cannabis plants—more specifically, humic acid derived from pure, mined mineral-rich organic plant active humates. (Humates have been shown to regulate the flow and enhance the transport of nutrients for some plants in certain cropping/growing systems.) In my book, I mention a commercial form of humic acid advertised as “[accelerating] nutrient absorption at the root boundary zone, where minerals enter the plant,” which continued into “Excellent for indoor and outdoor use.”

For a grower like myself focused on healthy plant growth, all these promises sounded fantastic and logical. Humic acid has been used on other crops for the purpose of increasing nutrient uptake and stimulating plant growth—why wouldn’t it work for cannabis?

While it is true concerning many other plants, it seems humic acid may have unwanted effects on the cannabis plant. A June 2019 study published in Frontiers in Plant Science titled “Impact of N,P,K, and Humic Acid Supplementation on the Chemical Profile of Medical Cannabis (Cannabis sativa L)” sheds light on the fact that humic acid impacts cannabis in more ways than just overall plant health.

Among other findings, the authors discovered that humic acid “was found to reduce the natural spatial variability of all the cannabinoids studied. However, the increased uniformity came at the expense of the higher levels of cannabinoids at the top of the plants, and THC and CBD were reduced by 37% and 39%, respectively.”

I mention humic acid because once upon a time it was thought of as a beneficial enhancement. But when applied to the cannabis plant, humic acid, at least according to this most recent study, can negatively impact cannabinoid production.

In turn, this finding also makes me question the use and application of fulvic acid on cannabis plants. Fulvic acid is thought to accentuate the production of nucleic acids and photosynthesis—and even made an appearance in my book: “Fulvic Acid will supercharge the entire plant from the roots to the growing tips,” I wrote.

Now I’m left to wonder whether, like humic acid, fulvic acid comes with drawbacks when applied to cannabis plants?

Perhaps in the future, data such as this will be utilized to purposely produce or manipulate certain desired cannabinoids or terpenes. There is much to prove and disprove regarding cannabis and its production. And with more and more researchers turning their attention to cannabis, it is no surprise that some things that once were considered to be true have now been proven false.

Perhaps we should take the time to re-evaluate how we’ve come to some of our conclusions, and whether that certainty is warranted. The way I test myself when I form an opinion is to immediately attempt to prove myself wrong. But even if a given opinion survives thorough scrutiny, it remains an opinion until scientific data can support the claim and is repeatable in the real world, not just a lab.

Kenneth Morrow is an author, consultant and owner of Trichome Technologies. Facebook: TrichomeTechnologies Instagram: Trichome Technologies k.trichometechnologies@gmail.com

Cultivar Details:

Plant physiology: Lemon Vuitton (LV) stays relatively short and stocky, with good, sturdy branching. The plants bush out well when only the terminal shoot is topped early. Growth is vigorous for a broadleaf type plant with internodal spacing wide enough for good airflow around the flowers late in flowering. Flowers are white/silver with extreme resin content.

Average yield: With a two-week vegetative growth time, expect 2 to 4 dry ounces per plant depending on many factors, including selection preferences. At nine plants per light, LV yields an average of 2.5 lbs./1000-watt light.

Flowering time: Swamp Boys harvest at 59 days in a greenhouse, although expect some selections to go to 63 days or more. Generally, the terpene profile gets deeper and more complex after 60 days if environmental conditions are kept at optimal levels—cooler temperatures with lower light levels or higher Kelvin (a unit of measurement used to express color temperatures).

Ideal light-intensity setting: LV can handle high light levels throughout the growth cycle, up to 750 watts per meter. That said, the flower structure has large bracts that prefer a slightly lower light level and cooler temperatures in the last 10 to 14 days. This protects the flower from drying out and ensures robust terpene production.

Ideal cultivation environment temperature: The LV plants Swamp Boys grew out did best at 78 degrees Fahrenheit daytime average, with cooler nights (72 degrees Fahrenheit) the last two weeks.

Ideal cultivation environment relative humidity: LV likes a relative humidity (RH) of 60% throughout most of its lifecycle, falling between 40% and 50% as Swamp Boys lower the temperatures during the last two weeks of flowering.

Water needs: On average, LV is a slightly light drinker depending on the leaf area index and level of crop steering. Phenotypes that prefer a slightly higher RH usually use slightly less water. LV struggles a little with aggressive dry-downs due to the plant structure and broad leaves, so the plant prefers a more vegetative irrigation model.

Nutrient needs: LV seems to perform best with a lower nutrient strength that corresponds with its irrigation preferences.

Cannabinoid profile: High THC (waiting for official test results).

Terpene profile: LV has a lemon, leather and fuel base with whiskey cream notes, which are some very interesting terpene combinations that immediately stood out. (Waiting for test results.)

Varietal resistance: LV flowers are fairly dense with large bracts, so humidity must be controlled during last stages to reduce the chance of Botrytis. Too much heat and high light levels in the later stages of flower can dry out bracts/flowers and reduce or change the terpene expression of this variety.

Plants, much like humans, require certain nutrients and minerals to thrive. These macro- and micronutrients help plants grow properly, obtain optimal yields, and complete their lifecycle. Aside from nitrogen (N), phosphorus is perhaps the most important of the macronutrients. Phosphorus (P) is essential for energy storage and utilization, root development and growth, flower formation, and metabolic activities in cannabis.

In plants, P is a mobile element that is necessary for many key metabolic processes, in addition to cell elongation and root growth and development. Therefore, P can be moved (translocated) from older growth segments of plants to satisfy demand in newer, developing plant portions. This is important to note, given that nutrient deficiency symptoms will appear in older leaves as the plant moves P resources into the new and developing leaves and other sinks such as developing flower buds.

Deficiency Symptoms

In earlier studies conducted at North Carolina State University (NCSU), researchers induced nutrient disorders in cannabis to examine how plants respond to various deficiencies. Those studies showed that symptoms of P deficiency will develop quickly in cannabis plants, especially during the mid-vegetative portion of the growth cycle and during the flowering stage. Plants first will appear stunted and develop more slowly compared to a healthy plant (Fig. 1). If the plant does not receive P, the older and/or lower leaves will develop olive-green spots in an irregular spotting pattern along the leaflet and margin (Fig. 2 & Fig. 3). As symptoms progress, the olive-green spots develop into larger olive-green spots that appear sunken and almost wet with some necrosis, or burning along the leaf margin (Fig. 3). In advanced symptoms, the yellowing leaves become severely olive spotted with large areas showing symptoms, and in severe cases, large necrotic portions develop (Fig. 4 & Fig. 5).

Phosphorus Fertility Rates

In a second research study, NCSU researchers looked at six rates for phosphorus fertility (3.75, 7.5, 11.25, 15, 22.5, and 30 parts per million (ppm) P). We explored the impacts of fertility rates on above-ground shoots, (i.e. leaf, stems) and below-ground roots, leaf tissue nutrient accumulation, total bud fresh weight, and cannabinoids.

For optimization of biomass production, a minimum concentration of 11.25 ppm P rate should be used for plants grown for flower buds. For mother stock plants, the rate may need to be slightly greater given that nutrient resources will be removed when each subsequent cutting is harvested. Additionally, researchers did not observe significant differences in root biomass production at any level of P fertility. This may largely be due to the fact that when plants are grown in a fixed substrate volume (within a container), they are limited in the amount of root growth and, consequently, more resources will be allocated to shoot growth.

Leaf Tissue Concentration. The rate of P fertility that accumulated in leaf tissue was not maxed out in this experiment. This is not surprising given the high levels of P fertility many cannabis growers provide plants. The highest P concentration (% dry mass) occurred with the 30 ppm P rate. This rate resulted in almost double the leaf tissue P as the lowest rate of 3.75 ppm. When plants take up extra nutrients but yield does not increase, it is defined as “luxury uptake.” Plants will take up the extra P if provided, but because flower yield did not increase, providing this extra P does not make economic sense.

Flower Buds. For sampling purposes, the shoot apical cola bud, three terminal axillary buds, and three interior branch nodal buds were sampled. The combined buds were pooled together to produce a sub-sample of buds and their fresh weights recorded at the time of harvest. The bud weights indicated that a fertility rate of 22.5 ppm P resulted in a significantly larger bud fresh weight, and almost doubled the fresh weights of the lowest two concentrations (3.75 and 7.5 ppm P).

When cannabinoids were analyzed, no increase in cannabinoid concentration occurred above the 11.25 ppm P concentration for the acid pools (CBDA, CBGA, THCA, total THC, total CBD, and total cannabinoids). Results varied for the active cannabinoids (decarboxylated cannabinoid pools) and thus are not highlighted here. More detailed results and data can be found in our recently published scientific journal article, “Impact of phosphorus on Cannabis sativa reproduction, cannabinoids, and terpenes,” noted in the sidebar. These data may indicate that a constant level of P fertility provided at 11.25 to 15 ppm P may be adequate to optimize cannabinoid production within the floral material (Graph 1).

Plants in this study received a constant feed fertility, and researchers did not alter P fertility based on growth stages, as is common in the industry. More research is needed to elucidate the impacts varying P fertility can have on cannabinoids. For these results, however, if a constant feed fertility is used, we would recommend a 15 ppm P concentration be provided, as this is above the threshold seen at 11.25 and would provide an extra fertility buffer to ensure maximum flower bud dry weight and cannabinoid production. This fertility level (15 ppm P) will allow for error and alterations given that different cultivars show varied architectural structures and may vary slightly in nutritive needs. More research also is needed to identify if cultivar variability exists due to plant architecture and bud types (Fig. 6).

Conclusions

A general recommendation of 11.25 ppm P for vegetative production appeared to be adequate, though P rates would need to increase as plants increase in size and age. For bud production, a P rate of 11.25 to 30 ppm P produced a similar amount of fresh bud biomass. For optimizing cannabinoid production, no increase in cannabinoid concentrations occurred above the 11.25 ppm P fertility rate. This indicates that in a constant feed fertility system, adding additional P resources above 11.25 may not increase cannabinoid production. The next steps in scientific inquiry would be to compare the constant feed fertility results against a program or programs that alter P fertility based on life stages. Additionally, species or type specific research is needed as different architectures, flowering shape and distribution, and cannabinoid profiles exist within Cannabis sativa.

Paul Cockson is a Ph.D. student at the University of Kentucky's Horticulture department. He is a part of the controlled environment horticulture (CEH) lab and is conducting research on plant nutrition and abiotic stress impacts on greenhouse vegetable quality and fruit development.

Dr. Michelle Schroeder-Moreno, Ph.D. is a professor of agroecology at North Carolina State University specializing in Mycorrhizal fungi and plant nutrition. She also is the Director of the Agroecology Education Farm. She was a team member on the NCSU phosphorus research project.

Patrick Veazie is an undergraduate researcher in the Department of Horticultural Science at North Carolina State University.

David Logan is an undergrad research assistant in the Department of Horticultural Science at North Carolina State University.

Dr. Brian E. Whipker, Ph.D., is a professor of floriculture at North Carolina State University specializing in plant nutrition, plant growth regulators and diagnostics. During the past two years, he co-authored eight scientific journal articles on the impact of fertilization with greenhouse species and three disorder diagnostic guides. Dr. Whipker has more than 28 years of greenhouse experience working with growers.