Article by Emma Stone, Weedmaps
Decades of prohibition have stunted scientific research into cannabis growth and production, but as legalization spreads, horticultural experts are again studying cannabis. Some are exploring how to optimize growing conditions to exploit the medicinal potential of cannabis.
A study published in the May 2019 issue of HortScience, a publication of the American Society for Horticultural Science, investigated the effects of targeted underwatering, or controlled drought stress, on cannabinoid content and dry weight. The researchers hypothesized that controlled drought stress might allow growers to enhance the quality of their cannabis crops.
Factors that Influence Cannabinoid Concentration
Cannabinoids are the oils concentrated in the glandular trichomes of female flowers that give cannabis its value as a medicinal and recreational product. Cannabinoids generally accumulate during the flowering stage of growth, but the timing of peak cannabinoid concentration tends to vary by chemotype and cultivar.
Cannabinoid concentration can be genetically influenced through breeding and phenotype selection. However, horticultural management methods such as fertilization, the choice of substrate, air temperature, lighting intensity, lighting quality, and photoperiod also can affect cannabinoid production.
Controlled drought, however, also can stimulate oil production. When exposed to intermittent watering and sunlight, herbs and spices cultivated in semiarid locations tend to be more aromatic and have higher essential oil concentrations. In one study of summer savory, a plant that was drought-stressed during the flowering stage had a 31% higher essential oil concentration than its watered counterpart.
Though drought-stressed plants may have a higher concentration of essential oils, the total yield sometimes is lower because conditions reduce growth and the amount of harvestable plant material. Careful manipulation of the level and the timing of the drought stress can help to minimize the loss of dry weight.
Tracking Effects of Controlled Drought
Fourteen-day-old cuttings taken from the same stock plant were transplanted into pots filled with a custom-blended organic growing substrate, watered by hand, and fertilized with a liquid organic fertilizer.
At fifteen days after transplantation, eight plants of similar height and canopy size were transplanted into larger pots and transferred to a walk-in growth chamber for flowering. This was considered the first day of the flowering stage. The plants were hand-fertilized and watered for another 10 days whenever the substrate moisture content reached 20%, and then cared for with various nutrient and element amendments until they were 39 days old.
On day 39, four plants were assigned to the drought group, and four plants were assigned to the control group. Water and fertilization continued to be provided for the control plants, but was withheld from the drought group.
Throughout the drying period, drought stress indicators such as volumetric water content, the moisture content present in the substrate, and wilting (measured by the angle of leaves) were recorded until the day after the drought group had received water and fertilizer.
Drought Increases Cannabinoid Concentration
The eight cannabis plants were harvested 54 days after flowering and hung to dry at 18 degrees Celsius, or 64 degrees Fahrenheit, for 12 days before being analyzed by an independent laboratory.
The major cannabinoids THC and CBD were detected in both the control and drought plants. Only one plant in the drought group had detectable concentrations of CBG and CBN, so the authors could not make comparisons with the control group. The drought treatment provoked a 12% increase in THCA concentration, and a 13% increase in CBDA concentration, but did not affect the concentrations of the other cannabinoids.
Drought also significantly affected the yield of certain cannabinoids. Yield was calculated as grams of cannabinoid from flowers per unit of growing area, with the following yield results in drought specimens compared with the control group:
- THC: 50% higher;
- THCA: 43% higher;
- CBD: 67% higher;
- CBDA: 47% higher.
There was no difference between the dry weight of the control plants and the drought plants, which was noteworthy because drought stress often results in reduced growth in other plant species.
The authors of the study also pointed out that there were no signs of nutrient deprivation or differences in plant appearance between the control and drought groups for nine days. From nine days until harvest, the drought plants showed the telltale paleness of veinal chlorosis, which arises from insufficient chlorophyll production in leaves.
Drought Could Be Beneficial
Although the study comprised a small sample size, the findings suggest that controlled drought conditions can be beneficial for cannabis plants, according to Russell Pace III, President of the Cannabis Horticultural Association.