Many growers believe that stressing plants, by drying them down, underfeeding them or hitting them with UV, forces them to make more resin. The research shows that stress can change cannabinoid levels, but the results are narrow and conditional, and sometimes the opposite of the folklore.
A controlled late-flower dry-down raised THCA and CBDA concentrations without reducing yield in one University of Guelph study. Underfeeding during flower lowered yield but raised CBD concentration in CBD-type plants. On the other hand, supplemental UV-B did not raise THC or yield, and in young hemp plants the LED treatment that produced the most CBD was also among the least stressful.
Water use matters too: at the leaf level, cannabis lost less water under elevated CO₂.
The common thread is control. These studies measured stress precisely, by substrate moisture, nutrient concentration or UV dose. Uncontrolled stress wasn’t what they tested, and it can easily cost yield.
Key terms
Water-use efficiency
How much a plant photosynthesizes for each unit of water it loses through its leaves.
THCA and THC
Living cannabis makes THCA, an acid form. Heat and time convert it to THC. CBDA and CBD work the same way.
Studies
5 studies
Increasing Inflorescence Dry Weight and Cannabinoid Content in Medical Cannabis Using Controlled Drought Stress
HortScience · 2019
Drug-type cannabis
Controlled experiment
A controlled late-flower dry-down raised THCA and CBDA without cutting yield.
Authors: Deron Caplan, Mike Dixon, Youbin Zheng
54(5):964–969
Study plant: One medical cannabis cultivar, drought stress applied in week 7 of flowering
Starting in week 7 of flowering, researchers withheld irrigation from one group of plants until they reached a defined wilting threshold, which took 11 days, then compared them with normally watered plants. The drought-stressed plants had higher THCA and CBDA concentrations without losing harvested dry weight.
Cannabis Hunger Games: nutrient stress induction in flowering stage – impact of organic and mineral fertilizer levels on biomass, cannabidiol (CBD) yield and nutrient use efficiency
Frontiers in Plant Science · 2023
Drug-type cannabis
Controlled experiment
Open access
One-third less fertilizer still delivered 95% of the CBD yield.
Authors: Danilo Crispim Massuela, Sebastian Munz, Jens Hartung, P. M. Nkebiwe, Simone Graeff-Hönninger
14:1233232
Study plant: CBD-dominant medicinal cannabis grown indoors
CBD-dominant medicinal cannabis was flowered on mineral or organic fertilizer at three strengths (80, 160 and 240 mg/L nitrogen) to test whether deliberately underfeeding in flower can save fertilizer. Underfed plants yielded less flower, but higher CBD concentrations made up most of the difference: they reached 95% of the CBD yield with one-third less fertilizer. The organic fertilizer was taken up and used less efficiently than the mineral one.
Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation
Frontiers in Plant Science · 2021
Drug-type cannabis
Controlled experiment
Open access
Supplemental UV-B didn’t raise THC, other cannabinoids or yield.
Authors: Victoria Rodriguez-Morrison, David Llewellyn, Youbin Zheng
12:725078
Study plant: Two indoor drug-type cultivars (‘Low Tide’ and ‘Breaking Wave’)
Two indoor cultivars received daily supplemental UV from 287 nm LEDs at a range of doses throughout flowering. Total yield fell in one cultivar as the UV dose increased, and the share of yield coming from the top buds dropped in both. UV did not deliver any commercially meaningful increase in yield, cannabinoids or terpenes.
Study plant: 25-day-old hemp seedlings in an aeroponic system, 20 days of light treatments
Young hemp plants were grown aeroponically under ten LED spectra plus natural light for 20 days, and researchers tracked cannabinoids alongside markers of plant stress. A red-blue-green mix (R7:B2:G1) gave the highest CBD with relatively low stress. THCA behaved most like a stress marker, while CBD and THC showed little link to light-induced stress.
Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2
Physiology and Molecular Biology of Plants · 2011
Drug-type cannabis
Lab study
Open access
Elevated CO₂ improved water-use efficiency by reducing water loss through the leaves.
Authors: Suman Chandra, Hemant Lata, Ikhlas A. Khan, Mahmoud A. ElSohly
17(3):291–295
Study plant: Leaves of four high-THC varieties, short-term gas-exchange measurements
Leaves of four high-THC varieties were measured at roughly ambient (390 ppm) and elevated (545 and 700 ppm) CO₂. Under elevated CO₂ the leaves partly closed their pores and lost less water, which raised their water-use efficiency.