The vegetative stage of cannabis plants grown indoors can be short in duration but is always impactful. At this stage, which lasts for about 2-8 weeks, plants develop stems and leaves and channel light energy into growing big and strong.
Do cannabis plants require high light intensities during the vegetative stage?
Light plays a key role in moderating plant morphology and yield. Manipulating light intensity (LI), spectrum, and photoperiod can be used to produce desired morphological characteristics in cannabis plants. Lighting intensity is also a consideration in indoor grows as the cost of electricity can be exceptionally high. 
During the vegetative stage, cannabis plants develop an increased demand for light energy due to vegetative growth (height, branch size, leaf size and thickness, etc.). However, there is insufficient evidence on the specific light intensities required to match vegetative stage production strategies.
A recent study published in Preprints (not yet peer-reviewed) aimed to shed light on the relationship between light intensity (LI) and the vegetative stage.  Specifically, the researchers explored the “opportunity to elevate [light intensity] during the vegetative stage to enhance plant structure and shorten the length of the vegetative stage.”
The researchers thus set out to investigate how cannabis plants in the vegetative stage respond to different light intensities.
Light intensity treatments were administered in randomized complete block design at different intensities (200, 450, 700, 950, and 1200 μmol·m-2·s-1). The researchers examined plant growth and leaf morphology under these light treatments.
The researchers used “Gelato” plants which were cultivated indoors for 21 days. Photosynthetic photon flux densities (PPFD) ranged from 200 to 1200 µmol·m-2·s-1 using light-emitting diode (LED) bars centered above plant plots. Average light accumulation per plant at the canopy level was determined; APPFD (average PPFD) ranged from 135 to 1430 µmol·m-2·s-1. Blue (445 nm) and red (665 nm) wavelengths were employed. The plants were exposed to a 16-hour photoperiod daily.
The researchers made the following key observations:
- The plant height increased quadratically (in a curved fashion).
- Growth predictors such as number of nodes, stem thickness, and dry weight above the ground increased asymptomatically.
- Foliar features showed different responses to light intensity.
- Chlorophyll content increased asymptotically.
- The size of the leaves decreased linearly while the weight increased linearly with increased light intensity.
- Internode length and water content decreased linearly with light intensity.
The results showed that PPFD levels of ≈ 900 µmol·m-2·s-1 resulted in “compact, robust plants that are commercially relevant.” There was a three-fold increase in biomass dry weight from the lowest to highest light intensity, but 90% of this increase occurred at 900 µmol·m-2·s-1.
On the other hand, PPFD levels of ≈ 600 µmol·m-2·s-1 resulted in more open plant morphology with increased airflow (and potentially reduced foliar pests) in otherwise compact genotypes. Maximum height and growth index were achieved at this level, possibly ideal for “open plant architecture and greater energy conservation.”
There was no obvious sign of transplant shock or light stress, even under the highest light intensities. Generally, higher light intensities produced plants that were shorter, denser, and more compact with more potential sites for flowers.
The researchers observed that PPFD levels ranging between 600 and 900 µmol·m-2·s-1 were ideal for vegetative cannabis. However, they noted that this can be affected by genotype and production scenario. 
Free-Photos via Pixabay
- Arnold JM. Energy consumption and environmental impacts associated with Cannabis cultivation. [master’s thesis] Arcata, CA: Humboldt State University; 2013. Cited by 9 (Semantic Scholar)
- Moher M, Llewellyn D, Jones M, Zheng Y. High light intensities can be used to grow healthy and robust cannabis plants during the vegetative stage of indoor production. 2021. doi:10.20944/preprints202104.0417.v1. Cited by n/a