Vegetative or generative: Growth strategies for cannabis

Mohyuddin Mirza
June 11, 2018
By Mohyuddin Mirza
Picture of a good vegetative growth in cannabis. Good branching, good internode stretch and nice leaf size are evident. The cuttings taken at this stage will root nicely. This plant is ready to go for generative or flowering state by giving 18 hours of light.
Picture of a good vegetative growth in cannabis. Good branching, good internode stretch and nice leaf size are evident. The cuttings taken at this stage will root nicely. This plant is ready to go for generative or flowering state by giving 18 hours of light.
Commercial horticultural crops have basically two stages of growth. The vegetative growth phase is where roots, shoots and branches are produced. This growth is needed for the plant to establish itself and have enough leaf area to carry out adequate photosynthesis.
Once the plant has established itself, it enters a generative growth phase, where the growth hardens off and plant produces flower buds. In plants like lettuce, leafy greens and transplants, we need the vegetative growth to dominate and for plants to stay in that stage. On other plants like cucumbers, tomatoes and peppers, we want the plant to be vegetative and generative at the same time.

In cannabis, we want the plant to stay distinctly vegetative in its early stages, and then generative in later stages. We do not want the plant to change direction of growth too soon or reverse from generative to vegetative state.

Strategies for growth

Vegetative
Light is ON for 18 hours. Duration of light or photoperiod is the biggest factor in keeping the cannabis plants vegetative. The picture of the plant in the beginning of this article shows how the photoperiod of eight hours made the plant generative, and by exposing the plant to more than 12 hours of light induced vegetative growth.

Keep the growing medium moist, consistently moist. Sometimes growers are worried with root diseases like Pythium root and stem rot and they go for a wet and dry irrigation regimes. This creates a water stress signal and plant will start showing signs of generative growth. Leaves will turn dark green and size will get smaller.

Make sure there is no nutrient stress on plants. Since we are applying water with nutrient solution on a regular basis, the chance of a deficiency is minimal

Cannabis plants are very sensitive to electrical conductivity (EC) values of over 2.5 mS. Higher EC values give a generative signal to plants. This can be observed by early development of trichomes on leaves.

Maintain carbon dioxide level of around 800 ppm. Over 1,000 ppm gives a generative signal to plants. There is a misunderstanding about the use of supplemental lights for cannabis. Higher levels are not better. Based on what we have learned from greenhouse vegetable crops, higher than 1,000 ppm causes the stomata to reduce in size and therefore affecting the transpiration. This is evident when top leaves are significantly smaller, compared to lower leaves, and are also thicker. Also, remember the way carbon dioxide is added into the air. On your computer the set point is 800 ppm. When the CO2 sensor senses the level has reached at the set point, the CO2 is shut off. Plant uses the CO2 and the levels drop to 600 ppm and it is injected again. Thus, on a graph it will look like “waves” not just a very straight line.

Use a nutrient program in which nitrogen to potassium ratio is 1:1, although I have seen growers using 1.2 to 1.0 as well. I prefer to use 1:1.5 N:K ratio so that growth is not soft. If you are growing cannabis in hot climates, adding small amount of ammonium nitrogen, less than 25 ppm, is beneficial for vegetative growth. Plants will grow perfectly if all the nitrogen is in nitrate (NO3) form.

Keep the VPD more close to 2 to 4 grams/m3 of air for vegetative growth.

Keeping mother plants/stock plants constantly in vegetative state is a challenge because the plants get root bound and that is a generative signal. The side shoots are used for rooting and they can be woody. Higher EC, over 4 mS have been found in mother plants and that will trigger early bud set. Keep these plants vegetative by using the strategies listed above. In addition, do not keep these plants for more than 60 days.  

Generative
The biggest trigger for inducing generative growth is reducing the photoperiod to 12 hours – that is 12 hours of dark period and 12 hours of light. Proper use of blackout curtains is essential with no light leakage.

The supporting strategies include:
  • Constant mild stress on the plants by using drier water regime
  • Using a “generative action” vapor pressure deficit. The normal range for generative action is around 5 to 7 grams/m3 of air.
  • Avoid climate conditions that promote powdery mildew.
  • Use higher potash to nitrogen ratio, closer to 2 to 1.
There are many strategies to keep cannabis plants vegetative or generative, and proper understanding of the effects of these strategies is important in decision-making.


Dr. Mohyuddin Mirza ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it )  is an industry consultant with more than 37 years of experience in crop development, production and marketing. He specializes in the technical aspects of hydroponics and systems for plant production.
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