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How much light do I need?

The previous post offered an introduction to PAR and PPFD. The question is how much is enough light and when is it too much. Supplying your plant canopy with an even and adequate amount of usable light is vital, but supplying your plants with more than they need can be a waste of energy and money. This can also lead to negative impacts on your plants.

All plants are different but can be placed into categories based on light needs per day. This is known as Daily Light Integral (DLI). DLI is the amount of PAR received each day as a function of light intensity and duration. This is known as moles per square meter per day (µmol/m2/d). A direct correlation between how much PAR your plants are exposed to and for how long each day. Generally, the vegetative stage of plant growth requires 16 – 24 hours of light per day, while the flowering stage requires up to 12 hours of light per day. Below is a general breakdown of how much PPFD is needed for plants to thrive during the different stages of growth.

Sustainable VegProduction VegFlowering Plant
50 – 200 µmol/m2/s150 – 400 µmol/m2/s 400 – 1 000 µmol/m2/s

These numbers vary based on type of plant and environmental variables like the introduction of carbon dioxide into the environment. 500 – 1000
µmol/m2/s has been noted as an optimum for growth. Less than 500
µmol/m2/s and growth will be slow. Plants require additional CO₂ to utilise more than 1 000 µmol/m2/s.

How much light per plant

Chandra et. al. concludes that peak growth of Cannabis Sativa takes place at 1 500 µmol/m2/s at 30ºC when supplementing with CO₂. Growth steadily decreases with more light or at higher temperatures, resulting in wasted energy.

References in order of appearance:

  • Next Light White Paper – “Helping Navigate the Ever Changing Grow Light Market”
  • Fluence Bioengineering White Paper – “High PPFD Cultivation Guide”
  • Suman Chandra, Hemant Lata, Ikhlas A. Khan and Mahmoud A. Elsohly – “Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions”