Active Member
Cannabis Fan

Hey everyone!  I learned a great deal about lighting from this website and various Bruce Bugbee videos.  I've read through the articles in the guide numerous times, but they all really just point you back to a calculator to make sure your lights are efficient for your grow space- not necessarily how to dial them in once you're using these specific measurements.

I've read that cannabis needs a minimum of 22 DLI with a max of 65 (although this level requires large amounts of CO2).  In the Optimal PPFD for cannabis section it states that the optimal levels without CO2 are between 500 PPFD (32.4 DLI @ 18/6) and 700 PPFD (45.36 DLI @ 18/6).  I cannot find the answer to when you can really utilize your lights and start hitting these targets without damaging plants.  Does anyone have a rule of thumb?  Maybe once you see x amount of fingers on a leaf?


Seedling Stage:  DLI Recommendation? (I've heard 10-20)

Early Veg:  ??

Vegetation Stage: 32-45?  

Flowering Stage:  45?


"The Optimal PPFD for Cannabis:

The data from Chandra et al. confirm that the optimal photon density for peak cannabis photosynthesis is between 500 and 700 µmol/m2 (PPFD). It also shows that we should avoid going over 1000 µmol/m2 (PPFD) which could lead to damage. With artificial lighting, the distribution of light is never perfect. Therefore, we want to ensure that all areas of the canopy get at least 500 µmol/m2 (PPFD) and that no spot receives more than 1000 µmol/m2 (PPFD). We recommend an average of 700 µmol/m2 (PPFD). With most grow lights, an average of 700 will ensure that you stay within the optimal range for peak photosynthesis in all regions of the canopy." (

Posted : 03/04/2021 6:29 am
Active Member
Cannabis Fan

Ah...  I'm noticing the data in the Optimal PPFD for Cannabis section doesn't specify what light cycle the optimal PPFD is referring to.  Are they recommending 500-700 PPFD on a 12/12?  It's been my understanding that DLI is the only factor that matters:

[(500 PPFD)(3600 seconds)(Lights On Hours, ie 18)]/1,000,000 which gives a DLI of 32.4


Dr Photon, can you help me please lol

Posted : 03/04/2021 6:50 am
Active Member
Cannabis Fan

I believe I found the answer in Dr Photons corner in the comments..  It appears Dr Photon found that DLI doesn't matter, just PPFD.  See below for anyone searching for the answer:


"Theres no missing data. The main vegetation and flowering light values are the same. As such the two are combined under the heading "main vegetation and flowering" where a recommendation of 700-1000umols is recommended. 

So you would be looking at

Seedlings and clones: 60-90umols

Early vegetation: 250-350umols

Main vegetation and flowering: 700-1000umols

There are no progressive steps after mature vegetation. Once leaves have developed to a mature age during main vegetation, the photosynthetic capacity does not change afterwards. Although they do age and so total photosynthesis declines over time, this has nothing to do with photosynthetic efficiency which is constant from the time the leaf switches from a sink to a source and is mature enough for full light intensity." (

Posted : 03/04/2021 7:27 am
Dr Photon
Member Moderator
Expert Grower
Completed Grows

Thanks for relaying the information heather, its good to see that it gets to the right people.

So yes, DLI has for the longest time has been misunderstood and been a contradicting metric that really, only applies to greenhouse applications.

The amount of light a plant can recieve, differs depending on certain conditions and so alters the DLI that a plant can recieve. For example, depending on the light cycle used. With 18/6, 20/4 and 24/0 being the most popular. All limit and change the amount of total DLI that can be provided. This is compounded even further during flower between photoperiods and autoflowers where autos can exceed the limit that photoperiods has and be allowed more DLI as a result.

Then there is the photosynthetic capacity of leaves, which during ambient co2 concentrations limits the photosynthetic rate to around 700 - 1000umols. But with co2 supplimentation this can be exceeded again and be allowed to provide even more light to plants and so increase the DLI.

Because of these variables and the nature of the photosynthetic pathways of cannabis leaves. DLI really has no purpose and benefit in our practice of cannabis cultivation. What is important and should be used by everyone, is the light intensity seen by leaves. Which dictates optimal photosynthetic efficiency and light absorbed by plants.

DLI has always been a parameter used by greenhouse cultivation, where the sun as the light source fluctuates and is monitored to determine whether supplimental lighting needs to be used. A vital aspect of greenhouse growing where DLI holds a purpose and significant benefit. Not like with indoor cultivation, where light systems are static and controlled.

Being wrong, is an opportunity for getting things right.
Posted : 03/04/2021 3:17 pm Chefomj liked