@Death The Cultivator @Delps8
I originally wrote this for the owner of my last commercial farm, and it was shared with a scientist from the University of California, Berkeley who also reviewed it, then shared out in multiple cannabis groups I moderate/admin for to get some ideas and feedback on (which the copy-and-paste here is more of an adaptation from where it went to the growers.)
The idea behind this wasn't to say "hey look at the guide I personally wrote," but instead "hey, there is a lot of information generally accepted to be true and here's the science behind the suggestions."
Would you guys be willing to look through this and see if you have anything to add/offer/critique to help make it more clear, or perhaps if some statements are too rigid, ways to properly explain without seeming too biased?
It's missing information about spectrum and individual effects, and I purposely didn't include some of the updated rhetoric from Bugbee (trying to have a better grasp myself.)
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Good morning growers!
Let's talk LIGHTING!
PAR, PPFD, DLI (oh my!)
I want to talk to you about some of the most important aspects of lighting, whether it's the sun or a grow light, and that is
intensity (how many actual photons are hitting the plant over a period of time) and
spectrum (the electromagnetic wavelengths of light produced by a light source to promote plant growth.)
In cultivation, this is defined by
PAR light.
PAR (which stands for
Photosynthetically Active Radiation) is light of wavelengths 400-700 nm and is the portion of the light spectrum utilized by plants for photosynthesis.
We quantify (or to put in a measurable form) PAR light using
PPFD (
Photosynthetic Photon Flux Density), which is defined as the photon flux density of PAR.
PPFD is measured in micro-mols per second (umol/m^2/s ), indicating how many photons in this spectral range fall on the plant each second.
You've likely seen maps of PPFD on descriptions of grow lights. These are generally square diagrams showing PPFD calculations of the light at different fixed heights (like 12" over the canopy, 24" over the canopy, etc) over an area (like 2 foot x 2 foot, 4 foot x 4 foot, etc.) These are very important, because it helps show the area of intensity coverage of your grow light. Many traditional grow lights are very intense in the center and tend to lose intensity towards the edges (much of this is in the design of the individual light.) This helps us plan our canopy space (and to make sure our plants on the edge of our space aren't being under lit.)
DLI (which stands for
Daily Light Integral,) is by definition the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period.
Instead of sticking to the old "status quo" of 18/6, 20/4, or 24/0 (traditional lighting schedules for autoflowers, or keeping photos in veg,) DLI can help us make more calculated determinations for how much light to shoot for in a 24 hour period.
There are some generally recommended guidelines for how much light intensity cannabis plants should have for optimal growth in different stages of their life (you will see some variation between these recommendations depending on the source, but they are generally very similar.)
IN SEEDLING, 100 TO 300 PPFD (6 TO 19 DLI)
IN CUTTINGS (CLONES,) 75-150 PPFD (5 TO 10 DLI)
IN VEGETATIVE, 300-600 PPFD (19 DLI TO 39 DLI)
IN FLOWER, 600+ PPFD (26+ DLI MINIMUM)
Cannabis in different from typical greenhouse plants in how it thrives under high-intensity light. It's one of the few plants which flourishes growth in full bloom with more than 900 to 1200 PPFD (38 to 52 DLI.)
However, as light intensity (PPFD) increases, photosynthetic rates also increase until a saturation point is reached. Every plant species has a light saturation point where photosynthetic levels plateau. Light saturation normally occurs when some other factor (normally CO2) is limited.
I've included a chart in the attached images that was produced by the engineers from Fluence Bioengineering, a very reputable lighting company in the cannabis industry, from their guide on Photobiology showing the percentage of relative photosynthesis based on average atmospheric CO2 levels in the air (around 400 ppm.) Photosynthesis plateaus in efficiency around 500 PPFD.
So without augmenting the CO2 levels in your grow environment, photosynthesis reaches a plateau at a certain light intensity threshold. This gives us a guideline for determining how much light is simply "too much light" based on the environment, as well as "how much is not enough." There is literally a point where you're just pushing photons against a plant that can withstand them, but not utilize them.
It's important to note that if temperature, humidity, CO2, nutrients, or even the moisture level of the grow medium are outside the optimum range for cannabis, these attribute to the limitations of photosynthesis.
OUTDOOR & GREENHOUSE LIGHTING
For outdoor growers, Jim Faust, a professor of Floriculture from Clemson University in South Carolina, developed a well known DLI map of the United States showing average DLI for every month of the year, which greenhouse and outdoor growers use as a guideline.
As an example outdoors (living in Colorado,) in the month of January we averaged around 15-20 DLI from the sun. By comparison, in June we'll see around 50-55 DLI average (which is the most light intense month of the year in the state.) Please note that these are outdoor values for DLI; there is light transmission loss between the the outdoor and indoors from the barriers the greenhouse material creates (which can be calculated.)
So in the winter months here, we're providing only about 1/3 of the actual light intensity the plants would normally receive during the summer months by relying on the sun only (which is why our plants get so much bigger in the summer vs winter months without supplemental lighting.)
This is also in part due to there being less light hours available during the day until winter solstice (the shortest day of the year in December.) In Denver in the month of January, the sunrise occurred around 7:20 am and will set around 4:53 pm. That's a total of 9 hours 33 minutes of lighting. Complicating matters further, during this period of lighting there are only around 4-5 hours of actual intense light (higher PPFD) that support optimal photosynthesis (intensity drops off rapidly in the afternoon until dusk.)
Conversely in the summer time (using June as our example,) the sun rises around 5:30 am and sets around 8:20 pm. That gives 14-15 hours of actual day lighting per day.
In august, the daylight hours drops to about 14, then 13 in September, to 12 hours in October (which helps correlate how photoperiods flower outdoor in our season.)
Photoperiod versus Autoflower
Photoperiods are considered a
short-day flowering plant.
Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. Photoperiod cannabis doesn't flower in response to the amount of light receives, but rather the amount of darkness it receives. So the amount of daylight hours versus darkness photoperiods receive, whether artificial or natural, is absolutely critical.
Autoflowers are considered
day-neutral flowering plants.
Day-neutral plants do not depend upon the amount of darkness or daylight hours, but instead flower out as a response to aging. As the plant matures, it begins to bloom. While genetics have the most influence on the timing of this, stress and environmental factors can have an impact on this mechanism.
Within these plants, there exists a further division that puts some plants into qualitative and quantitative groups that require specific conditions to perform certain biological tasks, or will still carry out tasks, but sub optimally until conditions are met.
Whether it's a photoperiod or autoflower, the type and quality of light you're giving the plant is very important during different stages of growth. I hope this helps explain things when you're looking at or considering grow lights, or maybe helps understand your own existing setups better!
Appreciated!
www.khanacademy.org
