Light is food for your marijuana plants, and the “feed program” for light is way more complicated than the feed program for nutrients fed through roots.

You can read this longer, more detailed article to get more info on two major concepts you must understand and manage to get the most from your marijuana growing.

The first concept is photosynthetic photon flux density (PPFD). The second is daily light integral (DLI).

PPFD is a measure of the intensity and density of photons hitting your plants per second. DLI is the measure of the total amount of photons per lights-on cycle.

If you give your plants too much or too little PPFD and/or DLI, it harms growth and productivity.

Too little PPFD and DLI creates slow-growing, fragile plants that will not yield large, potent harvests.

Too much PPFD and DLI creates stressed plants overdosed on light, and they too will not give you the largest, most potent harvests possible.

To properly feed light, you need grow lights that have the right light spectrum for cannabis. Most don’t, especially old-school dinosaur technology like high pressure sodium and metal halide.

This means you could be providing apparently sufficient amounts of PPFD and DLI, but because the lights are not delivering a comprehensive range of photonic nutrition from different light wavelengths, your plants suffer.

We’ve tested all the major and minor brand of grow lights and found that only one manufacturer has an ideal spectrum for marijuana. This is the Austrian LED grow lights manufacturer SANlight.

SANlight makes speciality lights with spectrum ideal for starting seeds and clones. And after seeds and clones are established, SANlight full-season grow lights provide all the photons your plants need from start-to-finish.

You should use a PPFD meter to accurately manage your light feeding. By far, the best we’ve found come from Apogee, founded by a scientist who probably knows more about how light affects cannabis than any other scientist in the universe.

Apogee makes several reliable meters, including some that calculate DLI in real time. When you have accurate PPFD readings, you can also use online and app DLI calculators to determine total DLI.

But none of this helps you unless you know DLI and PPFD target numbers. These numbers are generic and adapative, meaning their usefulness is modified by several grow op realities, including:

  • What strains you’re growing.
  • Whether you add C02 to the grow room or not.
  • How healthy your plants are.
  • What phase of growth your plants are in.
  • How well you keep VPD within ideal range.
  • Grow room temperature and humidity.

PPFD intensity should be:

70-200 for clones and early seedlings.

200-450 for well-rooted grow phase plants.

450-1200 for bloom phase plants.

DLI should be:

3-11 for clones and seedlings.

11-22 for established grow phase plants.

22-50 for bloom phase plants.

As noted previously, these ranges are generic. You must carefully observe plants, grow op conditions, and light levels to select the right PPFD and DLI.

For example, if you give plants too much PPFD, it can cause leaf bleaching. If you’re adding C02 to grow room air during lights-on, you can deliver higher PPFD and DLI—researchers adding C02 to achieve ambient C02 air concentration of 1000 or more ppm got massive yields and potency increases using as high as 1500 PPFD during late bloom phase.

One crucial key is to GRADUALLY increase light delivery as your plants get older. This is similar to how you increase nutrients dosage as plants age.

If you too-quickly add light intensity, it can shock plants.

Another factor is that when you go from 18 hours of grow phase light per day to 12 hours bloom phase light per day, increase PPFD to ensure plants are still getting target DLI—because you just lost six hours of light delivery.

Of note is that a sudden shift from 18 to 12 hours of light per day may shock plants and lead to problems such as hermaphroditism. When you are about a week away from starting bloom phase, you should decrease your light hours by about 50 minutes per day, so the light hours gradually taper to 12 rather than suddenly going from 18 to 12.

You want to be expert at observing your plants’ growth rate and structure and analyzing what their appearance and growth rate means.

For example, if grow phase plants are stretching, it’s often because PPFD is too low. If they’re stunting and/or leaves are burning, it could be that PPFD is too high.

If growth is slow, it could be caused by DLI being too low.

Of course, there are other factors that could be causing problems similar to those caused by incorrect light feeding, such as VPD out of range, inferior nutrients, grow op temperatures too high, inferior genetics, pests and diseases.

You could have a perfect grow op where every other factor is ideal, but photon feeding problems will work against growth rate, plant sturdiness, harvest weight, and potency.

We’re enclosing below a technical abstract of a scientific experiment measuring how light affects cannabis. New research is always adding to our understanding of how to feed photons to cannabis, and the following excerpt is very useful if you can understand all the abbreviations and measurement cues.

The bottom line is you could be doing everything else right in your grow room and still miss out on maximum rewards, just because of problems with photon feeding. Using the information in this article, you learn to avoid light-related problems and push your plants to deliver their heaviest, most potent harvests.

Now take a look at concise photon/cannabis science:

Effect of different photosynthetic photon flux densities (0, 500, 1000, 1500 and 2000 μmol m-2s-1), temperatures (20, 25, 30, 35 and 40°C) and CO2 concentrations (250, 350, 450, 550, 650 and 750 μmol mol-1) on gas and water vapor exchange characteristics of Cannabis sativa L. were studied to determine the suitable and efficient environmental conditions for its indoor mass cultivation for pharmaceutical uses. The rate of photosynthesis (PN) and water use efficiency (WUE) of Cannabis sativa increased with photosynthetic photon flux densities (PPFD) at the lower temperatures (20-25 oC). At 30°C, PN and WUE increased only up to 1500 μmol m-2s-1 PPFD and decreased at higher light levels. The maximum rate of photosynthesis (PN max) was observed at 30°C and under 1500 μmol m-2s-1 PPFD. The rate of transpiration (E) responded positively to increased PPFD and temperature up to the highest levels tested (2000 μmol m-2s-1 and 40 0C). Similar to E, leaf stomatal conductance (gs) also increased with PPFD irrespective of temperature. However, gs increased with temperature up to 30 oC only. Temperature above 30 oC had an adverse effect on gs in this species. Overall, high temperature and high PPFD showed an adverse effect on PN and WUE. A continuous decrease in intercellular CO2 concentration (Ci) and therefore, in the ratio of intercellular CO2 to ambient CO2 concentration (Ci/Ca) was observed with the increase in temperature and PPFD. However, the decrease was less pronounced at light intensities above 1500 μmol m-2s-1. In view of these results, temperature and light optima for photosynthesis was concluded to be at 25-30°C and ~1500 μmol m-2s-1 respectively. Furthermore, plants were also exposed to different concentrations of CO2 (250, 350, 450, 550, 650 and 750 μmol mol-1) under optimum PPFD and temperature conditions to assess their photosynthetic response. Rate of photosynthesis, WUE and Ci decreased by 50%, 53% and 10% respectively, and Ci/Ca, E and gs increased by 25%, 7% and 3% respectively when measurements were made at 250 μmol mol-1 as compared to ambient CO2 (350 μmol mol-1) level. Elevated CO2 concentration (750 μmol mol- 1) suppressed E and gs ~ 29% and 42% respectively, and stimulated PN, WUE and Ci by 50 %, 111 % and 115 % respectively as compared to ambient CO2 concentration. The study reveals that cannabis can be efficiently cultivated in the range of 25 to 30 oC and ~1500 μmol m-2s-1 PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2 concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO2 rich environment.