When your goal is growing marijuana perfectly, you keep records of plant performance, inputs, and grow room conditions, trying to make sense of differential performance between cannabis plants and strains.

We’ve seen individual marijuana phenotypes and even individual plants in a group of clones or seedlings respond differently to how much light they get per second and per light cycle. They also respond differently to light spectrum.

There are many problems that come from using light incorrectly, and many benefits from using it properly:

  • Giving too much or too little light intensity to seedlings and new clones can permanently stunt or otherwise sabotage structural development, including root growth, leading to poor crop outcomes. The same is true for established grow and bloom phase plants.
  • Intense light can drive crops to higher yields, or it can harm them so yields drop.
  • The total amount of light plants receive per day, which is partially but not exclusively determined by the number of hours grow lights are on, has big impacts on marijuana plant growth rate and profitability.
  • Light pollution during the bloom phase dark cycle can severely disrupt plant performance.
  • When you use grow lights with a spectrum tuned specifically for cannabis, and properly administer light, benefits include shorter internodes, faster root and above-ground growth, thicker stems, larger buds, faster growth, increased intake of water and nutrients, increased resistance to disease, stress and pests.

Fortunately, we’ve talked to the world’s most statured scientist studying how light affects cannabis plants: Dr. Bruce Bugbee, an esteemed professor at Utah State University.

Beginning years ago, Dr. Bugbee thoroughly studied how light affects plants and then created Apogee Instruments, a legendary company that makes premium horticultural light meters and other high-tech gear for growers.

Lately, Dr. Bugbee has been alerting growers to the fact that although cannabis responds to light intensity per second, it also responds to total light per day.

Light intensity per second is measured as “micromoles of light” (µmol) per square meter per second.

There’s another important measurement—daily light integral (DLI)—which is the moles of light per square meter per day.  

DLI is the total sum of photosynthetic lighting intensity measured as “PPFD” over a 24 hour period. Photosynthetic Photon Flux Density (PPFD) is the amount of biologically-active light radiation delivered as micromole-per-square-meter-per second (μmol/m2/s).

Apogee research, and our grow room experiments at Growing Marijuana Perfectly, show there is no one-size-fits-all formula for grow-light-to-canopy-distance, PPFD intensity, light spectrum, or DLI.

One crucial factor is that not all light affects cannabis metabolism and photosynthesis. Not all light is PPFD light, and some light wavelengths harm marijuana plants.

Only light in the range of 400-750 nanometers (nm) has the power to beneficially affect basic plant metabolism and function.

This is an expanded nanometer range, derived in part from Dr. Bugbee’s recent research showing that the old range of 400-700 is too narrow.

Dr. Bugbee’s decades of scientific discovery and meter innovations has taken us a long way from the days when growers used lux or camera meters to measure light coming from old-school metal halide and high pressure sodium bulbs that used to be the dominant type of grow light.

Thanks to his research and proprietary meters, we precisely measure photosynthetically active radiation and test how cannabis plants and strains respond to it.

Apogee’s Extended Photosynthetically Active Radiation, (ePAR) sensors give you high-accuracy total photon flux intensity measurements in the 400-750 nm ePAR range.

Their DLI meters use sophisticated algorithms and data logging to show how much total light your plants get per day.

Apogee also makes light pollution meters, which marijuana growers use to discover nearly-invisible and more obvious light leaks that subtly but harmfully affect bloom phase performance.

Many growers don’t realize how crucial the dark phase length and blackout ratio is for ensuring a successful bloom phase, or how small amount of light it takes to upset bio-hormonal rhythms that govern budding and resin development.

Light leaks create stress that can trigger hermie foliar structures, which may pollinate an entire crop overnight.

“Our Apogee DLI meters give you data as detailed as .1 hours of light that the plant has received at that spot during the 24 hour day,” explains Apogee technical expert Chris M. “These meters don’t continuously make measurements. They take a reading every three minutes and photoperiod hours of light data is accurate down to 6 minutes. In other words, 8.4 hours would be 8 hours 24 minutes of light during the day. These meters are very sensitive, detecting any light greater than .01 µmols.”

When you use an Apogee ePAR meter, you can precisely give your plants the recommended PPFD amount for marijuana growth phases:

  • Seedlings and new clones can handle 100-300 PPFD; their light-handling capability is strain and phenotype specific.
  • Established grow phase plants can handle 300-500 PPFD. PPFD amount has to be gradually increased as plants mature.
  • Bloom phase plants can handle 450-630 PPFD in a grow room with ambient natural C02 concentrations, and can handle 630-1100 PPFD in grow rooms with sufficient added C02. This would be C02 added so the grow room has about 1000 parts per million C02 in its air.
  • Maximum PPFD intensity only works well when plants are in an ideal grow room environment and being fed high quality nutrition.

Using Apogee DLI meters, you can accurately measure and dose the total light your plants get each day. In a grow room without added C02…

  • Seedlings should start in week one with 12 DLI. Week two is 16. Week three is 19.
  • Established grow phase plants need 30 DLI in week four, and 43 in week five.
  • When you flip your indoor marijuana plants to bloom, DLI should be dropped to 30 for week six, then gradually increased with 35 for week seven, 40 for week eight, and 45-55 for weeks after that.
  • Note: these are approximate DLI doses. Your plants’ genetics, phenotypes, grow room conditions and inputs all affect how much DLI is optimum in seedling/clone, grow, and bloom phases.
  • In a grow room with added C02, DLI can be increased by about 10-15%, as long as you closely monitor your plants and adjust all other inputs to match the C02’s metabolic effects.
  • Careful monitoring of growth rate, bud development, scent, leaf health, water and nutrients usage, and other plant health indicators is necessary to maximize DLI and PPFD.
  • Greenhouse and outdoor marijuana growers also benefit from managing DLI.
  • Using precise record-keeping, you discover significant differences in light-related plant performance and strain performance
  • Your goal is to find the sweet spot of PPFD and DLI so every watt of grow light electricity you use creates benefits for your marijuana plants, and you don’t use excess electricity that wastes your money and creates extra grow room heat.

Please watch the videos embedded in this article and check embedded article links so you see how important it is to fully understand and manage PPFD, DLI, and to ensure 100% total darkness during lights-off phase.

We haven’t spent time in this article dealing with the big impact grow light spectrum has on plants, but we’ve only found one grow light manufacturer who legitimately tested spectrum and intensity effects on cannabis plants and designed their LED array to deliver exactly the right ratios and amounts of various light wavelengths for maximal growth, potency, and yield.

As always, the marijuana growing community is indebted to Dr. Bugbee, the world’s leading scientist, educator, and grow op equipment manufacturer studying how light affects marijuana plants.

His technology is not cheap, but it is absolutely exclusive, easy to use, and reliable. We’ve used his meters several times per day in indoor and outdoor settings for years, and never once had a glitch. Your plants are worth the investment of procuring Apogee meters and other devices.

Dr. Bugbee’s Apogee team are very bright, professional, helpful people, so reach out to them to see what Apogee gear will best improve harvest weight, potency, and overall success of your cannabis growing seasons.