Originally written by Sirius J for High Times
Indoor grow manuals always mention the importance of using reflectors to help take advantage of every ray of light your growlamp puts out, but most amateur growers don’t realize exactly how much this really matters. Using some simple measurements and mathematical approximations, HIGH TIMES has determined roughly how much light you can save with some cleverly hung reflectors.
Cannabis ferociously devours light on a daily basis making it one of the most important limiting factors for yield. Simply put, the more abundant and better your light source is, the higher your yield. Without needing to hang more electricity-guzzling, heat-generating lamps in your garden, good reflector placement can increase light intensity in the canopy by up to 60%.
What, may you ask, do we use to measure light intensity for the purposes of plant growth? Photosynthetic photon flux, or PPF, is the sum of all the photosynthetically-active photons between wavelengths 400 nm (blue) and 700 nm (red), which roughly coincides with the spectrum of visible light humans can see. PPF is measured in units of micromoles of photons per second, μmol/s. Photosynthetic photon flux density, PPFD, is a measurement of the density of the photon flux falling over a given area and has units of μmol/s/m2. The daily light integral (DLI) is the sum of all the photosynthetically-active photons a given area receives in a day, and has units of moles/day/m2.
For the purpose of measuring plant-usable light that covers your garden’s canopy, PPFD is unit of choice. On the other hand, DLI is a tool that botanists use to compare the light requirements of different plants, and its what greenhouse growers measure to assess how much supplemental lighting they may need on shady days. For example, orchids only require a modest 8 moles/day/m2 for high quality growth, while tomatoes need to bask in up to 30 moles/day/m2. In the cold, dark month of December, Northern California experiences 10-15 moles/day/m2, but in June, this number can go as high as 55 moles/day/m2. Check out this publication for DLI maps of the USA, and this publication for a diagram of the DLI requirements of various common greenhouse plants.
If we knew what the DLI requirement was for cannabis we’d tell you; we’d shout it on the mountain tops from Malibu to Morocco, but the fact is nobody knows for sure. For all intents and purposes, it takes around 25 moles/day/m2 to yield decent harvests, but the more light the better. If a…