The industry and gardeners tend to interchange the two terms using the term PAR when they should be saying PPFD. I tend to say grow light hanging height is not a perfect science. Partial sun plants need a minimum of 150 to 200 mol/m²/s. We see that the PPFD values fall from 719 (highest) to 409 (lowest) even within the center 1×1’ square. Full sun plants need a minimum of 400 to 600 mol/m²/s. You will have trouble finding a PPFD value for most lights. PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”. PPFD stands for ‘Photosynthetic Photon Flux Density.’ PPFD is a measurement of light intensity, and it says how many light photons that fall within the PAR-range (expressed in micromoles) hit a given square meter per second: μmol m(-2)s(-1). So the logical solution to fix your problem is to determine the grow light height where your plants receive the ideal amount of light intensity for that phase of growth: 200-250 micromoles for seedlings and 300 to 400 micromoles of PPFD for vegetative phase plants. This list is not very accurate, but it’s still very helpful. A photon is the most basic and smallest unit of light. Here is a brief list of the PPFD (in mol/m²/s) requirements for plants: 1. The amount of light is actually measured as PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated to PFD. Even this has a problem. As you explore this world and unpack the different metrics and grow lights and systems that you need for optimum yields and healthy, flavoursome plants, you will hit a wall of acronyms. Full shade and other low-light plants need around 100 mol/m²/s. At the 2×2’ square we see PPFD values as low as 142 umol/m2/s. PPFD is a better way to measure the quantity of light for LED grow lights, than lumens. 2. https://bioslighting.com/horticulture-lighting/led-grow-light-distance-chart Just like humans count their energy needs in calories, PPFD or PAR is a similar measurement for plants. The PPFD value in the center, 719 umol/m2/s, is high enough to stimulate photosynthesis in flowering weed plants, but the light coverage area is rather poor. Many “tomato” (cannabis) growers run their plants at a PPFD of about 500-800 umol/m 2 /s (which is too bright for most orchids/houseplants). Your goal with the majority of tropical houseplants is to target about 20-40% filtered sun (removing 60-80% of direct sun intensity like leaves and clouds would in nature). 3. Some houseplants do well in a shady corner while other plants, especially fruit-bearing plants like cannabis, peppers, and tomatoes need a lot of light to grow to their full potential. 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