Blue photons do about 20% less photosynthetic work than red photons at 660 nm. Green photons trail red by up to 10%. Kusuma, Pattison and Bugbee measured both gaps, and that short ranking is the honest basis for naming a worst light color for plants. No cited source picks one out. The differences are real, but they are small, and they show up in a lab long before they show up on your windowsill.
So the green light plants get is not going to waste. Green beats blue per photon, which flips the old classroom lesson that leaves bounce green back and use none of it. There is a second claim that sounds like the same one and is not. UNH Extension says growth runs best under a red and blue mix with less yellow and green in it. That ranks a whole recipe. The 10% figure ranks single photons. Keep the two apart when you read a spectrum claim, and note that nobody has published a number for a yellow photon at all.
The diodes tell a different story than the photons do. A blue LED runs at 93% efficiency, ahead of red at 81%, far red at 77% and phosphor converted white at 76%. So the color that does the least work on a leaf is the most efficient to produce. Fixture makers work inside that tradeoff, which is why the spectrum you buy is a compromise rather than a best case.
Color does jobs that have nothing to do with raw output. Blue steers chlorophyll production, holds back stems that stretch too far, and controls the stomata on the leaf surface. Red drives photosynthesis and pushes flowering and fruiting. Far red earns a place too. A healthy plant wants red, far red and blue all supplied, so a spectrum judged on photon efficiency alone would leave out things the plant needs. Strip a color out and you lose the work it was doing.
Do not shop for a fixture that strips green out. The label hides the mix anyway. A Kelvin rating gives the approximate color of a lamp and not the wavelengths in it, so 4000K on a box is a sketch rather than a spec. Two lamps with the same number can hold different amounts of green and red inside them. If the recipe matters that much to you, ask the maker for a spectrum chart. The rest of the box is marketing.
Total light delivered beats spectrum tuning. Add hours on the timer or move the fixture closer before you pay extra for a custom color mix.
The red and blue light plants use is already sitting inside a plain white LED, and the amount you deliver matters more than the ratio. A 10% to 20% gap between photon colors sounds big on paper. You wipe it out by running the lamp a few hours longer or by dropping the panel closer to the canopy. Both fixes are free. Distance is the better lever of the two, since a panel parked far from the leaves hands them less light.
Blue lands at the bottom of the ranking, so it is the closest thing to an answer that anyone can point to. The gap runs 20% per photon. Blue also keeps your stems short and your stomata working, which is why no sensible spectrum leaves it out. Most plants that grow poorly are short on hours and parked too far from your lamp.
Read the full article: Grow Lights for Indoor Plants: What Really Works
