LED grow lights explained in plain English

What matters is how many particles of usable light hit the leaves each second. “Usable” traditionally means photosynthetically active radiation (PAR), defined as 400 to 700 nanometers. Plants also respond to wavelengths outside that band, so a PAR spec isn’t the whole story of how plants respond to light.

Four terms you’ll see

• PPFD: how much usable light lands on one square meter each second. Look for this number at a specific hanging height.
• PPF: total usable light the fixture puts out.
• Efficacy (µmol/J): how much light you get per unit of electricity. Higher is better. Good modern LEDs are around 2.5 or higher.
• Daily light integral: total light across the day. More hours at the same intensity means more total light.

What research found

• Under LEDs, flower yield rose in proportion to light intensity between 600 and 1,000 µmol/m²/s.
• A separate trial in the CBD-dominant cultivar Stillwater tested up to 1,800 µmol/m²/s. Flower yield and apical flower density rose, with no cannabinoid-concentration response. This isn’t a recommendation to run a home canopy at that intensity.
• In medical-hemp trials at matched PPFD, increasing blue photons from 4 to 20 percent reduced flower yield by about 12 percent. The researchers found fixture efficiency and purchase cost mattered more for return on investment than that spectrum difference. That doesn’t mean all plants should get as little blue light as possible.

What to look for when buying

1. A published PPFD map at your tent size and hanging height.
2. A dimmer.
3. Efficacy around 2.5 or better.
4. Full-spectrum white light, which also lets you see leaf color accurately.

The efficacy figures above are shopping benchmarks, not a current market survey or a performance guarantee. Check independent test data, coverage, electrical certification and the manufacturer’s installation clearances.