Every LED manufacturer claims their spectrum is "optimized for cannabis." Most are selling you a color temperature and calling it science. The reality is more interesting — and more useful. Different wavelengths of light trigger different plant responses: stem elongation, leaf expansion, branching, flowering speed, trichome density, and terpene production are all influenced by the light spectrum reaching the canopy.
Understanding the photobiology doesn't mean you need to buy expensive multi-channel LEDs. It means you understand WHY certain lighting choices work, and you can make smarter decisions with whatever light you have.
How plants see light
The photosynthesis action spectrum
Plants don't use all wavelengths equally for photosynthesis. The photosynthetic action spectrum — how efficiently each wavelength drives photosynthesis — peaks at two regions:
Blue peak (~430–450 nm): absorbed primarily by chlorophyll a and chlorophyll b. Drives photosynthesis efficiently but also triggers morphological responses (compact growth, thick leaves).
Red peak (~640–660 nm): the most photosynthetically efficient wavelength range. Red photons drive more photosynthesis per unit of energy than any other color. This is why most grow LEDs are heavy on red diodes.
Green (500–565 nm): contrary to the common claim that "plants reflect green light," green photons DO drive photosynthesis — just less efficiently than red or blue at the leaf surface. However, green light penetrates deeper into the canopy and through leaf layers. In dense cannabis canopies, green light reaches lower bud sites that red and blue can't. Modern research shows green may contribute 10–20% of total canopy photosynthesis in dense plantings.
Beyond photosynthesis: photomorphogenesis
Light doesn't just feed the plant. It shapes it. Different wavelengths activate different photoreceptor proteins that control how the plant grows:
| Photoreceptor | Wavelength | Effect |
|---|---|---|
| Phytochrome (Pr/Pfr) | Red (660nm) / Far-red (730nm) | Flowering, stem elongation, shade response |
| Cryptochrome | Blue (400–500nm) | Compact growth, stomatal opening, circadian rhythm |
| Phototropin | Blue (400–500nm) | Phototropism (growing toward light), chloroplast movement |
| UVR8 | UV-B (280–315nm) | Stress defense, trichome production, flavonoid synthesis |
The most important of these for cannabis growers is the phytochrome system.
The phytochrome system and flowering
How phytochrome controls flowering
Cannabis is a short-day plant — it flowers when nights are long enough. But it's not actually measuring darkness. It's measuring the ratio of two forms of phytochrome:
Pr (phytochrome red): the inactive form. Absorbs red light (660 nm) and converts to Pfr.
Pfr (phytochrome far-red): the active form. Absorbs far-red light (730 nm) and converts back to Pr. Pfr also slowly reverts to Pr in darkness (thermal reversion).
The trigger: when Pfr drops below a threshold ratio during the dark period (because it slowly converts back to Pr without light input), the plant receives the signal to flower. This is why even a brief flash of light during the dark period can disrupt flowering — it reconverts Pr back to Pfr, resetting the clock.