Direction and stem shape
Phototropism
Blue light tells the plant which way to grow and to track the sun
Stout stems
Red and blue favor short, thick stems
Stretch
Far-red promotes elongation
Shade cue
Extra far-red under a leafy canopy or trees lengthens internodes, especially on equatorial genetics
Red and far-red on flowering
Stop signal
Red near 680 nm can hold plants out of flower
Go signal
Far-red near 730 nm promotes flowering when red is absent
Night flash
A burst of red near 660-666 nm during a long night resets the flower clock unless far-red follows
Clock jump
Strong far-red at the start of night can shorten the dark need by about two hours
UV bands
UVA
About 320-400 nm. Helps reverse some UVB DNA damage, drives anthocyanins and other flavonoids, and can add to photosynthesis and phototropism
UVB
About 290-320 nm. Linked in the book to higher THC as dose rises; strongest outdoors early summer and a fraction of that by late September
UVC
About 200-290 nm. Ozone blocks most solar UVC. Germicidal lamps use it; it is harmful to people and animals
Atmosphere
Ozone strips the highest-energy solar photons below about 290 nm
Human-visible range reminder
Near-visible span
Roughly 200 nm (UVC) to 800 nm (far-red)
Human vision
About 400 nm (blue-violet) to 730 nm (far-red), peaking near 550 nm green
Plant priority
PAR 400-700 nm still does the bulk of growth work
Do not chase UVB unsafely
A broken metal-halide outer jacket dumps dangerous UVB and deadly UVC. Unplug it. Do not strip the glass to chase potency