Ask Ed: Questions and Answers – Light Deprivation, How to Ditch the Tarp

Question: Dear Ed,

First, let me thank you for your informative book. About your section on photoperiodism: I am flowering out a spring crop in California in a greenhouse. The hours are 12/12 now and I was wondering your thoughts on the use of 730nm far-red light instead of pulling a tarp. Is this even possible?


– Geoff

Ed’s Rosenthal: Dear Geoff,

The short answer to your question is yes.

The chromaprotein that plants use, phytochrome, to determine flowering is a “dimer.” It has two versions, an “on” and an “off” version which are switched on and off by light. When subject to red light the dimer stays in the “off” position. When red light (660-680 nm) ceases, gradually over a 2 hour period it turns to its active state, which forces the plants into flowering.

Far red light (730nm) has the opposite effect. When plants are exposed to it in darkness the dimer immediately becomes active, or “on.” If a far-red light is shined over the plants immediately after dusk or the lights are turned out it saves 2 hours of dark time that the dimer would spend in transition. If the plants are receiving 9 hours of darkness and 15 hours of light outdoors, and the far-red light is shined on the plants immediately after dusk, it is like adding 2 hours to the dark period, giving the plants the equivalent of 11 hours of darkness, long enough to force flowering.

Far-red LEDs can be special ordered, this will not work unless the light is truly “far-red” or 730nm.

For more, check out Ed’s Marijuana Grower’s Handbook.

Send your grow questions to Ed Rosenthal at [email protected].