Light deprivation is the removal of light from a plant to trigger flowering. Because cannabis is an annual plant, it relies on light levels and photoperiod to determine when it is time to switch into flowering mode and produce flower buds. Typically cannabis switches into flowering mode (in outdoor grows) during Fall, as the length of day starts to shorten. In commercial greenhouses and outdoor grows, however, it is more efficient to purposely deprive cannabis of light to trick the plant into flowering than simply waiting for Fall to roll around every year. Light deprivation allows growers to harvest cannabis once every 2-3 months instead of once a year. Special materials, often called “blackout” materials, are sold to block as much sun light as possible. Indoor grows, which already control how much light their systems put out, can simply switch the lights off.