Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

see differs from the light plants use to grow. Plants use the photosynthetically active response (PAR) portion of the spectrum. Humans use the central portion of the spectrum, while plants are able to use large portions of the spectrum not measured by light meters that record foot-candles, lux, and lumens.
    Light is also measured in spectrum with Kelvin temperature which expresses the exact color a bulb emits. Bulbs with a Kelvin temperature from 3000 to 6500 are best for growing marijuana. The PAR section explains that plants use specific portions of the spectrum–a complete range from blues to reds. Lamps with a spectrum similar to PAR-rated bulbs can use Kelvin temperature of a bulb to ascertain the approximate PAR rating of the lamp. Color spectrum results from a specific mix of different colors. High intensity discharge bulbs are very similar in spectrum. Making these safe assumptions, a rough PAR rating could be extrapolated from a Kelvin temperature rating.
    The Color Corrected Temperature (CCT) of a bulb is the peak Kelvin temperature at which the colors in a bulb are stable. We can classify bulbs by their CCT rating which tells us the overall color of the light emitted. It does not tell us the concentration of the combination of colors emitted. Companies use a Color Rendering Index (CRI). The higher the CRI, the better the bulb is for growing.
Light Meters
    Most commercial light meters measure light in foot-candles or lux. Both scales measure light to which the human eye reacts to “see.” They do not measure photosynthetic response to light in PAR watts.
    Light measurements in this book are made in foot-candles and lux. This information is still valuable, because it records the amount of light spread over a specific surface. The information is then coupled with the PAR rating of different bulbs. Regardless of the lamp, the amount of light emitted is constant. It only makes sense to use the proper reflective hood with a high PAR-rated bulb to grow the best garden.

    After all the talk about PAR watts, industry officials are unable to agree on a common scale of measurement. For this reason, we have decided to rely on Kelvin color temperature to measure lamp spectrum.
Photoperiod
    The photoperiod is the relationship between the duration of the light period and dark period. Most strains of marijuana will stay in the vegetative growth stage as long as an 18 to 24-hour light and a 6 to 0-hour dark photoperiod are maintained. However, there are exceptions. Eighteen hours of light per day will give marijuana all the light it needs to sustain vegetative growth.
    Flowering is most efficiently induced with 12 hours of uninterrupted darkness in a 24-hour photoperiod. When plants are at least two months old–after they have developed sexual characteristics–altering the photoperiod to an even 12 hours, day and night, will induce visible signs of flowering in one to three weeks. Older plants tend to show signs of flowering sooner. Varieties originating in the tropics generally mature later. The 12-hour photoperiod represents the classic equinox and is the optimum daylight-to-dark relationship for flowering in cannabis.
    Research has proven that less than 12 hours of light will not induce flowering any faster and reduces flower formation and yield. More than 12 hours of light often prolongs flowering. Some growers have achieved higher yields by inducing flowering via the 12-hour photoperiod, then changing to 13-14 hours of light after two to four weeks. However, flowering is often prolonged. I spoke with growers who increase light by one hour two to three weeks after flowering is induced. They say the yield increases about 10 percent. Flowering takes about a week longer, and different varieties respond differently.
    A relationship exists between photoperiod response and genetics. We can make generalizations about this relationship, because little scientific evidence documents the extent to which specific strains of cannabis are affected by photoperiod. For example, sativa -dominant plants that originated in the tropics respond to long days better than indica- dominant plants. On the equator, days and nights are almost the same length year-round. Plants tend to bloom when they are chronologically ready, after completing the vegetative growth stage. However, most growers are familiar with the pure sativa strain, ‘Haze’, which flowers slowly for three months or longer, even when given a 12-hour photoperiod. You can