Flowering is controlled by a plant’s natural response to light called photoperiodism. This complex process of interacting factors plays a pivotal role in reproduction, fruit and grain production, etc. Indeed, the entire world’s food supply relies on photoperiodism as much as it relies on the Sun rising every morning. Despite it’s fundamental importance to Life on Earth, the science behind this phenomenon remains mostly a mystery.
Scientists don’t know everything there is to know about how changes in day length induce flowering in plants, but the bits and pieces we do know have helped gardeners grow better pot more efficiently for decades. Don’t keep yourself in the dark; this added knowledge could help to understand your plants fundamentally and help you troubleshoot, or avoid altogether, any issues you might encounter while growing cannabis.
When it comes to photoperiodism, cannabis falls into the category of short day plants (SDPs), since cannabis initiates flowering with short days at the end of the summer and beginning of fall. Other plants initiate flowering with the coming of longer days after winter during the spring, and are known as long day plants (LDPs). In both cases, a mysterious hormone dubbed florigen induces the flowering response when the plant’s natural circadian rhythm coincides with the length of the day, or night.
Short day and long day plants differ in they way they flower, but the mechanisms of their circadian rhythm have some similarities. LDPs use the transition between night and day as their reference point, but scientists think SDPs use the transition between day and night as their reference. In either case, plants measure the length of the night, not the day.
This means a cannabis flowering cycle can be thrown off, or even reversed, if the plant experiences a flash of light at night. On the other hand, a bout of darkness during the day does not affect the plant’s circadian rhythm and will not throw off flowering. Dim lights at night, like that of a full moon, won’t throw off flowering in small amounts. Plants are mainly sensitive to red and blue light, and some growers use narrow-band green LED’s to work in their gardens when the growlamps are off.
Flowering happens when the day length coincides with the internal circadian rhythm of the plant. The plant marks time with an internal clock that sets its rhythm on a 24 schedule, no matter the season of the year or if it experiences constant conditions. That said, plants that use the photoperiod to regulate their flowering, like cannabis, are more sensitive to a disruption in their internal clock if grown under 24-hour lights.
Not only does constant lighting use exorbitant amounts of electricity, some research shows that plants can only absorb a certain amount of light every day. The daily light integral (DLI) sums all the light a plant receives in a day, and is a useful measurement for growers that want to optimize growth and electricity usage.
A cheap spectrophotometer can give you a decent estimate on your DLI. Some anecdotal evidence points to 25 mol-1m-2day-1 (25 moles of photons per square meter per day) as the “saturation point” for cannabis. After the saturation point has been reached, the plant doesn’t require further light except to maintain its lighting schedule. Greenhouse growers use the DLI to determine if they need supplemental lighting to assure high yields, but many indoor growers haven’t tapped into the full energy-saving potential of switching to low-wattage lights after reaching the saturation point.
With some clever lamp arrangements, you could save up to 60% on the operating cost of lighting an indoor garden. A better understanding of cannabis biology helps growers every step of the way; stay put for more information from HIGH TIMES on optimizing light usage in your garden.