Can plants tell time?

Most of the Northern Hemisphere recently undertook the practice of “falling back” or changing all the clocks to represent the loss of one hour, in order to make better use of natural daylight. But this process of daylight savings also affects the time of no light or light deprivation. Usually when the term deprivation is used, the end result is usually not one of ideal growth, but this is not always the case in terms of a plant’s performance and its flowering cycles. Light, and the lack of it, definitively has a direct and profound effect on the success of a plant's ability to flower and bud.

Plants can indeed sense time, and they have an inherent awareness to where they are in the day’s cycle. Plants achieve this because of something called circadian rhythms – defined by Merriam-Webster as the “inherent cycle of about 24 hours that appears to control various biological processes, such as sleep, wakefulness, and digestive activity”.

A plant’s phytochromes naturally sense not only the amount of light a plant receives, but also the tiny differences in light wavelengths. This fine tuned sensibility allows a plant to match its chemical reaction to its environment.

Just how does light affect plants? Well, it’s one of the most complex and discussed questions in the world of plant study. The four major aspects of plant growth most greatly affected by light are: Photosynthesis, Chlorophyll Synthesis, Photoperiod and Phototropism.

The term photoperiod refers to the length of day for a plant. All plants need several hours of light, as well as darkness each day, and the photoperiod is essential to the developmental response of the plant. There are different varieties of plants, which can be classified as night-long, short-night and night neutral plants.

Plants that bloom when they are exposed to darker periods, such as poinsettias, gardenias, Christmas cactus and Marijuana (Cannabis), will not bloom unless the nights are longer or equal to the days. These night-long plants are very light sensitive, and as little as a minute of light during the night could be enough to prevent them from entering the blooming or flowering stage. In general most night-long plants need about 11-13 hours of light per day to flower.

For example, if you live in the Northern U.S., you might find that a Christmas cactus next to a window blooms too early. You can add a light above the cactus and use a timer to keep it on for an hour or two after sunset each day in the early fall. Then gradually taper off the extra light to encourage the plant to start blooming. You will need to experiment to determine how much extra light to provide each day and when to start reducing it for your part of the country.

Many cannabis growers induce flowering by setting their grow lights on a timer to an even twelve hours on and twelve hours off. Additionally, they make sure the twelve-off period is support by the use of blackout systems, light deprivation curtains or blackout end walls. This is to make sure that there is absolute darkness and no light seepage, so that the plant does not remain in a vegetative state.

There are also short night plants, such as carnations grains, vegetables and annuals. These short night plants need about 14-18 hours of light per day in order to flower.

Then there are other plants that bloom regardless of the photoperiod length, so these are called night neutral. These species include plants such as roses, carnations, geraniums and household plants, like African violets.

By installing an inexpensive automatic timer on your grow lights, you can make sure that the plants receive the amount of light that they need and that they bloom when you want them to.

Usually, the best guide for your plants’ optimal lighting is the plant itself. Look for signs of too much sun, which include yellowing leaves or burn spots, or too little sunlight, which could include leggy growth. Ultimately, by determining your plants’ ideal light needs, you will be rewarded with blooming success.

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