A recurring series focusing on plant cultivation by university researchers

Cannabis is a short-day plant, similar to poinsettias and chrysanthemums. Consequently, it requires a night cycle long enough to induce flowering. The critical night length, or the amount of time required for the photoperiod response in Cannabis to occur in order to stimulate flowering, is between nine and 10 hours, which corresponds to 14 to 15 hours of daylight, but it varies by cultivar. To induce Cannabis flowering in greenhouse production, plants receive 12 hours of darkness.

For clonal propagation (vegetative cuttings for rooting), cultivators must grow mother stock plants in the fall and winter to ensure there are enough cuttings for them in the spring and summer, or they must grow them indoors under artificial light. This presents a unique challenge in that the lack of natural light the plants are subjected to during the fall and winter would cause flowering. As a result, growers need supplemental lighting to reach a day-length extension of at least 16 hours or to perform night interruption to keep mother plants vegetative for cuttings. This range of external lighting should be adequate, though cultivars may have lower or higher lighting needs.

Keeping plants vegetative can be accomplished in two different ways (Fig. 1): The first is by extending the hours of light plants receive, giving plants 14 to 15 hours of light continuously. Ideally plants should have more than 16 hours of light to be on the safe side; and in fact, most cannabis operations provide 18 hours of continuous light. This method adds hours of daylight as the sun sets or slightly before. The second method is “night interruption.” This method adds a period of lighting in the middle of the night, breaking up the night period to prevent plants from flowering.

Night interruption is a common practice in floriculture production to retain the vegetative state of mother stock plants. However, we could not find research reports that address the suitability of using night-interruption lighting for retaining the vegetative state of Cannabis. Therefore, researchers at North Carolina State University conducted a study that compared 16-hour day-extension lighting to a truncated night-interruption lighting of 12+4 hours of lighting, and with 12-hour short-day lighting on the effects of flowering and vegetative growth of Cannabis. Here are the results:

Figure 1: This diagram outlines the different lighting regimens utilized. The first regimen provided 12 hours of light and 12 hours of darkness, which resulted in flowering plants. The second treatment provided 16 hours of light and eight hours of darkness through daylight extension, and this kept the plants vegetative. The final treatment also provided 16 hours of light, but the supplemental lighting was provided after a period of darkness. The final treatment also resulted in plants that remained vegetative.
Diagram by Paul Cockson

How we conducted the experiment

Figure 2: View of Cannabis plants in the growth chambers at the North Carolina State University Phytotron
Photo by Brian Whipker

We transplanted rooted cuttings of CBD-type BaOx, Cherry Wine, Endurance, Stout and T1 cultivars into 6-inch standard pots and placed them in North Carolina State University’s Phytotron growing chambers, facilities for growing plants in strictly controlled environmental combinations and specifically designed to study the response of plants to their environments (Fig. 2). Three photoperiod treatments were provided: short days of 12 hours of lighting; a truncated night interruption of 12 hours of light during the day and four hours of supplemental lighting in the middle of the night; and, day-length extension of 16 hours of continuous lighting.

We collected data during the eight-week period to explore the impact the lighting had on inhibiting flowering. Researchers collected data on the date the first flower part appeared on the short-day plants and included plant height, diameter and shoot dry weight. The team analyzed the data to determine statistically significant differences among photoperiods and cultivars.

What we found

As expected, flowering occurred with the Cannabis plants grown under short-day conditions (Fig. 3). The date of the first visible flower varied by cultivar, with T1 being the quickest, followed by Stout, Endurance, BaOx and finally, Cherry Wine being last (seven days later than T1).

Flower development didn’t occur on plants grown with 16 hours of lighting provided by either night interruption or day-length extension. These results provide conclusive evidence that both lighting strategies (night interruption or day-length extension) inhibit the long-night induction requirement of Cannabis to flower.

Figure 3: Growth characteristics of Cannabis (A) Endurance and (B) Stout cultivars grown with a 12-hour short-day (left), a truncated 12-hour day and a four-hour night interruption lighting (middle) and 16-hour day-extension (right)
Photos by Brian Whipker

In addition to flowering control, we observed an unexpected impact between the two supplemental treatments related to plant growth. Plants grown with night interruption were 15.4% shorter and 8.6% smaller in diameter than day-length extension plants. However, the total plant dry weights were similar. Differences in growth may be attributed to the variation of growing temperatures used in the study of 72 degrees Fahrenheit from hours 6 to 22, and 64 degrees from hours 22 to 6. With the night-interruption treatment, the additional four hours of lighting occurred with cooler temperatures and potentially resulted in less cell elongation.

Currently, no plant growth regulators are registered for use on Cannabis, so more compact and robust plants produced with night interruption could be beneficial to growers to help maximize yield per square foot. Smaller plants have the advantage of allowing a higher pot density to be grown in the same unit area of greenhouse space, which can help increase profitability.

Conclusions

Growing Cannabis with 16 hours of lighting in a 24-hour period provided by either day-length extension or night interruption was effective in retaining the vegetative state of mother stock plants. Growers can use either lighting strategy to produce mother stock plants. They also can apply either lighting strategy in order to bulk up plants vegetatively prior to flower initiation. A full report of the study can be found in the journal Crop, Forage and Turfgrass Management: bit.ly/cannabis-night-interruption-NCSU-study.

Brian Whipker1,2, Paul Cockson1, Turner Smith1: 1Department of Horticultural Science, North Carolina State University, Raleigh, N.C.; 2Floriculture Extension and Research