Since the dawn of time, farmers have understood the role of light in plant growth; it wasn’t until the beginning of the twentieth century that we began to understand the importance of darkness. In 1913, the French graduate student Julien Tournois discovered that hops and hemp grown under glass would flower precociously in winter. He also observed that the plants would flower most rapidly when allowed only six hours of daylight[1].

Tournois’s research ended when he died on the front during World War I, but a few years later two American scientists, Wrightman Garner and Harry Allard, unwittingly expanded upon Tournois’ findings. Wrightman and Allard discovered that certain plants bud more readily when they sense a change in seasons, or rather: Certain plants will begin to bud when they sense a change in the ratio of daylight hours to nighttime hours. Garner and Allard immediately saw the implications for agriculture. They began experimenting on a range of plant species and discovered that day length influences many aspects of plant activity, including dormancy, flowering, and potential yield[2]. In 1920 they noted: “under the influence of a suitable length of day, precocious flowering and fruiting may be induced[3].”

Garner and Allard invented a word to describe a plant’s sensitivity to day length: Photoperiodism. Photoperiodism is a biological response to a shift in the proportions of light and dark in a 24-hour cycle. Photoperiodic plants measure
hours of darkness in order to keep track of the seasons and thus flower at an appropriate time of year.

The two scientists began classifying plants as long-day plants (LDP), day-neutral plants (DNP), and short-day plants (SDP). Day-neutral plants can flower at any time of year, depending on other conditions. Long-day plants flower naturally in high summer, when the nights are shortest. Short-day plants flower naturally when the nights are long: either in early spring or in late summer and early autumn. Short-day species include chrysanthemums, poinsettias, cosmos, globe amaranth, rice, hyacinth bean, and some varieties of marigold, orchid, and strawberry; as well as and a number of other high-value specialty crops.

Short-day is actually something of a misnomer: short-day plants sense darkness, not light. When sensors in your plant’s leaves indicate that each 24-hour cycle includes 12 or more hours of sustained, uninterrupted darkness, your plant’s apical meristems (growing tips) will shift priorities: instead of producing more leaves and stems, the plant will begin to produce floral structure.

In Photoperiodism in Plants, Thomas and Vince-Prue expand upon the concept: “Perhaps the most useful proposal is that of Hillman (1969), who defined photoperiodism as a response to the timing of light and darkness. Implicit in this definition is that total light energy, above a threshold level, is relatively unimportant, as is the relative lengths of the light and dark period. What is important is the timing of the light and dark periods, or, to think of it another way, the times at which the transition between light and dark take place.”

Biologist P.J. Lumsden also emphasized the importance of precise timing, noting: “…photoperiodic responses require a time-measuring mechanism, to which is closely coupled a photoperception system. Further, the time-keeping mechanism must operate very precisely and it must be insensitive to unpredictable variations in the

In other words: absolute darkness is not necessary to trigger a photoperiodic response in SDP, but consistency of dark-to-light ratios is essential. During a 1938 experiment on the effects of light on xanthium, Karl Hamner and James Bonner discovered that the benefits of a long night could be reduced or abolished if the darkness was interrupted for even a few minutes[4]. The converse was not true: the flowering process was not reversed when the daylight hours were interrupted with

Growers of SDP crops have been using light deprivation research to their advantage for decades. For example, poinsettia farmers use automated greenhouses to ensure that plants bloom for the Christmas season. More recently, light deprivation technology has caught on in other specialty gardening industries.

Light deprivation is an ideal method for farmers who want to bring a crop to market before the market floods during the harvest season. The method also allows farmers to avoid potential rain damage by harvesting when weather conditions are ideal. Perhaps more importantly, light deprivation offers the opportunity to plant and harvest twice during one growing season and thereby double annual yield.

To utilize light dep, farmers plant crops in hoop houses or greenhouses, which are covered with opaque material for a period of time each morning or evening. The goal is to block sunlight and increase the number of hours the crop spends in darkness: more than 12 hours of darkness will stimulate flower growth in most SDP plants. The challenge is to keep the schedule consistent and to ensure that the darkness is not interrupted, either by unseen rips in the covering, shifts in the covering caused by wind, or human error. As Hamner and Bonner demonstrated, interruptions or inconsistencies in the light deprivation cycle can confuse the plant and slow flower growth.

Many light dep farmers still work manually, a less than ideal situation. Hiring workers to pull tarps leaves ample room for imprecision in timing, not to mention the high labor costs of paying two or more employees to spend several hours a day arduously tarping and untarping hoop houses.

At Harvest Excel, we are changing the marketplace. Automated greenhouses minimize human error and scheduling problems to allow for precise timing and an easy, streamlined process that prevents rents and other forms of light leakage. By minimizing effort, farmers are maximizing yield. We create affordable and excellent light dep technology that will allow you to take full advantage of a photoperiodic crop’s potential.

[1]“Photoperiodism in Plants” by P.J.
Lumsden; Biological Rhythms; Narosa
Publishing, 2002.

in Plants by Brian Thomas and Daphne Vince-Prue; Academic Press, 1997.

[3]“Effect of the Relative Length of
Day and Night and Other Factors on Growth and Reproduction in Plants” by W.W.
Garner and H.A. Allard; Journal of
Agricultural Research, 1920

of Growth and Development in Xanthium by Roman
Maksymowych; Cambridge University Press, 1990