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If you live far above or below the equator, then you experience changes in the length of the day and the night as we go through a year-long period. Depending on the latitude you live at, then you also may have a growing season that enjoys longer days and a dormant or winter season where the nights are longer and considerably chillier. 

Photoperiodism in plants is when plants use the length of light and darkness to regulate flowering and other processes. Photoperiodism acts without any input from the ambient temperature or the weather. It is strictly a light/dark kind of thing. However, photoperiodism is dependent on the ability of the plant to sense light. Plants have photoreceptors or cells that contain a light-absorbing pigment. When triggered these cells will alter specific proteins within the plant which cause changes in hormone production, gene expression and in growth. What this means is photoperiodism is a response of plants to light exposure that then causes change within the plant.

Some plants are labelled as short-day plants and these plants will bloom when the length of daylight drops below a critical threshold, typically in the late summer or autumn. A good example of a short-day plant that you may be familiar with are fall blooming mums. If you live in a cooler climate, then it is often difficult to see your fall blooming mums (chrysanthemums) show colour before we have temperatures below freezing. Thus, the morden mums were developed to have a trigger to bloom that was related to light but also to temperature thus causing this change in the proteins within the plant prior to freeze-up. More indoor plants which are also considered short-day plants include the Christmas cactus and the poinsettia.

Long-day plants will flower only when the amount of daylight lasts longer than their critical threshold, which is often earlier in the growing season when our day length becomes longer. Common plants that are considered long-day plants include lettuce, spinach and radishes. So, next time you see your favourite salad greens bolting to flowers, it may be due to more than just the temperature!

Although we classify plants as short- or long-day plants, there are some short-day plants that actually measure the length of the night that consists of continuous darkness in order to trigger the bloom. Thus, many of our short-day plants will flower when the day is short and the night is long, but will not flower if the long night is interrupted by a brief period of light. A good example of this characteristic is the poinsettia which will not trigger the formation of the bloom if the darkness is interrupted.

When looking at long-day plants, some will measure the length of the night, and require that dark period to be shorter than or equal to a critical length of darkness. Those long-day plants measuring the night period are considered to be “dark-dominant” as it is the dark period that is important to flowering. Many of the long-day plants will instead measure the length of the day, rather than the night and these plants are considered to be “light-dominant.”

Just to muddy the waters a bit more, we also have day-neutral plants that are not regulated by day length at all. These plants bloom when they are mature enough to do so. Plants that are day-neutral include cucumbers, tomatoes and corn. One other item to think about is that plants are complex. Many outdoor plants use a variety of triggers to prepare for the end of the growing season and the ensuing winter deep freeze. Tuber formation in potatoes and trees that are in the process of preparing for winter are also mainly controlled by the length of daylight but other factors will also play a role.

Hanbidge is the lead horticulturist with Orchid Horticulture. Find us at ; by email at [email protected]; on facebook @orchidhort and on instagram at #orchidhort.

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