Larry Hodgson has published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings available to the public. This article was originally published in Le Soleil on November 13, 1993.
No one can deny that the nights get quite long in November, and the days consequently much shorter. What is surprising, however, is that plants also know this… and that this difference in the length of nights from one season to the next governs a good part of their lives.
In horticulture, the distribution of the length of day and night is called “photoperiod” and the response of plants to this phenomenon is called “photoperiodism”.
Contrary to what one might imagine, it is not the length of the days that primarily affects plants, but rather the length of the nights. For example, in Quebec, the red maple (Acer rubrum) changes colour and loses its leaves early in the fall, in September, because the nights lengthen rapidly at that time. The tree “recognizes” in the prolonged nights that it is time to prepare for dormancy. This same species also grows in Florida, but only loses its leaves in December. The difference between day and night is less marked near the Tropic of Cancer.
European Trees Keep Their Leaves Longer
In Europe, horticulturists complain that North American trees lose their leaves very early, whereas their own trees keep their leaves until November. This is because the North American climate, for the same latitude, is colder. Our trees have therefore learned, over millions of years of evolution, to lose their leaves as soon as the nights start to get longer. European trees, on the other hand, know that they can afford to keep their leaves on until the increase in night length is more pronounced.
Photoperiodism and Flowering
It is not only the preparation for winter that is governed by the photoperiod, but also the flowering. Indeed, some plants are said to be “long day”. In reality, it is to short nights that they react, but since the terminology is already fixed… They only bloom in summer, when the day is longer than the night. Many of our summer flowering annuals and perennials fall into this category.
Other plants are called “short day” plants: they only flower when the nights are longer than the days. Our autumn flowering plants all fall into this category: goldenrods, asters, chrysanthemums, etc. If they were grown in an artificial situation where the night would always last 12 hours or more, they would flower.
Finally, other plants are called “day-neutral”. Their flowering is not affected by the length of day or night. The African violet, for example, or the begonia semperflorens, bloom in summer as well as in winter. (If neutral day plants often bloom a little more in summer than in winter, it is because there is more solar energy available in summer… And more energy means more flowers: the length of the day does not directly influence this increase).
Outsmarting Mother Nature
Humans have learned to take advantage of photoperiodism to induce flowering at the right time. For example, when the Christmas cactus (Schlumbergera) is grown in our climate, it tends to bloom in October/November and then a second time in February/March. It reacts more to moderately long nights than to very long ones. But in the greenhouse, the length of the night is strictly controlled so that it blooms exactly at Christmas.
Photoperiodism also explains why the short-day poinsettia (Euphorbia pulcherimma) is so difficult to rebloom in the house. It is so sensitive to the length of the nights that even cutting its night short by turning on a lamp once during the flowering period can abort the process. In greenhouse production, they make sure no one turns on the lights between late afternoon and dawn. Once flowering is well underway, however, the poinsettia becomes indifferent to the length of the day. You can therefore buy it and place it in a lighted room at night without fear of losing its charming bracts.
It is by carefully controlling the length of the day that nurserymen manage to supply us with pots of Chrysanthemums all year round. If this normally autumn-blooming plant is given nights longer than 12 hours, it will flower in any season, even in summer.
Photoperiodism is a complex but fascinating aspect of gardening!