Plant science

The Effect of Night Length on Plants

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”.

Red maple
The red maple (Acer rubrum) changes colour and sheds its leaves early in the fall as the nights get longer. Photo: James St. John, Wikimedia Commons.

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.

The goldenrod only flowers when the nights are longer than the days. Photo: Botsci, Wikimedia Commons.

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.

African violet
The African violet is said to have “neutral days”: its flowering is not affected by the length of the day or night. Photo: Vladimir2366fa, Wikimedia Commons.

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).

Christmas cactus (Schlumbergera)
The length of the night is carefully controlled so that the Christmas cactus blooms exactly at Christmas (Schlumbergera). Photo: Peter coxhead, Wikimedia Commons.

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.

poinsettia (Euphorbia pulcherimma)
The poinsettia (Euphorbia pulcherimma) is VERY sensitive to the length of the night. Photo: TANAKA Juuyoh,

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!

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

4 comments on “The Effect of Night Length on Plants

  1. Tropical plants should be less responsive to photoperiodism. They likely are. However, even they seem to know when they do not get as much time in the sunlight as they should. They like greenhouses that keep them warm at night, even in the dark.

  2. I had no idea! This probably answers a question I’d always had about Four o’clocks and how they knew when it was 4:00! Ha! Great information!

  3. Every plant and animal evolved with a night/day cycle, with a whole nocturnal ecology evolving to exist at night. That whole nocturnal ecology supports the day ecology. For example, there are 12,500 species of moths that exist in North America that all have larval forms as caterpillars.

    Ninety six percent of our songbirds absolutely require nice soft, juicy, protein packed caterpillars to feed their young and continue their species. Light pollution should be controlled and minimized, lighting only the areas we need. If we want to disrupt our own natural light cycle, we should pull shades and only be turning on lights outside when absolutely needed. Light at night has a negative impact on human health as well. Some kinds of lights are worse than others. A yellow light attracts far fewer moths. Controlling light pollution saves money too!

    When I went to using outdoor light only when expecting visitors, coming and going, or suspecting unauthorized activity outside, and closing shades at night, plus not raking every leaf (only where absolutely necessary), the fireflies returned, and I now have 100s of species of amazing and beautiful moths. I don’t have to get in the car and spend money to go bird watching any more. There is, of course more to my effort than that, such as using many native plants, but controlling light at night is key.

    • Liz Hathway

      This has me wondering about some of my “habits” I live on the north shore of Lake Ontario and have been disappointed in the declining number of butterflies and birds the past few years. The back of my house is almost all large (24′) windows with no shades to obstruct the view or the passive solar function. Now I’m wondering if the evening light from these windows is affecting the insect and bird population.

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