1. Why Does Spinach Grown in Spring Suddenly Grow a Flower Stalk?
If you try to grow spinach from spring to early summer, the flower stalk may extend before the leaves fully develop. This phenomenon is called bolting. Agricultural research reports that because spinach flowers when days get longer, growers need varieties resistant to bolting for cultivation in this season [5].
Plants that flower when day length exceeds a certain threshold are called long-day plants. Besides spinach, wheat and lettuce are often cited as examples [3]. Some materials also list radish as an example of a long-day plant.
In contrast, short-day plants flower when days become shorter. Examples include chrysanthemums, poinsettias, Christmas cacti, soybeans, rice, and morning glories [3]. The difference in how they react to day length determines when their flower stalks rise or when they bloom.
But is this "day length" actually referring to the length of daylight?
2. Experiments Show Plants Measure the Length of Night, Not Day
In 1920, Garner and Allard from the United States Department of Agriculture reported an observation. When they grew a tobacco variety called Maryland Mammoth in fields north of Washington D.C., it did not flower during the growing season. Yet, it bloomed when grown in a greenhouse during winter [4]. They concluded that day length influenced this behavior and named the phenomenon photoperiodism (the term was suggested by W.W. Garner's colleague, W.C. Cook). Tobacco and soybeans were classified as plants that flower when days are short.
This observation showed the influence of day length, but later it became clear that night length is also crucial [2]. The name "short-day plant" remains based on daylight, even though darkness is the key factor.
A decisive experiment proves that night matters. Short-day plants may fail to flower if exposed to light for a few minutes in the middle of the night. For many short-day plants, what counts is the length of uninterrupted darkness [3]. Even if the total daylight hours remain the same, breaking the night into two parts means the plant does not experience a long enough night.
For long-day plants, flower buds form when the night becomes shorter than a specific limit [1]. Light that interrupts the night acts almost like a signal to long-day plants that "the night has become short."
Is this critical threshold length the same for all plants?
3. Each Plant Has a Different Critical Night Length, and Some Ignore Day Length
You often see explanations stating that plants bloom when days are "shorter than about 12 hours." This is a rough rule of thumb. The critical threshold differs depending on the species or variety [3].
For example, a summary from the Botanical Society of Japan compared two varieties of Chrysanthemum indicum that flower under short days. One variety, NIFS-3, has a critical day length (the limit) of about 12 hours, while another, Matsukawa, has a limit of about 13 hours [6]. Under conditions shorter than their respective limits, the activity of the gene CsFTL3, which is involved in flowering in leaves, increased in both varieties. There can be a difference of about 1 hour even within the same species.
Some short-day plants are even more sensitive. The morning glory variety "Murasaki" is said to set flower buds after being given just one long dark period. Morning glories are used as models for studying short-day plants.
On the other hand, some plants are hardly influenced by day length. Tomatoes, corn, cucumbers, and some strawberries are called day-neutral plants [3]. This shows that changes in day length are not a universal signal for all plants.
If leaves measure the "length of night," how does this information travel to the tip of the stem?
4. A Signal Travels From Leaves to Stem Tips in Both Long-Day and Short-Day Plants
While leaves measure the length of night, flowers form at the tips of stems. Scientists hypothesized that a signal must travel between the leaves and the stem tip, temporarily naming this substance "florigen."
In 2007, two separate research groups revealed the identity of one such signal in the same year. A team led by Corbesier showed in the long-day plant Arabidopsis that the FT protein is produced in the phloem (the nutrient transport pathway) of leaves and moves to the stem tip to promote flowering [7]. In Japan, a group led by Isao Shimamoto showed that florigen in the short-day plant rice is a protein encoded by the gene Hd3a (the rice version of FT). Their research indicated it is also produced in leaf vascular bundles and moves through stem vascular bundles to the tip [8].
Rice plants into which Hd3a was introduced flowered much earlier than usual. However, this result applies to genetically modified rice, not normal rice.
The genes that produce FT and Hd3a are homologous, meaning they share a common ancestor. Genes from the same lineage are used in both long-day and short-day plants. Moreover, some research explains that their reaction to day length is reversed. In Arabidopsis, FT activity gradually increases as days get longer. In contrast, rice Hd3a is thought to be controlled like a switch: it strengthens under short days and is suppressed under long days [9].
After reading this, you might wonder when the plants around your home bloom. The next section explains how to compare night length with flowering times.
5. How to Calculate Night Length and Observe Plants Near You
To calculate night length: Weather maps in forecasts, newspapers, or meteorological agency websites list sunrise and sunset times. You can find the length of night by subtracting the length of day from 24 hours (measuring from sunset to the next sunrise). Try writing this down weekly to see if nights are getting longer or shorter. You can compare this with when spinach or morning glories try to flower.
To observe local plants: Write down the date flowers appear in your garden, field, or potted plants. Do plants exposed to streetlights or house windows at night bloom differently from the same plants in dark places? Comparing them helps make the idea of "uninterrupted darkness" feel more real.
Place the dates when home or neighborhood plants bloomed on the same page as your record of night length. You may see whether they bloom when nights are getting longer or shorter. The Japanese summary for Chrysanthemum indicum [6] can also be read as a discussion about critical lengths.
Safety Note: Always conduct nighttime observations with an adult.