How do plants tell the time?

By Sofia Miettinen

Have you ever noticed that snowdrops and daffodils appear in the spring, but other plants do not flower until later in the year? How can plants tell the time like this and know when to grow and start producing flowers?

Plants produce flowers to reproduce and make more plants. Plants decide when to flower by using both the amount of sunlight in the day and the temperature around them. As plants cannot move themselves to a better environment, they have to make sure that they choose the right time to flower so they can survive.


Plants have special sensor cells in their leaves called photoreceptors that allow the plant to see sunlight. Humans also have photoreceptors, and these photoreceptors are found at the back of the eye in the retina. Photoreceptors in both humans and plants are able to detect light and send this as a signal to other cells. Plants use their photoreceptors to be able to tell how much sunlight or darkness there is during the day and decide on what to do next.


Some plants like to grow and produce flowers when there is lots of sunlight during the day, and other plants prefer when there is lots of darkness. Long-day plants only produce flowers after they have been in the sun for longer than half of the day, and short-day plants flower when more than half of the day is darkness. This is called photoperiodism. Examples of long-day plants are poppies and carrots, and violets and sweet potatoes are examples of short-day plants. Some plants such as tomatoes and dandelions do not care how much sunlight or darkness there is and these are called day-neutral plants.


When spring comes, there is more sunlight during the day and the days get longer, and the photoreceptors in the leaves of the plant can sense this and start to produce special signals. Scientists have discovered many special signals such as CONSTANS and the Flowering Locus T. The signals move from the leaves of the plants up to the shoot and tells the plant it is time to start producing flowers. Long-day plants such as poppies will start to produce flowers in the springtime.


As well as by using sunlight, plants tell the time by detecting how cold it is around them before deciding to start to flower. Some plants such as snowdrops will only start to flower when there has been a long period of cold weather; this is called vernalisation. This stops the plants from producing flowers when it is the winter and tells the plant to wait till it is spring as spring is the perfect time for many plants to start producing flowers. In the spring there is lots of water, sunlight and warmth that plants need, and there are many bees flying around to help pollinate the flowers when they are produced. Some gardeners will even put their plants in the freezer to make the plant think that it is winter, so that the plant will flower earlier!


Now that you know plants flower at different times of the year, why don’t you have a look outside your window and see what you can see growing throughout the year!