How do Plants know when to flower?

Spring has finally come, and everywhere you look flowers are beginning to bloom. But, how on earth do plants know its spring and that its time to start flowering?

Control over plant flowering time is extremely important. For example, insect pollinators are only around at certain times of the year. If a plant was to start flowering too soon, or late, pollination and the production of the next generation of plants wouldn’t happen.

To ensure flowering at the correct time, plants rely on a range of environmental factors which help to tell them when to start flowering including: food availability, light and temperature. These factors are all linked together in a complex network. This complex network also helps plants to not incorrectly respond to a single flowering environmental cue. For example, if a plant relied purely on temperature as a signal, it could end up flowering too soon- think of those odd ‘hot’ days in early winter, followed soon after by heavy frost or snow…

The question of how plants sense its time to flower is very complex, and is still being unravelled now. In this post I will talk very briefly about how daylight and temperature can affect plant flowering time, I will link a bunch of much more in-depth articles at the end if you would like to read into this more!


One of the most important factors controlling flowering time is the photoperiod- or the number of hours of light and darkness a plant is exposed to. Often it is thought that it is the number of hours of daylight which is important for the flowering of plants but it’s actually the length of darkness which is the most important!

A plant that needs a long period of darkness are called ‘short day’ plants. Many spring plants are short day plants, these require less than 12 hours of daylight a day, any more than this will stop flower formation.

Chrysanthemums are an example of short day plants (source:pixabay)

Long-day plants on the other hand will only start to flower once they start getting more than 12 hours of daylight a day. Most summer plants, and vegetables are long-day plants.

Strawberries, our favourite summer fruits, are day-neutral plants. (source: pixabay)

There are also so-called ‘day neutral’ plants which fall outside either category, meaning they flower regardless of day length probably relying on other environmental cues instead.

So, how do plants sense this change in night length? Many flowering plants use a photoreceptor molecule to sense the change in night length.

There are two types of photoreceptor molecules: Phytochromes which sense red/far-red light, and cryptochromes which sense blue/uva light.

The phytochromes allow plants to sense when it is day and night- via exposure to Red light, which is present in daylight. Phytochromes have two forms- the active red light absorbing form, and the inactive far-red light absorbing form. When the phytochrome is in its active form it promotes plant growth, whilst the inactive form prevents plant growth.

Diagram of how the photorecepto states links to flowering in short and long day plants. Pfr= phytochrome which absorbs far red light, and Pr= phytochrome which absorbs red light. (source:

Cryptochromes are another type of photoreceptor important for photoperiodism, linking the circadian rhythm with light. Circadian rhythm is when a biological process goes up and down within a 24 hr period.

These two receptors work together in the plant to help control plant response to daylight- this is a pretty complex process, and a lot of work is still being undertaken in this area to understand fully how these two photoreceptors control flower initiation.

Our knowledge of these photoreceptors, and the plants responses to red and blue light, is something we are able to exploit. We commonly manipulate light exposure of plants and the composition of unnatural light (i.e red/blue amounts in LED lighting) to stimulate blooming of plants out of their natural season this means we can get fruits, vegetables and flowers year round despite the changing day-night lengths.


Many plants have a sweet spot- a temperature at which their seeds germinate the best. Seeds set at a below optimal temperature (e.g too cold) will often not germinate. This means that plants need to take care when taking cues from temperature. The process of temperature sensing is a complex mechanism in plants- with plants sensing both air and soil temperature. The cell signalling pathway leading to flower development is often reliant upon the activation and deactivation of several molecules at the same time to ‘turn on’ the flowering pathway. This prevents flowers beginning to bloom prematurely for example in response to one hot day in early winter.

Many plants also use a process known as vernalisation for flowering. This is when a plant needs a period of prolonged cold before they are able to set seed and begin flowering.

This feature is something which may cause plants difficulty in the face of climate change. In the future we may well have milder winters, with a lack of long cold periods. As many plants require these cold periods for flowering in spring and summer, the absence of cold days may impact in the germination of plants. This is an especially important consideration with crop plants.

Vernalisation, and the impact climate on plant growth and flowering is an area of plant science which is currently being looked into in a lot of detail. Although we know quite a bit about how plants respond to temperature, there are still a lot of unknowns. If we are able to figure out these processes fully we may be able to help find ways to fully future proof our crops no matter what the seasons bring.

This is just a very quick introduction into plant flowering- make sure you check out some of the links below if you would like to find out even more about the complexities of flowering!
References – short article about the affect of Red light on plant growth. – in depth research paper about photoreceptors and regulation of flowering time – In depth article about vernalisation and flowering

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.