How do plants know when to flower?

This web page quickly summarises the key reasons why plants flower when they do. Click on the links for more details.
  • Plants have to be 'mature' to flower. There are three phases a plant goes through; [a] juvenile, [b] adult vegetative and [c] adult reproductive. Plants have to be in the adult reproductive phase to flower. Juvenility can be as short as a few days or up to several years, depending on the species. Juvenility is a key factor which influences flowering.
  • Sometimes flowers can revert from adult reproductive to juvenile phase, and this is called floral reversion. However, this is rare in nature.
  • Many plants use day length (or more correctly night length ) to measure the time of year and therefore determine when to flower. This is called photoperiodism. Plants are classified into two broad groups; short-day plants (SDPs) and long-day plants (LDPs). Flowering in SDPs is triggered by increasing night length and flowering in LDPs is triggered by a reduction in night length. Once the night length is above or below a critical point, then flowering will commence. Plants measure night length using a chemical called phytochrome which is found in the leaves of plants.
  • Some plants are dependant on temperature to stimulate flowering. Plants don't measure day and night temperatures to determine the time of year (and thus whether they will flower) because this can be very inaccurate, especially when there is a warm spell or cold snap. However some plants need to be exposed to a sustained period of cold in winter in order to flower in spring or summer. This requirement for a cold treatment is called vernalisation.
  • Some plants will flower a few weeks after germination regardless of the weather or other environmental stimuli. This is called autonomous flowering and is common in species which have a short lifecycle, such as annuals (including ephemeral species). Most plants are not autonomous flowerers.
  • There are other factors which will also influence whether a plant flowers, such as genetics and internal hormonal levels. These are often affected by the above-listed environmental influences. Some genes promote flowering and some inhibit it. Hormones are chemicals which control the growth of plant cells, and these also have both promoting and inhibitory effects.
  • Because flowering is such a complex process, scientists are still trying to understand all the mechanisms which influence when and why a plant flowers when it does.
  • Potassium does not stimulate flowering in most species.
This website simply summarises some of the knowledge about floral initiation and development.