Question: how do flowers know when to wither in winter and when to grow back in spring?

Isabel Webb answered on 12 Mar 2014:

Plants can tell the season by both the length of the day/night, and the temperature.
Plants can sense the length of the day and the night, and so can tell what time of year it is (days are longer in the summer and shorter in the winter). They use compounds called ‘phytochromes’ which determine the amount of light and then send signals around the plant, telling it to either grow more leaves, or let them wither and die.

Plants also sense the cold – not just the temperature, but also how much cold there has been. Plants will lie ‘dormant’ (when their leaves have fallen and they conserve their energy in the winter) until they have had the correct number of cold days to know that spring is coming. They then send signals around the plant telling it to start growing back ready for the return of the warm weather.

Plants also use the day length to determine when to start flowering, which is why different plants flower at specific times of year. Some plants flower in the summer – “long-day plants”, and some flower in the winter – “short-day plants”.

It is important for scientists and farmers to understand what causes these changes, so that we know when to plant our crops and plants for research, to be able to get the healthiest plants we can!