Understanding photoperiodism

Steve and Val Bradley explain how daylength plays its part in plant growth

- Steve & Val Bradley

THE trouble with us gardeners is that we tend to think of time in terms of hours, days and weeks. Plants have no capacity for thinking like that they have to respond to environmental factors and one of the most important is light. This allows 'plant time', where an inbuilt natural clock and calendar is regulated by the time of year and, more precisely, by the duration of day and night length.

Plant's responses to daylight

Photoperiodism is the term used to describe a plant's various responses to daylength. These include leafy versus reproductive growth (flowers), seasonal changes like leaf fall and dormancy, development of food storage organs such as tubers, and vegetative reproductive structures such as strawberry runners.

Detecting the length of day or night, along with the temperature and water availability, enables a plant to tell the time of year and modify its growth accordingly to meet these conditions.

A plant reacts to the day/night cycle by detecting the ratio at dawn. Shorter nights mean spring is approaching and longer ones indicate autumn. The advantage to this response is an increased chance of survival for the plant, by adapting to falling temperatures and maximising the chance of reproductive success. Daylength is more important for the plant than the aboveground temperature, which can very widely from year to year.

Critical daylength