How does a plant know when it's time to flower?

A plant knows the season by measuring the dark. Day length changes through the year, and plants read that change, a skill called photoperiodism, the reaction of an organism to the length of light or dark periods. Plants are sorted into short-day plants, long-day plants and day-neutral plants.

There is a twist in the name. In 1920, W. W. Garner and H. A. Allard published their discoveries and believed it was the length of daylight that mattered. It was later found that the length of the night is the controlling factor. Plants are still called long-day or short-day, even though the night is what counts, because of that early misunderstanding. A long-day plant flowers when the night falls below its critical length, typically in late spring or early summer, while each species has its own critical night length.

The proof is a clever experiment. A short-day plant will not flower if you switch a light on for just a few minutes in the middle of the night, and a long-day plant can flower if you give it red light in the middle of the night. A brief flash of light is enough to cut the night in two.

How does a leaf sense light? With photoreceptor proteins. Phytochrome has two forms, and red light, present during the day, converts it to the active form, which can trigger flowering, germination or branching. Plants also get more far-red light in shade, which switches phytochrome back off. Biologists believe that plants measure the length of the night by matching these light signals against the rhythms of an internal circadian clock.