How Do Plants Know When It's Spring?
If you live in the Northern Hemisphere, you’ve probably had some warmer weather lately. You may have noticed the sun rising earlier or setting later. It’s definitely spring.
You are probably expecting plants to start blooming soon, but how do they know when it’s time?
Plants have noticed the early sunrise and late sunset too. More specifically, they respond to the number of hours of sunlight each day. Due to the Earth’s tilt on its axis, we get seasons, which comes along with seasonal changes in the number of hours of daylight. In the summer, the sun sweeps a longer arc through the sky (well, no, the Earth is actually moving with respect to the sun, but you get the idea), so there are more hours of daylight. The longest day of the year is the summer solstice. The shortest day of the year is on the winter solstice.
Plants show a behavior called photoperiodism, which is a response to length of day or night. Each species of plant has a different photoperiod, or day length signal. Plants bloom, or flower, or even have leaves change colors, when the number of hours of daylight reaches a certain high or low threshold. Long-day plants, such as carnations, lettuce, and wheat, flower when the length of daylight exceeds a certain amount. Short-day plants, such as cotton, rice, and sugar cane, flower when the length of daylight drops below a certain amount, which means winter is approaching.
This time of year, plants are starting to notice the longer days, just like you might be.
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