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Dr. Universe, How do plants know when it’s spring? Hannah, Age 3

Dr. Universe: How do plants know when it’s spring? Hannah.

Dear Hannah, When we see little green plants sprouting up from the soil, it is indeed a sign that spring has arrived. To find out how they know to grow, or germinate, I asked my friend Camille Steber.

In her U.S. Department of Agriculture lab at Washington State University, she studies the source of almost all plants: Seeds.

A seed holds pretty much all the information a plant needs to know how to grow. The shell that holds all the seed parts inside even contains food for the plant. But the seed is very dry and needs to sense clues from outside to know when to germinate at the right time.

You may have heard that some living things go dormant or hibernate in the winter, grizzly bears and some kinds of frogs, for example. They stay dormant to save energy and survive the season.

Seeds can go dormant, too — and they can’t germinate until spring. For a seed to know when it’s the right time, it first has to experience a cold and wet season.

In the winter and fall, rain and snow provide a lot of water for the seed. Maple tree seeds, for example, need to experience a couple of months of cold weather before they are ready to germinate. A change in temperature is one signal that helps plants know when to grow.

As plants sense temperatures rising, they release a combination of chemicals called hormones. The hormones help tell the seed to start producing different parts, such as roots, stems and leaves.

The length of the day is another way a plant knows when to grow. Just like your eyes have receptors that can sense light and help you see, plants also can sense light.

As Earth travels around the sun in its orbit, the length of days and nights changes from season to season. Plants can sense when the days are shorter in the winter. They also can sense when days are longer in the spring and many begin to grow.

There are some exceptions, though. The Christmas cactus, a plant that comes from rainforests in Brazil, flowers when days are short and nights are long. The ability to bloom in winter when days are shorter helps them keep from flowering when it is too hot. You can persuade the cactus grow by putting it in a dark closet.

A plant’s ability to sense these changes in temperature and daylight has to do with its genetic code, or DNA. DNA tells your hair and eyes to be a certain color — or when to have a growth spurt. Just as your genetic code is different from your friends’, a daffodil, a tulip, or a daisy are also different.

One of the first flowers to bloom in the spring where I live is the crocus. It’s a bell-shaped flower that comes in yellows and purples. What flowers are popping up in your neighborhood? Tell us about it at Dr.Universe@wsu.edu.


Dr. Universe
Do you have a question? Ask Dr. Universe. Send an email to Washington State University’s resident scientist and writer at Dr.Universe@wsu.edu or visit her website, askdruniverse.com.

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