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Flowers tend not to own calendars, so how do they know when to bloom?

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It’s spring here in the northern hemisphere, which means there are colorful flowers bursting out everywhere— tulips and daffodils and crocuses, you name it.

Oh! You!

You're back! But for a long time, botanists have been trying to figure out exactly what’s going on inside the plants while this is happening. Because plants don't have calendars.

How do they know when it’s time to bloom? Well, it turns out the answer involves a lot more than April showers. Now, scientists have known for a long while that plants respond to things like daylight and temperature, and somehow that stimulates flowering.

They weren’t sure how this process worked, but they thought it might involve some kind of hormone that traveled from the leaves to the shoots to get things going. They called this hypothetical hormone “florigen.” Recently, though, researchers have been able to put together the puzzle of gene activation and deactivation that actually makes it all happen. And it turns out that there are thousands of genes involved in the timing and production of flowers.

Plants’ leaves are like tiny chemical factories, and changes in temperature and the length of the day affect which molecules the active genes pump out. One of these molecules is a special type of RNA, the stuff in cells that helps make proteins from genes. This messenger RNA turns out to be the reality behind the mysterious “florigen.” It travels from the leaves to the plant’s meristems, the spots at the tips of the shoots where new growth happens.

There, it relays the message to important “master” genes with names like LEAFY and Apetala1: spring is here! These genes tell the meristem to stop producing leaves and start making flowers instead, getting the plant ready to reproduce. Despite these advances, though, a new factor is messing up plants’ sense of time: climate change.

As temperatures warm up earlier in the year, plants in some areas are flowering sooner and sooner. This can throw plants out of sync with the pollinators they need to reproduce, and with the animals that depend on them for food. So the next time you stop and sniff a rose, remember: there is more to a flower than just beauty.

It’s the product of literally thousands of genes switching on and off, and passing messages from one part of the plant to another, all working together to grow a sex organ. Thanks to two of our Patreon patrons for asking this question, and thank you to all of our patrons, who keep these answers coming. If you’d like to submit a question to be answered, you can go to

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