# How do plants know when to bloom?

**URL:** <https://boards.straightdope.com/t/how-do-plants-know-when-to-bloom/444575>\
**Category:** Factual Questions\
**Created:** [April 9, 2008, 7:02am UTC](https://boards.straightdope.com/t/how-do-plants-know-when-to-bloom/444575 "2008-04-09T07:02:08Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\
**Post date:** [April 9, 2008, 7:02am UTC](https://boards.straightdope.com/t/how-do-plants-know-when-to-bloom/444575/1 "2008-04-09T07:02:08Z")

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It seems like each species of flower around where I live each has it’s own bloom time, even if the individual plants are in completely different locations. While the average temperature obviously is fairly consistent when you measure it over a period of many years, in any particular year there plenty of hotter than average days before a given date, and colder than average days after a given date. So how do the plants decide when to bloom? Do they have some sort of internal clock? How accurate is it? Does it have some kind of correcting mechanism? Does the clock change gradually from north to south?

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**Author:** ![puppygod](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@puppygod](https://boards.straightdope.com/u/puppygod)\
**Post date:** [April 9, 2008, 7:38am UTC](https://boards.straightdope.com/t/how-do-plants-know-when-to-bloom/444575/2 "2008-04-09T07:38:12Z")

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Check [this wikipedia](http://en.wikipedia.org/wiki/Flowering#Flowering_transition) link and following one about plant hormones for basics.

> [@](#):
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> (…) To meet these needs a plant is able to interpret important endogenous and environmental cues such as changes in plant hormones levels and seasonable temperature and photoperiodchanges.

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**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\
**Post date:** [April 9, 2008, 9:09am UTC](https://boards.straightdope.com/t/how-do-plants-know-when-to-bloom/444575/3 "2008-04-09T09:09:25Z")

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Plants don’t have any internatl clock beyond the standard daily ~25 hour clock that most organisms have, and that would be quite useless at timing flowering. Plants where all indivdiuals flower at the same time regardless of environment are flowering in response to the number of hours of daylight. Nothing very tricky.

Note that most pants aren’t just or even primarily photoperiod sensitive. If you look at plants of the same species growing in differrent microclimates even in the same general area you will usually find considerable variation in flowering onset. That’s because these plants are either responding direstly to climate (eg they flower as soon as the temperature or soil moisture is right) or they are limited by daylength but dertermined by enviroment, iow they flower after the first shower of spring, or they flower as soon as the daytime maximum falls below 15o in autumn.