Sign In to join the community | Help
Lawn & Garden

How Do Plants Know When To Bloom?


I had the question asked to me the other day,” How do trees know that it is time to change color and drop leaves when it is still so warm outside?”. I then went on a tangential rant about short day plants and how trees and plants recognize when daylight hours get shorter and night becomes longer. They will behave differently once this happens.


It is for this reason that your Christmas cactus will really only bloom around Christmas time. It is also for this reason that the bracts on a poinsettia turn red and why chrysanthemums bloom when they do. Plants that need 12 or more hours of darkness are considered short day plants. There is also a name for this. Photoperiodism is the amount of light and dark that a plant is exposed to. Short day plants are the plants that you will find blooming towards winter or early spring.

On the other end of the spectrum there are long day plants that have a blooming response once night time hours get shorter. These plants are the ones like gardenias and coneflowers, which will be blooming in summer.


For some plants, photoperiodism or the length of the day and night is not a factor. These plants are considered to be day neutral. Plants like tomatoes, cucumbers and corn are considered to be day neutral plants.


It is for this reason that some growers will force bloom plants by manipulating light cycles for certain plants. You will find blooming gardenia plants in the grocery store in the early spring, long before they normally bloom. Putting a grow light on a long day plant like this for a few extra hours each day will trick the plant into thinking it is summer and time to bloom.


Growers will cover plants like mums and poinsettias each day to create more dark hours, to bring them to garden centers earlier in the season for sale.


As far the changing of colors on trees, photoperiodism is the key component, along with temperature and photosynthesis. Other factors like rain and drought dictate when and to what degree the colors will change. This fall in Atlanta has been very warm and we have not seen rain now for over 30 days. The leaves that should be on the ground by now, are still in the trees and green.

Not what you were looking for ? Try posting a question
Posted 2016-11-03T18:10:51+0000  by Ingar_HD_ATL Ingar_HD_ATL