Photoperiodism is the actual duration or length of the day (photoperiod) and is a significant factor in the growth and flowering of a wide variety of plants. The controlling effect of photoperiod is called photo-periodicity and can be controlled by the addition or reduction of light. Most floriculture crops have one of three flowering responses to photoperiod:
| Long Day Plants | Short Day Plants | Day Neutral Plants |
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| Long day plants are plants that bloom when light duration is more than 12 hrs per day. Day extension can be achieved with low light levels of 10-15 foot candles. Examples: Petunias, carrot, oats and rye. Long days can be manipulated by the addition of light. | Short day plants are plants that bloom when light duration is less than 12 hours per day. Examples: Poinsettias, Tobacco, Dahlia and cosmos. Short days are typically produced by removing the light source to the plant. This is typically achieved by blanketing the plants with a “black out” curtain system. |
Day neutral plants are plants that show little response to length of day light. Examples: Cucumber, cotton, tomato and potato.
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