Photoperiodic lighting is used to create long days for flower induction of long-day (LD) plants or to delay the flowering of short-day plants. Generally, long-day plants will flower when the daylength is longer than 14-16 hours (night length of less than 10 hours). Therefore, long-day lighting should be used from around Sept. 1 to April 15. Note that the critical daylength is likely to be different for each species.
Photoperiodic lighting differs from supplemental lighting, the latter is used to increase the quantity of photosynthetic light during a day or to extend the day. Supplemental light is used during short winter days or very overcast days, and requires high intensity light (typically 300 – 600 footcandles ( f.c.) for 6-12 hours).
Creating LD for photoperiodic lighting can be achieved using several possible lighting regimes:
- Day continuation lighting (also called day-extension lighting) - lighting from sunset until later in the night. (10 pm).
- Pre- dawn lighting - lighting from 2 am until sunrise.
- Night interruption lighting (also called "night break" lighting or “mum lighting”) - lighting in the middle of the night, usually 10 pm until 2 am. Fewer hours of lighting are needed for night-interruption compared to day continuation or pre-dawn lighting, so this is more economical and is used most often for lengthening the daylength.
- Cyclic or intermittent light –periodic lighting in the middle of the night, such as light on for five minutes every 20 minutes during a 4 hour period. Generally, plants need to receive at least 10 f.c. for a minimum of five minutes every half hour. This technique has been used to save energy using INC.
Incandescent (INC) light bulbs with reflectors have traditionally been used to create long days using night interruption lighting to regulate flowering of greenhouse crops. Unlike supplemental lighting, photoperiodic lighting requires only low-intensity light (typically 10 f.c.). Typically, a string of lights (60-watt INC bulbs spaced 5' apart) is hung 3’ above the tops of plants per bench or bed that measures 42-48" wide. A 24-hour timer is used to control the lighting period, turning lights on at 10 pm and off at 2 am.
As compact fluorescent (CFL) bulbs became available, growers replaced their INC lamps with CFL to save energy. CFLs are more energy-efficient than incandescent bulbs, however, their spectrum is not as effective at controlling flowering of some long day (LD) plants. CFLs are low in far-red (FR) light, which is required for rapid flowering of some LD crops, for example pansy and petunia. One solution has been to alternate CFL with incandescent bulbs in fixtures within the greenhouse to provide the necessary light spectrum for effective photoperiodic lighting and still achieved some energy savings.
Light-emitting diodes (LEDs) are emerging as a source of greenhouse lighting. LEDs have a long lifespan, are energy efficient and the spectrum of light emitted can be adjusted. The LED replacements for INC bulbs are more expensive than INC or CFL bulbs, but they last longer and are more energy efficient. Currently, LED bulbs are being successfully used in photoperiodic lighting for some crops. Research at Michigan State University tested LEDs developed by Philips Lighting (Philips DR+W+FR) as a replacement for 100 or 150 watt INC used for photoperiodic lighting. The lamps were installed 3-7 ft above the plant canopy and 3 to 10 ft. apart and used night interruption lighting (10 pm to 2 am). Flowering of bedding plant plants tested was similar as conventional INC lamps.
Resources on Lighting
Lopez R. 2013. The Basics and Beyond: Understanding the Difference Between Photoperiodic and Supplemental Lighting. Greenhouse Grower. Nov. issue.
Lopez R. and C. Currey. 2014. Managing Photoperiodic Lighting. Grower Talks. March issue.
Runkle E. 2014. Replacing INCs with LEDs. GPN Magazine. Nov. issue.
Photoperiod Control Systems for Greenhouse Crops. UMass Extension fact sheet
Mattson N. Greenhouse Lighting. Fact Sheet. Cornell University
Meng Q. and E. Runkle. 2014. Control Flowering with LEDs. Grower Talks. Feb issue
Tina Smith, UMass Extension and Dr. Douglas Cox, Stockbridge School of Agriculture