Daylength exerts profound effects on the growth and flowering of many plant species, and manipulation of daylength is essential for scheduling several greenhouse crops. The responses of plants to daylength may appear to be a perplexing subject, but it is easy to understand once some basic concepts are mastered. This article briefly reviews photoperiodism and photoperiod control systems for greenhouses.
Terminology. The terms photoperiod and photoperiodism are commonly used in extension bulletins and other literature, but what actually do these terms mean? A photoperiod is the duration of light within a particular time span, usually a 24-hour period. For example, a 12-hour photoperiod consists of 12 hours of light and 12 hours of darkness, whereas an 8-hour photoperiod consists of 8 hours of light and 16 hours of darkness. Photoperiodism is the term for responses of plants to the relative length of the light and dark periods. These terms have been used for many years and imply that the light period is more critical than the day period, but research has shown that it is the dark (or night) period that is more important than the light (or day) period for controlling photoperiodic responses.
The duration of daylight is defined as the interval between sunrise and sunset. Under natural conditions, the duration of daylight varies over the earth's surface, and is dependent on season and latitude. The duration of daylight for 42o N. latitude (the Springfield/Boston area) is presented in Table 1. At 42o N. latitude, the duration of daylength varies from 9 hours and 7 minutes on December 21 (winter solstice) to 15 hours and 15 minutes on June 23 (summer solstice) (Table 2). Civil twilight is defined as the interval between sunrise (or sunset) and the time when the sun is 6o below the horizon. We experience two periods of civil twilight in each 24-hour period: before sunrise and after sunset.
Let's suppose that the sun rises at 6 am and sets at 6 pm, and we would all agree that the duration of daylight is 12 hours. The question we need to ask is, do photoperiodically responsive plants perceive this as 12 hours of light? The answer is no. The light intensity at sunrise and sunset is often greater than 20 footcandles (ft-c). Light at very low intensities, i.e, civil twilight, is sufficient to induce photoperiodic responses in plants. For example, light intensity of less than 2 ft-c is sufficient to inhibit normal flower bud initiation in poinsettia, a short-day (SD) species for flowering. Thus, for photoperiodic responses, photoperiods are estimated by adding the periods of civil twilight to the duration of daylight. The duration of daylight plus civil twilight for 42o N.
|Day of Month||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.|
zDuration of daylight = the interval between the sunrise and sunset.
latitude is presented in Table 2. At 42o N. latitude, the photoperiod (=daylight plus civil twilight) varies from 10 hours and 11 minutes on December 21 (winter solstice) to 16 hours and 25 minutes on June 23 (summer solstice) (Table 2). It should be noted that local conditions, i.e., shading from trees or adjacent buildings, persistant cloudiness, or fog, may make the actual photoperiod shorter than estimated photoperiod (Table 2). Another important question is, can moonlight affect photoperiodic responses? The answer is no. The maximum intensity of bright moonlight is 0.02 ft-c, and this is not enough to be perceived as daylight by photoperiodically responsive plants.
|Day of Month||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.||h.||m.|
zDuration of daylight plus civil twilight = the interval from when the sun is 6o below the horizon before sunrise until the sun is 6o below the horizon after sunset.
Methods of photoperiod control. Photoperiod control is necessary for scheduling greenhouse crops such as mum, kalanchoe, Holiday cactus, and poinsettia. The latitude and species will determine whether shortening or lengthening of the daylength is needed. There are two types of photoperiod control systems used in commercial greenhouses: (1) using incandescent (INC), compact fluorescent (CFL), tube fluorescent lamps, high-intensity discharge (HID) lamps, or light-emitting diodes (LEDs) during naturally short days (SD) to create artificial long days (LD); and (2) using opaque black cloth (or similar material) during naturally LD in order to create artificial SD.
Artificial light sources
- Incandescent light bulbs with reflectors have traditionally been used to create artificial long days. INC light has a broad range of wavelengths but is low in blue light, emits large amounts of heat and are energy inefficient.
- Compact fluorescent lights are more energy efficient than incandescent bulbs, have blue and red light but are low in far-red (FR) light which is required for flowering of some LD crops.
- Tube type fluorescent lamps are large in relation to their output and are most often used in growth chambers and in multiple tier applications since their cool operating temperatures allow them to be mounted in close proximity to plant surfaces. Tube fluorescent lamps are available in a range of spectral qualities.
- High-intensity discharge (HID) lamps (metal halide and high pressure sodium) are primarily used as supplemental light to increase the rate of photosynthesis, such as for daylength extension during short winter days or supplemental light during very overcast days.
- Light-emitting diodes (LEDs) is emerging technology for greenhouse lighting. LEDs have a long lifespan, are energy efficient and the spectrum of light emitted can be adjusted. The LED replacement for INC bulbs are more expensive than INC or CFL bulbs, but they last longer and are more energy efficient. Currently there are LEDs commercially available that are successfully being used in photoperiodic lighting for some crops. See links under resources for more information.
Lengthening the daylength (artificial LD). Incandescent (Inc) lamps are installed to provide a minimum of 10 ft-c at the tops of plants. Fluorescent or HID lamps can also be used for lengthening the daylength. However, Inc lamps are used most often because they are efficient generators of red light (the LD response is most sensitive to red light) and Inc lamps are inexpensive and easy to install. Compact fluorescent (CFL) bulbs can also be used. CFLs are more energy-efficient than incandescent bulbs, however, CFLs are low in far-red (FR) light, which is required for rapid flowering of some LD crops. One solution has been to alternate CFL with incandescent bulbs in fixtures to provide the necessary light spectrum for effective photoperiodic lighting and still achieve some energy savings. For benches or beds that are 42-48" wide, use 60-watt incandescent lamps with reflectors (or alternate with CFLs). Space lamps 5' apart and 3' above tops of plants. A 24-hour timer is used to control the lighting period.
There are three possible lighting regimes: (1) lighting from sunset until later in the night (10 pm) is referred to as day continuation lighting (also called day-extension lighting); (2) lighting from 2 am until sunrise is referred to as pre- dawn lighting; (3) lighting in the middle of the night, usually 10 pm until 2 am, is referred to as night interruption lighting (also called "night break" lighting). Fewer hours of lighting are needed for night-interruption compared to day continuation or pre-dawn lighting, so the former is more economical and is used most often for lengthening the daylength.
Cyclic (intermittent) lighting. In mums, it is not necessary to continuously operate Inc lamps from 10 pm to 2 am each night to achieve LD conditions. With 20 ft-c intensity from Inc lamps, lights need to be on only for 5% of a 30-minute period (28 minutes dark/1 minutes light) from 10 pm until 2 am to insure vegetative growth. With 10 ft-c intensity from Inc lamps, lights need to be on only for 20% of a 30-minute period (24 minutes dark/6 minutes light) from 10 pm until 2 am to insure vegetative growth. Thus, the percentage of time that lamps are operating is dependent on the light intensity received by the plants. In commercial operations, growers use night-interruption lighting (10 pm until 2 am) and cyclic lighting (24 minutes dark/8 minutes light) with Inc lamps at 10 ft-c. A cyclic lighting programmer allows for 4 separate lighting circuits, and each circuit can be turned on for 8 minutes (1r8, 9r16, 17r24, and 25r32 minutes); this allows cyclic lighting for 4 separate zones. The cyclic lighting programmer can reduce photoperiod lighting costs by up to 75%.
Shortening the daylength (artificial SD). Plants are covered with an opaque material that reduces the light intensity. To be effective, the light intensity must be decreased to less than 2 ft-c under the material. In most operations, plants are covered at 4-5 pm and shade is removed at 7-8 am to provide an 8-hour day. A 4' height is recommended between the top of cloth and the bench (bed) top to reduce heat buildup around plants; tall crops (such as cut mums) require more height. The material must be placed over the plants continuously (every evening) until the response is maximal. Several types of materials can be used to cover plants, and include the following: (1) black sateen cloth is a jet black woven cloth made from cotton or a cotton/polyester blend; (2) polyolefin sheeting is a woven material that is tear-resistant and waterproof; and (3) 4-6 mil black plastic can be used but is not tear-resistant. Tears and worn spots should be repaired, and overlaps of shading materials should be properly sealed to minimize light leakage. Occasional 24-hour skips in shading are not catastrophic, but flowering can be delayed. For mums, every day that shading is skipped delays flowering by one additional day. Failure to shade consistently may also cause irregular opening of flowers.
Application of artificial SD treatments during warm weather may result heat delay. Heat delay can arise when temperatures are excessively high under shading materials. This commonly occurs during late spring, summer, and early fall. High temperatures (in excess of 85oF) for long periods can delay or completely inhibit flower initiation and/or development in some greenhouse crops. Some cultivars of kalanchoe and mums exhibit heat delay. If temperatures exceed 85oF during shading, pull shade over plants at the usual time (late afternoon), but remove the shade at twilight and replace it before sunrise. Alternatively, a grower can select cultivars that are resistant to heat delay.
Published: Boyle, T. Photoperiod Control Systems for Greenhouse Crops. 1992. Floral Notes, May-June 4(6):2-4.
Updated 2015, 2017
- Hanan, J.J., W.D. Holley, and K.L. Goldsberry. 1978. Greenhouse Management. Springer- Verlag, New York.
- List, R.J. 1951. Smithsonian Meteorological Tables. 6th edition. Smithsonian Institution, Washington, D.C.
- Mastalerz, J.W. 1977. The Greenhouse Environment. John Wiley & Sons, New York.
Lopez R. and C. Currey. Managing Photoperiodic Lighting. Grower Talks Magazine (added 2015)
Lopez R. 2013. The Basics and Beyond: Understanding the Differences Between Photoperiodic and Supplemental Lighting. Greenhouse Grower, Nov.
Mattson N. 2017. How Many Light Fixtures Do I Need. E-Gro Alert, March.
Runkle E. and M.Blanchard. Use of Lighting to Accelerate Crop Timing. Department of Horticulture, Michigan State University (added 2015)
Lighting Fact Sheets for Greenhouse Crops from Michigan State University Extension (added 2015)