Greenhouse Lighting
Length of the Day (Photoperiod)
In addition to light quality and light intensity, the length of the day (photoperiod) can affect the development of some crops. Most importantly, photoperiod can affect flowering. Other plant growth and development processes that are affected by photoperiod include vegetative growth, internode elongation; tuber, rhizome and bulb formation, sex expression, the formation of pigments such as anthocyanin, the number and size of root nodules, fruit set, leaf fall, and dormancy. Photoperiod refers to the period of time in 24 hours that plants are exposed to light. 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. 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. In temperate regions, where greenhouse crops are grown, day length changes seasonally. While we refer to photoperiod when discussing plant development, it is actually the uninterrupted period of darkness that controls plant responses to day length, not the length of the light period. The natural dark period can be extended (to create a shorter day), shortened, or interrupted by providing light (to create a longer day) to manage photoperiodic plant responses such as flowering. Knowing the photoperiodic response of different greenhouse crops can help to schedule plants to flower for specific dates and also reduce production time.
Plant Response to Photoperiod
Depending on their response to day length, plants are classified as either long-day plants (LDP), short-day plants (SDP), or day-neutral plants (DNP). LDP and SDP apply to crops that are sensitive to photoperiod while DNPs are not sensitive, that is, they do not exhibit photoperiodism. Long-day plants flower only if the day length is longer than a critical number of hours. For many long-day (LD) crops, a long day occurs when the photoperiod is at least 14 to 16 hours. More correctly stated these plants flower when the length of the dark period is less than eight to 10 hours long.
Creating Artificial Short Days
When the natural day length is long, there is only one way to create a short photoperiod in the greenhouse and that is by using an opaque material that does not allow light to penetrate, which is commonly referred to as “black cloth” or “blackout cloth.” Many growers use blackcloth to provide short days to induce flowering of poinsettias, chrysanthemums and other short-day plants. In addition, short days may be desirable when one wants to delay or prevent flowering of long-day plants.
Operation of Black-Cloth Curtains
The black cloth can be pulled over the plants manually at a specified time in the afternoon (usually following the workday) to truncate the natural day length or automatic blackout curtain systems can be used to enclose individual benches or an entire greenhouse. Larger growers use power-operated shading.
Blackout Materials
Blackout curtain materials include polyethylene film, knitted polyester, and composite fabrics where all the strips are either aluminized or opaque. Most blackout materials attempt to reduce heat buildup when the curtain system is covered for day length control in summer. So-called black/white poly is a polyethylene film that is co-extruded with white toplayer for light reflection, and a black bottom-layer for opacity.
Greenhouse Temperture Regulation
Application of artificial SD treatments during warm weather may result heat delay. Heat delay can arise when temperatures are excessively high under blackout materials. This commonly occurs during late spring, summer, and early fall. High temperatures (in excess of 85 deg F, 29 deg C) for long periods can delay or completely inhibit flower initiation and/or development in some greenhouse crops.
Creating Artificial Long Days
Under natural short days, long days can be created by lighting at the end of the day, known as day-extension (DE); by lighting during the middle of the night, known as night-interruption (NI) or by intermittent photoperiodic lighting, known as cyclic lighting. Each method has advantages and disadvantages, but generally, they are similarly effective. Both methods require you to deliver at least 10 foot-candles (fc) or 1 to 2 μmol·m−2·s−1 of light when measured at plant level.
Day-Extension
Day-extension (DE) lighting is the practice of delivering light to extend the length of the natural day. The length of time you light will depend on the period of darkness the plant requires and the natural day length.
Night-Interruption
When natural photoperiods are short, night interruption lighting (night-extension lighting) is used to promote flowering in long day plants and prevent flowering in short day plants. Night-interruption (NI) lighting is the practice of providing low-intensity light to plants during the middle of the night.
Cyclical Lighting
Cyclic or intermittent photoperiodic lighting is an alternative to the long day lighting strategies discussed above. This technique uses a series of short alternating light and dark cycles to substitute for one continuous light break. It is the use of periodic lighting in the middle of the night, where lights are on for 10 minutes and off for 20 minutes during a four-hour period. Generally speaking, plants need to receive at least 10 fc (~2 μmol·m−2·s−1 PPFD) for a minimum of 5 minutes every half hour.
Lighting Options for Controlling Photoperiod
Incandescent Light Bulbs
Incandescent light (INC) bulbs are commonly used in greenhouses to provide DE and/or NI lighting. Incandescent light bulbs may be used for cyclic lighting, because the frequent on-and off will not affect light life or fixture longevity. Incandescent light bulbs have a broad range of wavelengths but low in blue light, emit large amounts of heat, and are energy inefficient.
Compact Fluorescent Lights
Compact fluorescent (CFL) lights can also be used. CFLs are often used alone or mixed with incandescent or halogen incandescent bulbs to extend the day length in some photoperiodic crops.
High-Intensity Discharge Lamps
High-intensity discharge (HID) lamps lamps, such as metal halide and high-pressure sodium (HPS) lamps, may also be used effectively. There are several ways to use these lamps to provide DE and NI lighting.
Light-Emitting Diodes
Light-emitting diodes (LEDs) are an emerging light source with promising plant applications, including the regulation of flowering. Their long lifespan, energy efficiency, and ability to target specific wavelengths of light make them a viable option for managing photoperiod.
Supplemental versus Photoperiodic Lighting
In commercial greenhouses, photoperiodic and supplemental lighting are two strategies used to better meet plant growth needs throughout the day, grow cycle, and season.
Carbon Dioxide and Supplemental Light
For photosynthesis, plants need both light (PAR) and carbon dioxide. Both need to be available in sufficient quantities for either one not to become the limiting factor (if there is enough light but not enough carbon dioxide, carbon dioxide becomes the limiting factor and vice versa). Therefore, when using supplemental lighting to increase plant production, it is important to maintain sufficiently high carbon dioxide concentrations inside the greenhouse.
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