Light and Lighting Control in Greenhouses
Light duration refers to the period of time in 24 hours that plants are exposed to light. In temperate regions where greenhouse crops are grown, day length changes seasonally. This change results occurs because the earth's axis is tilted 23½ degrees from a line perpendicular to the plane of the earth's orbit about the sun. Photoperiod describes the duration of time that light shines on a plant each day, and the term photoperiodism describes the response of plants to the relative lengths of light and dark periods within a 24-hour cycle. While we refer to photoperiod when discussing plant development, it is actually the uninterrupted period of darkness that controls plant responses to day length. 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. 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. Knowing the photoperiodic response of different greenhouse crops can help to schedule plants in flower for specific dates and also reduce production time. For example, many bedding plants grown in spring are long-day plants and providing photoperiodic lighting can greatly accelerate time to flower and thus reduce production costs.
Variation in Photoperiod
The duration of the light period varies according to such factors as geographic location, distance from the equator, and time of the year. Variations in day length are relatively small in the tropics. At the equator, the day length is relative constant at 12 hours and 7 minutes during the year.
Photoperiodic Plant Response Types
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. Examples of long-day plants are tuberous rooted begonias and spinach. Short-day plants flower when the day length is shorter than a critical number of hours.
Decreasing Day Length
Many growers use an opaque material that does not allow light to penetrate, which is commonly referred to as “black cloth” or “blackout cloth” to create a short photoperiod in the greenhouse. 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 an automatic blackout curtain systems can be used to enclose individual benches or an entire greenhouse. Larger growers use power-operated shading. The cost of automatic shading systems might seem high, but the advantages easily sell these systems. At the flip of a switch or a computer signal, acres of greenhouse can be covered in a few minutes, eliminating hours of manual labor. This savings occurs twice a day. Care must be taken to open the cover gradually in the morning in the winter months. This slow exposure prevents the plants from being suddenly chilled by cold air that accumulates above the blanket during the night.
Extending Day Length
Under natural short days, long days can be created by lighting at the end of the day, known as day-extension (DE), or by lighting during the middle of the night, known as night-interruption (NI). 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. Some long-day plants flower if the photoperiod is at least 14 hours, whereas others require 15 or 16 hours of light.
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-extension (NI) lighting is the practice of providing low-intensity light to plants during the middle of the night. By interrupting the dark period, the plant will not perceive a long night, but a short night (or long day) instead.
Cyclic or intermittent photoperiodic lighting is an alternative to the long day lighting strategies discussed above. 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) for a minimum of 5 minutes every half hour.
Photoperiodic vs. Photosynthetic 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. Generally speaking, if there is a need to extend daylight hours or increase DLI for improved or accelerated plant growth, supplemental lighting is probably the best solution. If an identified need involves triggering the transition to a late-stage growth phase or improving deficiencies during a specific lifecycle phase, adding photoperiodic lighting may improve performance.
Lamp Types/Light Sources
There are several options you have when looking at light sources for use in photoperiodic lighting, each with advantages and disadvantages. Unlike supplemental lighting, photoperiodic lighting requires only low-intensity light (typically 10 foot-candles or 2 to 3 µmol?m−2?s−1). Photoperiodic lighting is only used to accelerate flowering of long-day plants and inhibit flowering of short-day plants; it does not increase growth. Incandescent lamps are often used since they are inexpensive and easy to install. Unfortunately, incandescent lamps emit a high proportion of far-red light, which in many plants promotes stem elongation.
Incandescent (INC) Light Bulbs
These 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. However, incandescent light bulbs are used most often because they are efficient generators of red light (the long day response is most sensitive to red light) and incandescent light bulbs are inexpensive and easy to install. Incandescent light bulbs are inexpensive and emit an effective spectrum, but they are very energy inefficient and require high power availability.
High-intensity Discharge (HID) 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.
Compact Fluorescent (CFL) 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.
Light-emitting Diodes (LEDs)
This technology is 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. Light-emitting diodes also provide the opportunity to adjust the ratio of red (R) and far-red (FR) light for desired plant responses.
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