Greenhouse Lighting
Daily Light Integral
While light intensity is important, daily light integral (DLI) is another effective means of monitoring photosynthetic light in the greenhouse. Light intensity is a snapshot in time; throughout the course of the day in greenhouses, it can change with the time of day, position of the sun in the sky and cloud cover. DLI in a greenhouse is obviously influenced by the greenhouse glazing, overhead obstructions, shading curtains, and high-intensity supplemental lighting (and distance from the canopy) when employed. Therefore, the cumulative amount of photosynthetic light a crop receives during the day can be a more meaningful way to measure and manage light in greenhouses. Daily light integral is the amount of usable light (expressed as PAR or photosynthetically active radiation) the crop received inside the greenhouse as a function of light intensity during a 24-hour period (day). The common DLI unit is moles of light per square meter per day (mol∙m–2∙d–1) or as described in this book, moles/day). Photosynthetic photon flux density (PPFD), on the other hand, is a measure of the number of photons in the photosynthetically active radiation (PAR) range (400–700 nm) that reach a given surface per unit time, expressed in micromoles of photons per square meter per second (µmol m-²s-1).
Maximize Transmission of Sunlight
Before installing supplemental light to increase the DLI it is important to first consider the greenhouse itself and how it can affect the amount of light reaching the plants. Greenhouse characteristics such as the structure and its orientation, glazing material, gutter height, composition of side and end walls, and type and placement of heating and other equipment influence how much solar radiation reaches crops inside. Consider the effects of these decisions on the transmission of sunlight to the crops inside. As mentioned, glazing materials have different light-transmission properties. Those with an initial and sustained highlight transmission (such as glass and acrylic) can slightly decrease the need for supplemental lighting in temperate regions.
Calculating Daily Light Integral
There are three ways to calculate the DLI in the greenhouse: (1) quantum sensors, (2) foot candle meter, and (3) DLI maps.
Quantum Sensors
A quantum light sensor will measure the light that is used for photosynthesis, or photosynthetically active radiation (PAR). Quantum sensors measure instantaneous light, reported in micromoles (µmol) per square meter (m–2) per second (s–1), or: µmol·m-m–2∙s–1 of PAR, which allows the summation into mol·m–2∙d–1 or moles/day. From this information the grower can then calculate the DLI. Many greenhouse growers use automated systems with quantum sensors connected to a data logger (Figure 8.4). If connected to a computer control system such as those from Argus, Hoogendoorn, or other similar products, the grower can easily monitor and automatically calculate the DLI, making sure plants get the light they need.
Foot Candle Meters
Many greenhouse growers in the U.S. make use of foot candle meters. The DLI is calculated by summing all the measured values made throughout the day and multiplying this value by the number by the time interval between each measurement. To work out the approximate DLI inside a greenhouse, under natural light only (excluding light from any HID or LED lamps) using a foot candle meter, the following procedure can be used.
DLI Maps
The DLI at your location can also be estimated using DLI maps (See Figure 7.2) that were developed by Jim Faust at Clemson University. Faust determined the average DLI outdoors during every month for the contiguous United States. Growers can use the maps to estimate the DLI delivered to their crops based on their greenhouse light transmission percentage. The daily light integral maps only describe the amount of light delivered from the sun to the outside of the greenhouse.
Managing Daily Light Integral
Commercial growers who routinely monitor and record the DLI received by their crops can easily determine when they need supplemental lighting or retractable shade curtains. Many greenhouse growers use automated systems with quantum sensors and computer controllers that calculate an area’s DLI and graph the results. However, smaller-scale growers can make use of portable DLI monitors, which can be moved around indoors to get an approximate reading for the DLI at the end of each 24-hour period. The DLI can also be calculated using hand-held light meters with some basic conversion factors for different types of lamps.
Daily Light Integral Requirements for Greenhouse Crops
The DLI has a significant impact on a number of plant variables, including root and shoot growth, stem thickness, plant height, branching, number of flowers, and flower timing. All these factors have a big impact on yields and overall crop quality. For propagating seedlings and many young cuttings, a low DLI of 6 to 8 mol∙m–2∙d–1 is recommended, which should increase to 10 to 12 mol∙m–2∙d–1 for older transplants, flowering annuals and small herbs. Many shade-loving indoor plants and ornamentals require a relatively low DLI. African violets and Phalaenopsis orchids prefer an average DLI of 4 to 6 mol∙m–2∙d–1. Many ferns perform best at a DLI of 4 to 6, cyclamens at 6 to 8, fuchsias at 10 to 12, chrysanthemums at 10 to 14, petunias at 16 to 18 and cut-flower rose plants at 18 to 22 mol∙m–2∙d–1.
The Effects of High Daily Light Integral on Plants
Excessively high light may result in a change in leaf orientation and shape. Leaves grown under excessively high light produce a more vertical and curled leaf blade in order to avoid light interception.
The Effects of Low Daily Light Integral on Plants
Very low light conditions (below 5 mol∙m–2∙d–1 or 500 to 1,000 fc) typically result in poor-quality plant growth and flowering. Under low light conditions, the plant lacks sufficient energy to produce a high-quality plant. The plants often have just one thin primary stem with very little lateral branching. There may be insufficient light to support flowers, so flowering may be delayed, flowers may be very small, or few, if any, flowers may be produced.
Daily Light Integral and Greenhouse Temperature
The quality of the growth that occurs under low light, largely depends on the greenhouse temperatures. Under cool growing conditions (< 65°F, 18°C), plant quality can be quite good. For example, northern European growers compensate for low light conditions by growing their crops cool. Crop time is increased, but the quality is good. Cool temperatures allow the leaves and flowers to develop slowly, which allows the plant more time to accumulate energy from sunlight to produce healthy leaves and flowers.
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