Greenhouse Light Quality and Intensity
J. Raymond Kessler, Jr.
Solar radiant energy is the dominant factor in the greenhouse environment. Sunlight is not only essential to the growth and development of plants but also to how growers manage the greenhouse day-to-day and season-to-season.
– Green plants are dependent on solar radiant energy to manufacture carbohydrates as a source of energy for respiration and growth.
– Spectral quality and length of the light period modifies plant development in terms of vegetative growth, flowering, seed germination, and other morphological traits.
– Solar radiant energy entering the greenhouse is responsible for an ever-changing energy balance within a greenhouse which is manifested as temperature patterns. As a consequence, day/night temperature adjustments are needed daily and seasonally.
– The frequency of irrigation varies with solar radiant energy, especially as it affects leaf and air temperature and the rate of evapotranspiration.
– Fertilization frequency and rate is closely related to the rate of plant growth which, to a degree, is determined by solar radiant energy. Therefore, adjustments to fertility programs should be made at least on a seasonal basis.
Terminology and Units of Measure
The electromagnetic radiant energy emitted by the sun includes gamma rays, ultraviolet light, visible light, infrared red light, and radio waves. Of these ultraviolet, visible, and infrared light are of primary importance to plants. Some definitions:
Light: The portion of the electromagnetic spectrum perceived by the human eye.
Radiant Energy: The form of energy that is propagated through space as electromagnetic waves. Solar radiant energy is the preferred term for sunlight.
Radiation: The process by which objects generate and emit radiant energy. This has to do with the source of radiant energy.
Irradiation: The process of interception of radiant energy by an object. (May be viewed as the converse of radiation)
Irradiance: The amount of radiant energy incident on a surface. This is a general term for how much radiant energy an object receives per unit area per unit time. A measure of light intensity.
Several methods for measuring irradiance have been devised. No single method has been found to be ideal for all the physiological processes that occur in plants. Devices for determining irradiance were originally designed to measure interior lighting needs for humans or for measuring total solar radiant energy by the weather service. Each of these devices and their units of measure a different portion of the spectrum.
Footcandle (ft-c) - A footcandle is the amount of light projected on the inside of a sphere one foot from a standard wax candle. A foot candle is equal to one lumen per square foot. Devices designed to determine irradiance in foolcandles were developed for determining lighting needs for humans inside buildings. Therefore, they measure the spectrum that the human eye sees. These devices and the footcandle is still extensively used by the greenhouse industry.
Watt per Meter Squared (W∙m-2) - This is a total energy unit used mainly by the weather service and measures the entire spectrum of sunlight.
µmole m-2 s-1 - Photosynthetically Active Radiation (PAR) refers to that portion of the spectrum utilized by plants in photosynthesis and is measured in µmole m-2 s-1. This is a scientific unit that measures irradiance of photon per square meter per second within the 400 to 700 nm range.
Properties of Radiant Energy
1) Spectral Quality (color)
Spectral quality of radiant energy refers to the wavelength or color of light. The unit of measure for wavelength is the nanometer (nm) or one billionth of a meter. All light sources emit radiant energy in different combinations of radiant-energy wavelengths. The sun emits all known wavelengths from a single source, but humans can see only a small portion of the total spectrum. The longer the wavelength, the lower the energy and the shorter the wavelength, the higher the energy. Plants utilize a different portion of the total spectrum which, in part, overlaps human vision. Plants can utilize the ultraviolet, visible, and the infrared portions of the spectrum.
The characteristics of glazing materials modifies the spectrum of solar radiant energy entering the greenhouse. Glass excludes a major portion of the ultraviolet portion of the spectrum while polyethylene absorbs some of it and transmits the rest. Absorption of ultraviolet light by polyethylene causes degradation of the material. The primary function of photosynthesis is to produce high-energy molecules, mainly carbohydrate, from carbon dioxide and water. Fundamentally, photosynthesis is a series of oxidation / reduction reactions that is driven by the intensity and quality of radiant energy. The primary peeks of photosynthesis, base on CO2 assimilation, fall within the 400 to 700 nm range or the red and blue wavelengths. Different plant species have slightly different action spectrums depending on the mix of pigments present in the leaves.
When the light supplied to plants is mainly blue, internodes and total height is short and growth is hard and dark green in color. When the light is mainly red, internodes are long and growth is soft. Greenhouse growers have little choice about the spectral quality of sunlight that enter the greenhouse. Glazing material alter the light spectrum in a few limited ways but this cannot be used as a tool by the grower.
2) Irradiance (Intensity)
The majority of greenhouse crops require full sun to grow and develop. Full sun ranges from 8000 to 10,000 ft-c. during the summer to as low as 500 to 2000 ft-c in the winter. Light intensity varies considerably with latitude and time of the year. This is a result of the inclination of the earth and rotation around the sun. Along the equator, light intensities remain more uniform with the change in season. Moving northward, light intensities are higher in the summer and lower in the winter. Growth rates of greenhouse crops generally follow the light intensity changes that occur with a change in season. Growers must adjust cultural practices to compensate for these light intensity changes. Light intensity also varies during the day and on cloudy verses sunny days.
Light Compensation Point
Photosynthesis is a food-manufacturing process while respiration is a food-utilizing process that provides energy for growth. For any plant species, there is a minimum light intensity at which the amount of food produced equals the amount used by respiration. This is the light compensation point.
Light Saturation Point
As light intensity increases during the day, photosynthesis increases up to a certain light intensity and then levels off. This maximum light level is the light saturation point or where a further increase in light intensity does not result in an increase in photosynthesis. Light saturation points can vary widely depending on plant species.
Greenhouse crops may be classified as high, medium or low light plants. High light plants can be grown in full sun while growth of shade plants (low light) is reduced or plants are damaged by light intensity above a level that is species specific. Individual leaves of full sun plants may light saturate at 2000-3000 ft-c, but they can tolerate intensities of 8000 to 10,000 ft-c. In contrast, leaves of shade plants light saturate at a lower intensity, 500 to 1000 ft-c. Shade plants have much less tolerance of full sun so steps must be taken to regulate light intensity during the year.
In sun plants, full sun promotes root development and increases root/shoot ratio. Leaves in full sun are thicker and small than in shade. Generally, high light decrease plant height and internode length, and increases dry weight and stem thickness. Plants grown in inadequate light are taller, with thin weak stems, and lower dry weight. It is important for the grower to learn to recognize the symptoms of too much or not enough light for a given crop.
Plant density and spacing patterns can have a profound effect of yield and quality of greenhouse crops because of the possibility of competition for light and mutual shading. Plants spaced too close together will loose lower foliage because of mutual shading. Many crops will also stretch when crowded. Equally important, however, is the net return per square foot of growing area. Some markets demand the highest market quality, e.g. Florists. While others are willing to accept lesser quality for better price, e.g. mass market. A greenhouse business must weigh quality and profit per square foot in deciding on spacing for a given crop.
Up until plants compete for solar radiant energy, decreasing plant spacing will increase yield per square foot without reducing quality. However, when competition for light begins, quality will be reduced. Plant spacing should be increased in the winter when light intensities are low and can be decreased in the summer
Maximizing Light Intensity
It is important to ensure the highest light intensity possible during the darker portion of the year from mid-fall through early spring except for shade plants.
The simpler the greenhouse support structure, the greater the light intensity inside. Since the 1950's, support structures have moved from wood to strong, light-weight metals. This has reduced the amount of structure needed and allowed an increase in glass size. The introduction of light-weight plastics has further reduced the need for support structure.
Shadows are cast from the greenhouse frame. The magnitude of these shadows depends on the angle of the sun and the season of the year. Single greenhouses should be oriented east to west along the long axis above 40° latitude so light can enter from the side in winter when the sun is low in the sky. Below 40° latitude, a single greenhouse should be oriented north to south.
Light incidence angle refers to the angle that sunlight strikes the glazing of a greenhouse. If light strikes the glazing at too steep an angle, it will be reflected. Light striking the greenhouse glazing at a 60° angle results in the least reflectance. Therefore, a roof angle of about 35° results in satisfactory light transmission.
Many greenhouses are shaded during the summer using a mixture of latex paint and water to reduce light intensity. Normally, most of this wears off with time and weather. However, a residue may remain through the winter. Dust can also accumulate on the glazing surface resulting in as much as a 20% reduction in light transmission. Dirty glass should be washed before the darker season of the year using a commercial glass cleaner. Rigid plastic glazing can also be washed with detergents.
Reducing Light Intensity
Shade adapted plants require varying degrees of light reduction during different times of the year. Many can withstand full sun during the winter but need protection during other seasons. Propagation areas also need light reduction to reduce transpiration when cutting have few roots. Light reduction is usually accomplished in one of two ways: 1)spraying a liquid shading compound on the glazing surface, or 2) installing shade cloth over the greenhouse or in the greenhouse above head height.
This involves spraying a white mixture latex paint on the glazing surface. This can be purchased from greenhouse supply companies (“Cool-Ray”). Shading compound can also be made by combining one part latex paint with 15-20 parts water. This is then sprayed on the glazing using a hydraulic sprayer. It is important that the applicator develop a smooth, uniform action and moves at a consistent speed to get uniform coverage. In large operations, shading may be applied by helicopter. Shading is usually applied in one or two application in late spring or early summer. Most shading compounds intended for greenhouse are designed to wash off with weathering during the summer and into the fall. However, it is a common greenhouse chore to clean the greenhouse galzing in the fall to insure maximum light transmition during the winter.
Shading can also be accomplished by covering a greenhouse with fabrics made of polypropoylene, polyester, or saran. These fabrics come in weaves of various densities to accomplish varying degrees of shading from 20 to 90 percent. For small applications, fabrics provide greater flexibility the shading compound.
The problem with the above two methods is that they cannot be adjusted for cloudy days or in the morning and evening on clear days. Many modern greenhouses have equipment that draws a sunscreen above head height and below the greenhouse structure. These may be operated manually by closing the screen gradually during the morning and gradually opening the screen in the afternoon.
Sunscreens can be automated by attaching the equipment to a photocell that continuously senses the light intensity and adjusts the screen according to set points for the crop being grown. Complex decisions about light intensity can be made by associating the operation of the sunscreen with an environmental control computer. The sunscreen can also double as a heat blanket by closing the screen at night.