Supplemental Light

J. Raymond Kessler, Jr.


      Supplemental light refers to the addition of artificial light from electric lamps to ambient sunlight entering the greenhouse through the glazing material. Supplemental light involves adding to available ambient light for more rapid plant growth. This should not be confused with photoperiodic lighting which is used to modify the day length to control growth and development. Supplemental lighting involves higher light intensities (300-600 ft-c at plant height) from High Intensity Discharge (HID) lamps that are operated during the daylight hours. HID lamps are often turned on for 14 to 18 hours per day from 6:00 AM to 8-12:00 PM. The spectral output of HID lamps are mainly in the range useful to plants in photosynthesis (400-700 nm). Photoperiodic lighting involves much lower light intensities (7-10 ft-c) from incandescent lamps operated for four hours during the middle of the dark period. The spectral output of incandescent lamps is mainly in the red range that effects phytochrome.

When and How to Apply

      Supplemental light is mainly used during the time of year when ambient light intensity may limit plant growth. In north American, low light during winter in the greenhouse is due to a shorter day length, lower angle of the sun, and an increase in the number of cloudy days. In northeastern parts of the United States, light levels may be 20-30% of summer intensities. Overhead structures and glazing can decrease intensities by an additional 10%. Plants receiving inadequate light show increased time to flower, a reduced growth rate, stretched narrow stems, poor lateral branching, smaller leaves, and reduced flower size, e.g. reduced quality.

Example: Suppose seed of petunia are sown in a greenhouse every two weeks for 52 weeks of the year with the goal to bring the crop into flower in 10 weeks. Those crops that spend the principle amount of growing time in the summer will be ready to transplant in less amount of time than those grown in the winter. Those spending the principle amount of growing time in fall and spring will have an decreasing or increasing production time, respectively. The change in the amount of production time required through the seasons is largely due to changes in natural light intensity and duration. This presents the greenhouse producer with a scheduling problem. Crops sown in the late fall will have to be sown earlier, say allowing 12 weeks production time, while those sown in late spring will have to be sown later, say allowing an 8 week production time. The goal of supplemental light is to add to the limited natural light available during the winter to shorten production times and smooth-out the curve presented above.

      In the southern US, supplemental light may only be useful from late November through early March. Even then, it may be more cost effective to turn the lights off during the middle of a sunny day. Total light intensities (supplemental plus natural) above 5000 to 8000 ft-c probably justify turning the lights off, depending on the crop.

      It is usually not cost effective to supply supplemental lighting to crops in the finishing stage. The large number of lamps required to light a large area with relatively low plant density and the high cost of electricity make the operational cost verses profit from the crop cost prohibitive. When supplemental lighting is most useful:

          Supplemental lighting is more effective on young plants than older plants. It is most often used on young seedlings or in cutting propagation.

          Supplemental light is most economical when plant densities are high, e.g. young seedlings and cutting propagation. In other words, the greater the number of plants per unit area, the fewer number of lamps and electricity will be required to obtain a set light intensity per plant.

Supplemental light should provide light in the photosynthetically active radiation range (400-700 nm) of the light spectrum. Light intensities of 300 to 600 foot candles are frequently used in the greenhouse and 400 foot candles is a good starting point.


Bedding Plant Seedlings

      On plug seedlings, supplementary light is generally applied 16 to 18 hours per day for four to eight weeks beginning soon after germination. For bedding plants, application is usually most efficient during the first half of the production cycle. Most bedding plants may be classified as 8, 10, 12, or 14 week crops. This refers to the time from sowing to flower. A 10 week crop such as impatiens or petunia would receive five weeks of supplemental light. However, optimum supplementary light intensity and duration can vary by crop type, geographic location, and time of year. Durations longer than 18 hours per day for more than eight weeks generally produces diminishing returns or may be harmful.

      It is both convenient and appropriate to apply supplemental light during the plug flat stage of bedding-plant seedling production. Application during this time accelerates seedling growth and reduces the time from sowing to ready to transplant.

      For day-neutral plants, flowering is a matter of the plant obtaining a certain minimum size or stage of development. For these plants, time to flower has been found to be inversely correlated to cumulative photosynthetically active radiation (CPAR). In other words, the higher the total light a plant receives, the greater the dry weight gain (growth) and the fast the plant will obtain the minimum size to flower. Supplemental light accomplishes this by reducing the time of the vegetative phase (time from sowing to floral initiation) but has little effect on floral development.

      Several studies have shown that application of supplemental lighting to plug-grown seedlings under winter greenhouse conditions can result in accelerated time to transplant, increased dry weight, increased leaf area, and more branching. Excellent results have been obtained on begonia, geranium, ageratum, petunia, and salvia. Begonias provided 0, 50, 125, or 200 µmol m-2 s-1 metal-halide supplemental light in the greenhouse for 2, 4, 6, or 8 weeks reached a transplantable size from sowing after four weeks at 125 µmol m-2 s-1. Additional light intensity and/or duration did not result in faster transplant times. Considering that six to eight weeks may be required in the greenhouse during winter, supplemental light could result in a two to four week reduction in crop time.

      The benefit of supplemental light applied in the plug stage often carries over to finish plants. This carry-over effect once plug seedlings are transplanted to final containers, results in finish plants that obtain a marketable size and flower sooner than untreated seedlings. Plants that received supplemental light as seedlings often show greater branching, greater dry weight, thicker stems, and thicker, larger leaves at finish.


Lamp Types

      HID type lamps are generally preferred for supplemental lighting because of high efficiency, uniform light distribution, and lower amount of shading from the fixtures.

Incandescent: These are not used for supplemental light purposes because of excessive heat, poor light quality for growth, and low efficiency (conversion of electricity to usable light is about 7%, the rest is lost as heat). High red light and the absence of blue light causes excessive internode elongation and small leaves at growing intensities. However, this is the lamp of choice for photoperiodic lighting where red light is desired and low light intensities are used.

Fluorescent: These lamp are most often used in growth chambers (rooms) or in small seed germination setups. The most efficient of these lamp types are cool-white and warm-white bulbs, able to convert about 20% of the electricity they consume to usable light. Light emitted by these bulbs is predominately in the blue spectral region. Special fluorescent bulbs with phosphors emitting light in the red region have been developed called grow-bulbs. The spectral output of these bulbs more closely compares to the action spectrum for photosynthesis. In many studies however, plant growth is not any better under grow-bulbs than under cool-white. The ballast and fixtures for fluorescent lights are large for the amount of light produced so to install enough fluorescent lamps in a greenhouse to supplement natural sunlight would completely shade the benches, therefore is not practical.

High-pressure Mercury: An older kind of HID lamp with a spectral output similar to fluorescent. These lamps are available in sizes up to 1000 W. This bulb type has been largely replaced by more modern types because of its low efficiency (13% conversion).

Low-pressure Sodium: These types of lamps are not used for supplemental light purposes because of poor spectral qualities and low intensity. However, this is the most efficient lamp for lighting with a conversion rate of 27% and a long bulb life. These lamps are available in sizes from 35 to 180 W.

High-pressure Sodium: These lamp types are the most widely used for greenhouse installations. They have acceptable spectral output and intensity, are inexpensive to purchase and install, have long bulb life, and have high efficiency (25% conversion). High-pressure sodium lamps are available in 250, 400, and 1000 W sizes. The 400 W is most commonly used because the fixtures are compact and less expensive.

Metal-halide: These lamps are the lamp of choice for most new installations. The spectral output of these lamps is the best for plant growth than any others. These lamps are slightly less efficient than high-pressure sodium (20% conversion) but this is being improved.

      New lamps and fixture designs are being developed all the time that use less electricity, that are more efficiency and operate cooler. Over the last ten years, fixture designs with optimized geometric shapes for reflectors have maximized light reflectance so that 90% of light generated by the lamps is directed on the plants. This has resulted in the need for fewer lamps. Horticultural fixtures have been developed to direct the light in a square pattern as broadly and uniformly as possible while minimizing fixture size. Special fixtures have even been developed for the ends and sides of the greenhouse that directs light into the greenhouse that would otherwise strike the wall.


Installation Layout

      General recommendations for lamp number and spacing for a supplemental light installation are difficult to provide. This may depend on;

          Lamp type, wattage, output intensity, and lamp efficiency.

          Fixture and reflector design and reflection efficiency.

          Crop type and intensities needed for growth.


      Companies specializing in horticultural lighting should be consulted for optimal designs for a particular installation. However, the number of lamps per unit growing area can be estimated.


1.   Start with the number of foot candles recommended for the crop to be grown and convert ft-c to lux. Ex. 1ft-c = 10.8 lux, if 240 ft-c is needed; 240 × 10.8 = 2592 lux

2.   Calculate the square feet of growing area to receive 240 ft-c supplemental light. Ex. a 4 × 8 bench is 32 square feet.

3.   Determine the luminous flux (in lumens) output of the specific lamp to be installed (see table). Ex. Metal Halide (400 W) = 36,000 lumen

4.   Calculate:


Lamp Number = [Light Required (lux) × Lighted Area (ft2)] / = Output of 1 lamp (lumens) =

      [2592 lux × 32 ft2] / 36,000 lumen = 2.3 lamps


Lamp Outputs for Horticultural Lighting

Lamp Type

Lamp Wattage

Luminous Flux (lumen)


150 W


Fluorescent Cool-White

40 W


Fluorescent Cool-White HO

110 W


Fluorescent Cool-White VHO



HID Mercury Vapor

400 W


HID Metal Halide

400 W


HID High-Pressure Sodium

400 W


HID High-Pressure Sodium

1000 W




      Economic considerations in choosing a supplementary light installation are important. First, determine if the benefits obtained from installing supplementary lighting will justify the investment. Some regions of the country have adequate sunlight for good growth so the benefits could be negligible. Next, choose a lighting fixture type that efficiently converts electricity to usable light, provides a usable light distribution pattern, is designed for the greenhouse environment, and is cost effective to install and maintain.


Total installation (fixture/ wiring/ bulb)

                                                                  Per fixture

      High-pressure Sodium 250 W            $247

      High-pressure Sodium 400 W            $248

      High-pressure Sodium 1000 W          $385

      Metal-halide 400 W                            $251


      One source estimates that it costs about $2 / sq.ft. for installation or about $250 per lamp/fixture. About 30% of that cost is for the bulb. Payback for a supplemental light system, given substantial crop improvement, has been estimated to be from 2½ to 3 years.

      Another estimate sets the cost at $2.25 / sq.ft. for a high-pressure sodium installation. A 400 W fixture costs $150 to $170 and the bulbs $50 to $75. One lamp provides a light intensity of 400 ft-c over a plant area of 100 sq.ft. A 400 W bulb used about 465 W of electrical energy per hour. Cost per hour for a particular can be calculated:


      Cost/day = ((# of lamps × 465 W/h × operation hours/day)/1000) × cost $ /KWH