Chapter Ten - Artificial Lighting

      Greenhouse gardeners are used to harvesting summer crops one or two months earlier than their neighbors. Later in the season, the greenhouse environment allows them to harvest one or two months after other gardens have closed for the winter.
      With just a small investment of time and energy, any greenhouse can be used to grow summer as well as winter crops nearly all year round, or until freezing temperatures kill summer plants.
      In temperate zones the amount of light reaching sea level can vary considerably. In midsummer at noon on a clear day, I’ve recorded over 10,000 lumens at the 40th parallel. (A lumen is a measure equal to one footcandle per square foot.) In midwinter at noon, the same area received only 950 lumens. Fruiting plants require 2,000 to 3,000 lumens to produce, and 1,000 to 2,500 lumens to maintain life processes and slow growth. Non-fruiting plants, such as greens and root crops, generally require from 1,000 to 2,500 lumens.
      Many greenhouses are already insulated and solarized to take advantage of the “greenhouse effect.” The only thing that stops their plants from producing “summer” crops throughout the year is an adequate amount of light to support rapid growth.
      Standard fluorescent fixtures are not the answer. The fixtures themselves block all sunlight that would reach the plant directly. Only ambient light, coming from the sides, gets through. The light is distributed unevenly, since the ends of a fluorescent emit considerably less light than its center area.
      One way of increasing the efficiency of fluorescents is to build your own fixtures so that the spaces between the tubes are not blocked, and more direct light reaches the plants. Aluminum foil can be shaped to make reflectors, so that light that normally escapes from the top or sides of the tube is reflected downward. Use the dull side of the foil as the reflective surface. It reflects as much light as the bright side, but distributes it more evenly.
      Lighting technology has increased the options of the home gardener. Besides the standard fluorescent, he/she can choose from
      Very High Output (VHO) fluorescents, mercury vapor lamps, metal halide lamps, high-pressure sodium vapor lamps, and low-pressure sodium vapor lamps.
      VHO fluorescents look just like the standard fluorescents, but use about three times the energy and emit about 2½ times the light. Although they are not as efficient as the standard lamps, they are much more convenient to use. Two of them replace a bank of five standard tubes. VHO tubes require a different ballast than do standard ones, but are the same size, so that more natural light is allowed to reach the garden. The tubes are positioned 6 to 12 inches from the tops of the plants.
      A fruiting garden lit entirely by fluorescents requires about 20 watts per square foot of growing space. For example, a 4 x 8 foot garden (32 square feet) needs about 640 watts. At 72 watts for an eight-foot standard tube, this means that about nine tubes would be required. Most ballasts light pairs of tubes, so eight or ten would be the actual number. Eight tubes would be spaced at two per foot of width. A VHO fluorescent, on the other hand, uses 215 watts, so only three would be required. Even if four were used, there would be only one per foot of width.
      Almost all gardens have bright spots and darker areas. Plants that require less light can be placed in the darker areas: for example, beneath the ends of the tubes, where the light emissions decrease dramatically.
      Since greenhouse gardeners can count on some natural illumination, they do not need as many lamps. Almost all naturally lit gardens can get by on about 10 watts of electric light per square foot, so that the 4 x 8 foot garden in a greenhouse actually requires only half the number of tubes calculated above. Of course, if more light is used, the growth rate increases.
      Fluorescents come in a number of different spectra, such as daylight, warm white, cool white, deluxe warm white, or deluxe cool white, as well as purple grow tubes. Many other spectrum choices are available. Each of these tubes is coated on the inside with a slightly different phosphor, which glows when it is activated by electric current. Each phosphor emits a unique spectrum of light, with the cool whites tending toward the blues, and the warm whites toward the reds. The term “deluxe” indicates that more redemitting phosphors have been added. Although a garden using only one kind of tube will do fine, for best results the tubes should be mixed.
      Grow tubes emit relatively high amounts of red and blue light. They were developed after it was discovered that chlorophyll uses red and blue light most efficiently. These lights were designed to approximate the photosynthesis (or chioroplast synthesis) curve. However, they emit less than 65 percent the light of other tubes (1,950 lumens). In experiments and informal inspections, I have observed that the responses of plants vary more with respect to the total lumens received than to the particular spectral pattern of the light. As long as the plants receive some light from each part of a broad spectrum, they will adjust and grow well. In practice, grow tubes do not work as well as other fluorescents.
      Fluorescent tubes emit a lessening amount of light over the course of time. After a couple of years, they may emit less than 65% of their initial output. On-off cycles wear the tube out faster than continuous use.

      Fluorescent lights use a ballast to convert electricity to a high-voltage, low-power current. Older ballasts were often insulated in PCBs, so it pays to buy new equipment, rather than risk a health hazard. Usually it is cheapest to buy a unit and rewire it onto the new frame. Every ballast has an easy-to-follow wiring diagram attached to it. The ballasts are the heaviest part of the fluorescent unit. For easy handling, it is best to separate the ballast from the lights. The ballasts can be placed on a board in a remote location, and the lights attached when the setup is in place.
      A convenient way to light the garden is with a bank of U-shaped tubes, or with 8-inch round fluorescent converter fixtures. These units are designed to change a standard light fixture to a fluorescent one with a built-in ballast. A 2 x 3 foot board can be used to mount standard fixtures, and then the fluorescents can be screwed in. They can also be hung independently from a rafter or frame, using a thin string, with each fixture attached to its own electric cord. I grew a garden in which I suspended these lamps so that they hung between the plants, not directly above them. This brought the lights very close to the flowers.
      Metal halide lamps are the white-light street lamps used to illuminate everything from parking lots to night events. They are available in a number of wattages, but the two most convenient ones to use are the 400 and 1,000 watt sizes. They emit a mean of 32,000 and 92,000 lumens respectively, and are slightly more efficient than a standard fluorescent. Mean lumens are used because the the lamp output changes as it wears out. 400 and 1,000 watt metal halide lamps have an initial output of 40,000 and 110,000 lumens respectively. A 400 watt metal halide lamp can be used as a supplementary source of light for an area of about 7 foot x 7 foot, or about 50 square feet. A 1,000 watt halide can provide illumination for about a 10 foot x 10 foot area, or a garden of about 100 square feet.
      Plants seem to respond better to strong sources of illumination which are dispersed rather than to a moderate source which is emitted over a wider area. More light penetrates to the inner foliage. A metal halide’s efficiency is increased when the halide shuttles back and forth, illuminating each garden spot brightly for a limited period of time. The area that the lamp can illuminate may be increased by about 30%, or the lamp can cover the same area with more light.

      Metal halides promote vigorous growth. Fruiting patterns, which are somewhat slower using fluorescents, quicken; fast growth can be maintained even in midwinter. When I lived in an apartment, I grew tomatoes and herbs using halides in an unused hallway. I was eating juicy, ripe tomatoes 120 days after planting the seeds.
      Sodium vapor lamps are the amber or orange-looking lamps used to illuminate streets and roads. Their spectrum is centered in the orange to red bands, but they emit some blue light. These lamps, too, come in 400 and 1,000 watt sizes. The 400 watt size emits 50,000 lumens initially, and has a mean rating of 445,000 lumens. The 1,000 watt lamp emits about 140,000 lumens initially, and about 126,000 mean lumens. It is about 1½ times as efficient as a fluorescent.
      I have grown plants from seed using these lamps as the sole source of illumination, and the growth rate was about the same as with metal halides. Some gardeners claim that flowering and fruiting are increased because of the emphasis on the red spectrum. When the lamps are used as a supplemental light source, their spectrum becomes less important, since natural light will supply an adequately wide spectrum.
      Low-pressure sodium vapor lamps emit an orange light that covers only a narrow portion of the visible spectrum (680-690 nanometers). They are very efficient. The largest size, a 180 watt unit, emits about 33,000 lumens, and is more than twice as efficient as a standard fluorescent. The lamp is encased in a three-inch-wide glass cylinder and consists of a U-shaped tube with the diameter of a neon light. It is powered using a ballast.
      This lamp cannot be used as a primary source of light because plants grow stunted at first and eventually die. Low-pressure sodium lamps do work well as a secondary light source, though, and are an efficient way to increase the total number of lumens reaching the plant. They are especially good at brightening up dark spots in the garden.
      Mercury vapor lamps also come in 400 and 1,000 watt sizes, and emit 12,800 and 47,700 lumens respectively. They emit a white light similar to a metal halide’s, but are not nearly as efficient, so they should not be purchased new. If they are available used, though, they may be worth having.
      For the most efficient use of any of these lamps, the reflectors should direct the light to the plants, not to the walls or ceiling of the room. Some commercial reflectors do not cover the lamps entirely, so that much of the light is directed horizontally rather than vertically or diagonally. Reflectors can be modified and customized using aluminum foil. It is easily held on by tying down with thin wire such as picture-hanging wire, which is not affected by the high temperatures.
      Artificial light should be provided while natural light is present, so that the plants get as high-intensity a light as possible. If the garden is partially shaded, for instance, in the early morning or late afternoon, the lighting system may be used on a multi-cycle each day, being turned on and off several times, to supplement the natural light during the darker periods.
      It is almost essential to put the lamps on a timer, so that they go on and off with regularity. As the natural light cycle changes with the seasons, the electric lights should be adjusted. During the late spring, supplemental lighting may be appropriate only at the beginning and end of each day. The lamps are generally not needed during summer. In early fall, they may be used during cloudy weather and at the beginning and end of the day. By late fall they may be used to supplement the weak sunlight all day, and may continue in this fashion until early spring.
      Metal halide, low and high pressure sodium lamps, as well as accessories to shuttle the lights, are available from a number of horticultural supply companies.