Greenhouse-Garden-Grass, 7(2): 4-7 (1968)
Growth and flowering of African violets under artificial lights.
Vernon A. Helson

Young plants of the saintpaulia varieties Pink Beauty and Blue Boy were grown under different light intensities from different sources in an experiment to determine the light requirements for growth and flowering.

Standard gro-lux fluorescent lamps, a light source providing high energy in the red and blue regions, and low energy in the green region of the spectrum were designed and marketed by the Sylvania Company especially for growing plants under artificial light (Sylvania Lighting Products Eng. Bull. 0-230, 1961). The object of this investigation was to determine whether the growth and flowering of African Violets (Saintpaulia ionantha) were better when grown under gro-lux lamps as compared with cool-white lamps, and whether the far-red from incandescent lamps was necessary for better growth and flowering of the plants.

An experiment was set up to determine the light intensity required for flowering of African Violets. Young plants of Pink Beauty and Blue Boy, well established in John Innes potting soil in the greenhouse, were placed on a rack at different differences from a light panel in a growth room of the Plant Research Institute Phytotron (Fig. 1) to give 1000, 400, 200 and 100 ft-C of light from a mixture of cool-white and incandescent lamps, as measured with a Weston model 765 illumination meter. The photoperiod was 16 hours and the temperature was held constant at 25°C. After 66 days under these light intensities, the most abundant flowering for both Pink Lady and Blue Boy was under the 400 ft-C and flowering had started under the 200 ft-C (Fig. 2 and 3). These results indicated that the optimum light intensity for African Violets was around 300 ft-C.

Next, an experiment was initiated to compare gro-lux and cool-white fluorescent lamps with and without incandescent as light sources for the growth and flowering of African Violets. Blue Warrior plants were propagated from leaf cuttings in the greenhouse. Six months after placing the leaves in sand, the small plants were transplanted into 4 inch clay pots of soil and 8 plants were placed under each of the four different light sources. Four temporary 28 inch cube cabinets were made up with aluminum angle for framing, aluminum building paper for the sides, and expanded aluminum for the bottom and top (Fig. 4). The light panel in each cabinet consisted of three F20T12 fluorescent tubes spaced at 9 inch centres. When incandescent was used, two 10 watt bulbs were placed between the tubes so that the gro-lux and cool-white panels supplied 11 watts/sq. ft. and the gro-lux cool-white with incandescent supplied 15 watts/sq. ft. The plants were placed 20 inches from the cool-white fluorescent with incandescent to give a light intensity of 300 ft-C. The plants under the cool-white fluorescent without incandescent were raised to give the same light intensity. Plants under gro-lux lamps with and without incandescent were placed the same distances from the lights as those under the cool-white lamps. Light intensity readings from gro-lux lamps as compared with those from cool-white lamps have little meaning as the photocells in illumination meters have high sensitivity in the green portion of the spectrum where energy emitted by gro-lux is low. The temperature of the cabinets was not controlled but the cabinets were located in a large room where the light temperature was around 25°C, the dark temperature around 20°C, and the relative humidity about 30%. The lights were on for 16 hours each day.

Conclusions

  1. Fifteen watts per sq. ft. of fluorescent light giving about 300 ft-C are adequate for good vegetative growth and flowering of African Violets.
  2. Gro-lux fluorescent lamps with some incandescent light produce more vegetative growth and more flowering than cool-white with incandescent or cool-white or gro-lux alone.
  3. Flowering of African Violets is increased when the far-red from incandescent light is added to either grow-lux or cool-white fluorescent light.
  4. Also, gro-lux lamps may have an aesthetic value for the hobbyists in that the high blue and redemitted by the lamps causes the leaves to appear darker green and the flowers a brighter red or blue.