 plants have long petioles, large thin leaves, and few, if any flowers. Shading can be applied to the greenhouse glazing in March and a second application, if required, in May to reduce light intensity. This should be removed in October. The installation of an automated shade cloth system can provide a finer degree of control over light levels. Shade systems are often placed under computer control.
Growing African Violets under artificial light has been used effectively by amateurs and professionals for years. Commercial growers desiring to make more efficient use of greenhouse space have developed multi-layer bench systems in which plants on top receive natural light and those on lower layers receive fluorescent lighting. Installations for this purpose should be designed to provide 600 to 900 foot-candles with a photoperiod of 14 to 18 hours per day. This may be accomplished by mounting two, dual-bulb, 8-foot fixtures 10- to 12-inches above the plants. Lighting companies manufacture special fluorescent bulbs for growing plants, however, several studies have been unable to show that these lamps improve plant growth compared to cool-white or warm-white bulbs. Flowering of African Violets may also be improved by adding incandescent light at 10% of the total wattage. HID lighting from high-pressure sodium or metal halide lamps have also been used to grow African Violets either as a sole light source or to supplement to natural light.
The desired greenhouse humidity depends on light and temperature. African Violets usually grow well at a 50% to 70% relative humidity. During the summer, every effort should be made to keep the humidity up when temperature and light are high. Low humidity at this time can desiccate flower petal margins resulting in petal burn. Humidity may be raised by using evaporative cooling or by simply wetting walks with a hose several time a day. In the winter, high humidity, especially at night, can lead to disease problems. When the humidity approaches 100% at night, Botrytis can infect the flowers. One solution to this problem is to run a ventilating fan at night controlled by a humidity sensor.
Supplementing the greenhouse atmosphere with additional carbon dioxide to 800 to 1000 ppm increases African Violet growth and may allow plants to be grown at a lower light intensity without sacrificing quality. This technique is especially applicable during the winter months when light levels are lower and less frequent ventilation is required.
African Violets have a very fine root system and therefore require a well-aerated, well-drained medium high in organic matter. Media may be commercially available mixes or be prepared on-site. In either case, the medium should have a low soluble salts and a high water- and nutrient-holding capacity. A recommended starting media for mixing on-site would be either the Cornell A mix or the Cornell Gesneriad mix, which consists of peatmoss plus vermiculite and/or perlite. These media should be amended with dolomitic limestone to a pH of 5.8-6.2. Some growers include low rates of micronutrients and superphosphate, while others supply major and minor elements exclusively through the liquid fertilization program. Soil-borne fungal diseases can be a serious problem so many recommendations call for media pasteurization.
African Violets can be classified as light feeders, preferring a low steady supply of nutrients from a balanced fertilized. Preferably, this should be a liquid fertilizer with low salts index, high nitrate, and low ammonia and urea, such as 15-16-17 peat-lite special, 14-12-14, or 15-15-15. A constant fertilization rate of 100-125 ppm nitrogen is adequate, along with one clear watering per week. Soluble salts should be about 0.8-0.9 mmhos/cm (2:1 extract) for young plants, 1.2-1.4 mmhos/cm for plants about six weeks after potting, and no higher than 1.7 mmhos/cm at the finish. In general, newly potted plants should not receive fertilizer until the second or third week or when roots reach the side and bottom of the pot.
African Violets thrive best when the soil is 