African Violets, particularly in the leaf flat and plug flat stages. Maintaining a medium temperature of 68F can reduce production time by two weeks and improve plant quality and flowering.
Temperature is the main factor used by growers to speed up or slow down flower development as the crop approaches finish. African Violet flower development can be divided into nine stages:
1. Bud visible in the leaf axil (2 mm long).
2. Flower stalk begins to elongate.
3. Flower stalk begins to bend.
4. Flower stalk curves over to protect primary bud.
5. Flower stalk completely curved.
6. Inflorescence pokes through leaf canopy, flower starts to straighten.
7. Flower stalk straightens out.
8. Primary flower opens.
9. Five flowers open per plant.
If the crop is not on schedule, temperatures may be raised to speed flowering or lowered to delay flowering by identifying the stage of flower development and using the temperatures in Table 1. Where possible, the temperature can be reduced a few degrees in the last week of a crop to enhance flower color and size.
|Table 1. Days from visible bud to five open flowers on African Violets.*|
|Temperature||Inflorescence Development Stage|
* Adapted from Faust and Heins, 1994.
One reason African Violets do well in most homes is because they are basically shade-adapted plants. Light intensity is the primary factor regulating both the time to flower initiation and the number of flowers produced. The usual recommendation is for 800 to 1200 foot-candles measured in the middle of a clear day. However, this depends on the temperature and season. During the winter, higher light intensities (1200 foot-candles) can be used when temperature is more controllable and the number of hours of natural light is short. In the summer, lower light intensities (800 foot-candles) are best to help control heat.
Plants receiving too much light produce hard brittle growth and are generally stunted, with short petioles and bleached foliage. Excess light can actually burn both flowers and leaves. Without enough light, 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 flow