ers. 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 maintained uniformly moist, but not saturated for any length of time. When the media is allowed to dry to the wilting point, growth is stunted and the plants never seem to fully recover. This is often because of root damage from concentrated soluble salts. On the other hand, saturated medium deprives the roots of oxygen. Root damage from either moisture extreme provides an opportunity for crown rot disease to develop.
In all seasons, it is a good practice to overhead water early in the morning so that the foliage drys quickly. Overhead watering can be used on African Violets up to flower opening. However, it is of utmost importance that the water be tempered close to the foliage temperature. If the water is too cold or too hot, chlorotic circles appear on the leaves, called 'ring spot'. Many greenhouses temper the water with large water heaters or heat exchangers associated with the boiler heating system. A good rule of thumb is to maintain the water temperature 8F plus or minus the air temperature. A 65 to 75F water temperature is generally safe. Once flowers begin to open, many growers use tube watering or some form of subirrigation such as capillary matting or ebb-and-flow systems, because the lifetime of the bloom is decreased by directly applying water overhead. The frequency of fertilization should be cut in half when supplied through subirrigation.
African Violets can be propagated from seed, but only a few cultivars are currently available that will come true from seed. The major means of propagation is by leaf cuttings, though some progress is being made using tissue culture.
Growers who propagate using leaf cuttings maintain an extensive stock plant program and allocate a large area to leaf and plug flats. This requires investment in greenhouse space and labor both to perform the propagation and to maintain the stock plants. Many of the cultivars better suited for commercial production are patented. Therefore, propagation should not be done without a propagators licence. The decision to propagate in-hou