Plants use sunlight for photosynthesis and are exposed to ultraviolet (UV) radiation that is present in sunlight. UV radiation is divided into 3 classes: UV-A, UV-B, and UV-C. The Ultraviolet-C (UV-C) region of the UV spectrum includes wavelengths below 280 nm; these highly energetic wavelengths are absorbed by ozone and are not present in the sunlight at the earth's surface. Under normal growing conditions, effects of UV-C light are not seen on plants. This research examined the effects of the application of ultraviolet-C irradiation (UV-C) on greenhouse ornamental plants and demonstrated very promising uses of UV-C as a treatment to increase branching and reduce the height of plants, and in some situations, affect the rate of flowering. Several conclusions have been made from this research that are consistent with multiple plant species. First, the amount of UV-C light that a greenhouse plant receives is critical to its response. The proper weekly dosage, for as little at 15 min a week, will control a plant's growth response. In addition, too high a dosage of UV-C irradiation will burn plants and too low will have no effect. Second, proper applications of UV-C light decreases final plant height. Several plant species have responded to applications of UV-C light by growing shorter than the control plants that receive normal greenhouse lighting. Third, UV-C light increases branching of greenhouse plants. At appropriate dosage rates, UV-C light increases branching on some species and increases the number of flowers that are produced. This avoids the need to pinch plants and to apply plant growth regulators. Fourth, the application of UV-C light can affect flowering time. The application of UV-C irradiation can either delay flowering or cause earlier flowering depending on plant species and dosage rate. In some cases, the increased branching is accompanied by delayed flowering.
