Does a Black Light Help Rose Bushes to Grow Better?

Black Light

• Black lights, or ultraviolet lights, emit light waves with a shorter wavelength than human eyes can perceive. Most black lights that are commercially available also give off some purple light. Small amounts of ultraviolet light can have some beneficial effects on roses, but too much ultraviolet light is harmful.

Effects

• The photosynthesis process is reduced in roses growing under black lights, which means that they don't grow as much. According to Ann E. Stapleton of the Department of Biological Sciences at Stanford University, ultraviolet lights cause mutations by damaging plant DNA. However, ultraviolet light found in sunlight can also help roses survive by stimulating them to deposit compounds that absorb ultraviolet rays in the outermost layer of cells. Exposure to ultraviolet light also encourages roses to produce enzymes that help repair damage caused by those lights. One side effect of their response to ultraviolet light is an increased ability to repel disease and pests because their biochemistry is modified.

Colors

• Sunlight emits all the colors in the visible spectrum, along with ultraviolet and infrared light, but only certain colors, like red and blue, are very important for plant growth. Both red and blue lights affect growth and photosynthesis. Red light controls the production of seeds and flowers, so it's especially important for roses. Blue light controls the development of leaves. Plants that receive too much red light grow spindly, while plants that receive too much blue light don't produce many flowers and become stocky.

Light Types

• Black lights don't provide roses with the amount of red and blue lights they need for photosynthesis. According to plant education author and consultant Dr. David Hershey, plants die when grown under just black lights. Incandescent lights emit a lot of red light, but not much blue. In addition, they produce a lot of heat, which can burn plants. While incandescent lights can be used to provide foliage plants with supplemental lighting, they are a poor choice for flowering plants like roses, which have higher light requirements. A combination of cool white and warm white fluorescent lights, on the other hand, provides a good mix of blue and red lights. Special plant-growing fluorescent lights are also available that that emit more red light, which is especially good for growing rose blossoms.