Light Harvesting

• The formal name for black light is ultraviolet light, and it's actually invisible to both human eyes and plants. Black light features wavelengths too short to register within the visible light spectrum of colors that range from red to violet. Plants have never developed an ability to harvest black light and convert it to chemical energy through their photosynthetic processes precisely because of black light's short wavelengths. Common "black lights" are often actually deep blue or deep violet and not true black lights.
  
  

Plant Exposure

• If a plant were to be grown exclusively under true black light it would be unable to engage in photosynthesis to any great extent. Plant growers who experiment with commercially available ultraviolet lights for plant growth are benefiting from the blue light found in those lights. In fact, the shortest wavelength, 200 to 280 nanometers (nm), of black light is highly toxic to plants. Light isn't even helpful to plants until its wavelengths reach 380 nm.
  
  

Optimal Growth

• Plants will grow their best when exposed predominantly to red light and blue light sources, such as sunlight and even some fluorescent light sources. Violet light, because it's a part of the blue band in the light spectrum, can be partially absorbed by plants. However, plants grown under violet light won't grow very robustly or very vigorously. To ensure a minimum of plant growth from light, try to obtain light from sources in the 400 to 700 nm range.
  
  

Considerations

• Black light at 200 to 280 nm is UVC light while 280 to 320 nm is UVB light, and both damage plants. Black light/UVA light, which has a wavelength of 320 to 380 nm, neither helps nor hurts plants. Because the colors within the visible light spectrum blend one into another, plants are usually assured of benefiting at least minimally from any light shown on them. Plants partially absorb yellow, orange, indigo and violet light, but will reflect green light away.