The quantity of light plants receive and the quality of that light can influence growth and stem extension.
Quantity. One of the biggest mistakes Runkle says he sees is too much supplemental light being used in the greenhouse on sunny days. In the average crop, photosynthesis increases to a certain point, the light saturation point. At that level, around 3,000 footcandles, the extra light is no longer taken up by the plant as energy.
Finding the right balance is critical, however, as more light can means more beneficial characteristics. In research on New Guinea impatiens, more light on cuttings meant more rooting more quickly.
What if you can’t use supplemental lighting over the whole range? When is the best time to give more light to young plants? Research on petunia ‘Madness Red’ shows that if divided into thirds, the best third of the propagation cycle to use supplemental lighting with high-pressure sodium lamps is the last third. If able to light for longer periods, supplemental lighting during the last two thirds was better than their first two thirds.
Quality. Did you know that lamp type can affect stem elongation? Incandescent lights, which emit a lot of far-red light, have been shown to produce taller plants. Plants spaced close together in the greenhouse exhibit more stretch, because the plants are completing for light.
For more details on temperature and light, go to the Michigan State Floriculture page.