Photoperiodic Stimulation of Growth by Artificial Light as a Cause of Winter Killing.

The results of several recent investigations indicate that length of day or photoperiod is an important factor in determining the length of growing season in certain species of woody plants. The short days of late summer and autumn stop stem elongation and the production of new leaves in a number of species long before temperatures are low enough to check growth. This has been strikingly demonstrated by a hedge of Abelia grandiflora growing on the Duke University campus. Abelia is ordinarily quite hardy, but it has been observed that the plants in this hedge near the electric lights winter-kill badly, while those midway between the lights are never injured. A study was therefore made to determine the cause of this difference. The entire hedge was trimmed September 25 and at that time it was quite uniform in appearance. By October 20, when the first light frosts occurred, the plants on each side of the lights for a distance of about five yards were distinctly different in appearance from those farther away. They bore numerous new shoots which were soft and succulent, and covered with pale, green leaves which contrasted strongly with the short, woody shoots, and bronzed, dark-green color of the leaves farther from the light. Difference in color and amount of growth was so marked that it could be seen at a distance of over 200 yards. The number of new shoots produced since pruning was counted in a strip one foot wide at yard intervals from the lights. The results of 14 such counts were averaged and are shown in figure 1. It will be observed that, beginning five yards from the light, the number of new shoots decreased rapidly with increasing distance up to about 15 yards, where the number became constant. It was also observed that the plants near the lights continued to flower later in the season than the plants at a distance. Late in November, after several hard freezes had occurred, counts were made of the number of frosted shoots at yard intervals from the lights. It was found that the tips of all the growing, succulent shoots had been killed back two to six inches. The averages of ten such counts are shown in figure 1. It will be observed that the greatest number of dead shoots occurred at three yards from the lights, which is also the point of greatest light intensity, and that no dead shoots were found at a distance of nine or more yards from the lights. The light intensity amounted to only about two foot-candles at the brightest point, so it could not be measured satisfactorily with an ordinary 881