The surprisingly low intensities needed for satisfactory rooting under favorable colors of light augur well for the possibilities of commercial application of,this system of propagation. Intensities between 100- and 200-foot candles have been used most frequently, but excellent results have been obtained with lower strengths. Two miniature propagating cases that have given outstanding results have provided intensities of 80- and 110-foot candles on the cuttings. With cold cathode tubes, normal rooting was often obtained under only 40-foot candles. In one experiment, using helium-filled, nonfluorescent cold cathode tubes, moderate rooting resulted in the normal period of time with only 10-foot candles. Cuttings root well under 16 hours of light a day or continuous light.

Well-insulated opaque structures provide an easy control of humidity that will usually balance naturally at a relatively high level. They also require very little heat in winter. A semi-underground shed, 10 by 36 feet, constructed of cinder blocks was operated during the winter as a propagating house for both cuttings and seedlings. Although the insulation was not perfect in many respects, the lowest temperature recorded inside after subzero nights was 58° F., and usually the temperature ranged from 60° to 68°. Aside from the lamps, no heat was provided other than a small 1-kilowatt electric heater, which was operated as needed. A brief check-up once a day by one man was the only attention given.

The requirements for producing good seedlings under fluorescent lamps are considerably different. Much higher light intensities are needed; most seedlings require at least 600- or 700-foot candles for proper development. This amount of light will produce excellent seedlings if temperatures are kept from exceeding those proper for growing. Raising the temperature may increase the light requirement above that which can be supplied from several fluorescent tubes. Distances of less than 1 foot from several 40-watt tubes in a reflector are most useful for growing seedlings, and for best results the seedlings can be placed so they almost touch the lamps. Fifteen or so hours of light a day is satisfactory with seedlings.

The quality of light is most important in growing good seedlings. Too much red will produce somewhat soft, blanched, sickly growth. But a predominance of blue light tends to produce short-jointed, stocky plants. Thus the type of seedling produced can be varied somewhat by changing the spectral balance of the light. The daylight and the soft white tubes produce excellent seedlings. Combinations of white and daylight tubes and also combinations of 3,500° white and blue tubes are useful.

Both seedlings and cuttings have been grown for long periods under artificial light of good spectral balance, like that provided by mixtures of white and daylight tubes, and have performed normally when moved into conventional greenhouses.

THE AUTHOR

F. C. Bradford is a senior horticulturist in the Division of Plant Exploration and Introduction, in the Bureau of Plant Industry, Soils, and Agricultural Engineering, He is a graduate of the University of Maine and has taught horticulture and has done research on that subject at the University of Missouri and at Michigan State College, He is the co-author of two books: Fundamentals of Fruit Production and Orcharding.