Summary: Plant light has 3 dimensions or qualities. Understanding these lighting dimensions, the type of plants grown and how all these factors work together can help you enjoy your plants much more.
In years of drought, with little moisture to produce morning mists, and with no clouds in the sky, both duration and intensity of light are enough to keep them from flowering satisfactorily.
The sensitivity of some plants is amazing.
If certain annuals or perennials fail to bloom, check their surroundings. Often a grove of trees to the east or west, or tall buildings, will reduce morning or evening light intensities enough to throw them out of step.
The human eye is such a sensitive instrument for the detection of light and adapts itself so readily that it is a poor medium for judging whether light is adequate. We need a kind of light meter that will read the spectrum to which plants respond, so as to measure effective light quality.
Light Composition
This brings us to our third dimension of light, that of composition. Only the visible spectrum is useful to plants. Different colors of light can affect the plants differently in growth and development- blue light, red light, green light, white light, etc. Blue light helps encourage vegetative leaf growth. Red light, together with blue allows plant to flower. Plants do not respond to invisible light rays in the ultra violet range, and invisible red is actually heat.
Ordinarily, composition outdoors is not a problem. When we grow plants indoors, however, composition immediately becomes important.
An artificial light source, either fluorescent or incandescent, a combination of the two is best.
For rooting cuttings under artificial light, where the heat from incandescent light bulb is excessive, the warm white or soft white fluorescent tubes are used because they supply more of this red quality.
This is at best a sketchy explanation of the role or effect of light on plant growth. It is presented with a hope that it will stimulate further study by interested gardeners.
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Images: Forban300 | Sean McGrath