Erica's Original Notes The length of day or night triggers Nature's switches, as in winter when the sap thickens, promoting resistance to cold. Plants may belong to one of the following groups:
Short day: These plants need a long uninterrupted night in order to form buds; violets and goldenrod belong to this group; just a flash of light will upset the rhythm. Chrysanthemums can tell the difference between 14 and 15 hours of light; growers use these time clocks in greenhouses to control flowering, and this is one of the boring reasons we find mums in florist shops throughout the year!
Long day: These plants are typified by irises and clover.
Neutral day: Roses, carnations and African violets are in this group, which needs 12 hours of night. Nighttime is plant growth time, while daytime is for photosynthesizing, a time for assimilating vitality. Arctic plants are stunted by light due to the long summer hours.
Rhythms: There are many rhythms affecting the plant world: circadian (daily), lunar, solar, and seasonal; so far, only the hands of the biological clock have been studied, but the nature of the clock itself is unknown.