Temperate plants have evolved under a 24 hour cycle of light and dark and 12 month seasonal variations in day length. These natural cycles of light provide cues for regulating circadian rhythms, seasonal phenology and the expression of phenotypic variation in plants. Artificial lighting has been shown to disrupt these natural cycles.
Illumination created by artificial lighting varies from place to place, but can be comparable to daylight.
(Bennie et.al., 2016)
Plants use light to generate energy through photosynthesis and also to gain information about the environment, such as season, which in turn determines the timing of key physiological processes such as germination, bud-burst, flowering and senescence. Street trees in urban areas often leaf several weeks earlier than in rural areas, due to the presence of artificial light and some trees keep their leaves almost all year. The effects of artificial lighting on trees have been observed in plane trees, horse chestnut and silver birch (Matzke 1936; Schroeder 1945; Briggs 2006).
Certain ornamental plants are also sensitive to artificial lighting, with levels < 5 lux influencing flowering times and growth (Cathey & Campbell, 1975a,b). Light sources with a high proportion of red and far-red, such as high-pressure sodium lighting, have the greatest effect.
Changes in the timing of plant processes can affect the overall health of plants in a variety of ways. For example, early bud-burst can expose the leaves to frost and pathogens, and flowers may be too early to coincide with the arrival of pollinating invertebrates. Whilst the overall impact of artificial lighting on plants is complex and somewhat unpredictable, new lighting schemes should be designed to avoid the illumination of natural habitats and landscaping planting.