Artificial light lengthens the growth cycle of plants in urban areas

Artificial light affects the growing season in urban environments up to three weeks longer than in rural areas , according to a new analysis led by Vanderbilt University (Nashville, USA) and published in Nature Cities . To reach this conclusion, they used satellite data from 428 urban centers in the Northern Hemisphere collected over seven years.

Rapid urbanization is leading to warmer, brighter cities. Buildings and concrete in particular absorb and radiate heat, creating urban heat islands . In these areas, atmospheric temperatures are higher during the day and night than in the surrounding areas.

Similarly, the amount of artificial light at night has increased by 10% in cities over the past decade. Light and temperature largely regulate plant growing seasons. For example, increased light and temperature cause trees in cities to bud and flower earlier in spring and change color later in autumn, compared to trees in rural areas . However, the research did not examine in depth the magnitude of their individual or combined impacts.

The team, led by researcher Lin Meng, analyzed satellite observations taken between 2014 and 2020 in 428 cities across the Northern Hemisphere—including New York, Paris, Toronto, and Beijing—along with data on artificial nighttime light, near-surface air temperature, and plant growing seasons.

The authors found that the intensity of artificial light at night increases exponentially from rural areas to urban centers. According to the researchers, this increased amount of light has a greater influence on the start and end of plant growing seasons than the temperature difference between rural and urban areas .

They also observed that the effect of artificial light is especially pronounced at the end of the growing season, rather than at the beginning. Specifically, the start of the season occurs on average 12.6 days earlier in cities compared to rural areas, and the end is delayed 11.2 days in the urban environments analyzed .

While these general patterns hold true for cities in the Northern Hemisphere, the authors also identified differences across continents. The scientists observed that the season begins earlier in Europe, followed by Asia and then North America , although North American cities recorded the highest levels of light.

The effect of nighttime lights is strongest at the beginning of the season in some climate zones—such as those with temperate climates and dry summers, or cold climates without a dry season—while the impact at the end of the season shows greater consistency across cities.

The authors caution that the effect of artificial light on the growing season may become even more complex due to the relatively recent shift from high-pressure sodium lamps to LED lighting, to which plants may respond more sensitively.

Meng and his team emphasize that future urban infrastructure plans should consider lighting that minimizes its impact on vegetation while also meeting functional requirements.