Gardeners in northern climates know the power of artificial light. Lamps specially tuned to the needs of plants can give people a head start on the growing season, enabling them to raise their first lettuce and peas indoors.
On a larger scale, cities are doing much the same. The proliferation of streetlights and other illumination has turned urban areas into what amounts to giant well-lit greenhouses, helping to add nearly a month to growing seasons in cities.
Our ability to alter when plants in the natural world bud or shed their leaves has been known for decades. But the main culprit has long been seen as rising temperatures, brought on by a combination of global greenhouse gas pollution and the local effects of heat-absorbing pavement.
But new research, published recently in Nature Cities, finds that in many cities in the northern hemisphere, artificial light has played a bigger role than heat in changing plant behavior in recent years. Together, the two forces appear to be causing plants to start growing in northern cities nearly 13 days earlier and end the season 11 days later, the scientists reported.
The phenomenon could boost urban plants by giving them more days to grow. But there could also be down sides. It “can put a strain on the energy and water balance of plants, which also have to contend with water shortages in increasingly dry cities,” said Franz Hölker, a researcher at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, who worked on the new study.
Urban heat and artificial lights go hand in hand, making it hard to tease apart the relative role of each one in altering the seasons. The scientists turned to a combination of satellite and ground observations and statistical analysis to gauge their separate effects.
The group selected 428 cities in the northern hemisphere and separated each city into 10 zones, from most rural to most urban, based on the density of impervious surfaces such as roads, roof and sidewalks. Then they looked at satellite records between 2014 and 2020 of temperature, artificial light, and when vegetation took off in the spring and faded in the fall.
Their ability to see the differing effects hinged partly on how temperature and lighting patterns differ in cities. Temperature increased most during the shift from the country to suburbs, while the amount of light soared in the move from the suburbs to central cities.
The scientists found that in 279 of the cities they observed, either artificial light or temperature played an bigger role in the start of the growing season. But the split was almost even – light was more of a factor in 135 cities. Air temperature ranked higher in 144 cities.
The outsized effect of artificial light was most clear in the end of the growing season. There, higher amounts of light were correlated with prolonged growth in 72% of cities, while higher temperatures only showed a clear relationship with later onset of fall loss of vegetation in 118 cities.
This stronger effect of artificial light could be due to plant physiology. Previous studies found the length of daylight carried more weight than temperature in controlling when plants stopped growing, the authors wrote.
The continued urbanization of the planet, as well as a shift to brighter, more energy-efficient bulbs, could both add further fuel. “We expect the effects of artificial light on the growing season to increase even further in the future,” said Hölker.
He was involved in earlier research that found the spread of LED lights was making artificial lighting brighter – a 2% annual increase in outdoor lighting brightness between 2012 and 2016.
While it’s unlikely the world is going to turn off the lights for the sake of plants or wildlife, there are ways to ease the problem. For instance, some of the same researchers found that streetlights designed to shine only where needed and minimize spillover of light into the surrounding area significantly reduced how many flying bugs they attracted at night.
If people can do the same for urban trees and other plants, maybe they can be more like their rural cousins, and less like tomatoes in a greenhouse.
Wang, et. al. “Artificial light at night outweighs temperature in lengthening urban growing seasons.” Nature Cities. June 16, 2025.
Image: ©Anthropocene Magazine / AI-generated
