Plants that live in cities have much longer growing seasons now

Cities are becoming more than just human-built environments. Their light and heat now influence how plants grow.

A major international study confirms this shift. Artificial lighting lengthens the growing season in urban areas more than heat does.

The researchers analyzed satellite data from 428 cities across the Northern Hemisphere between 2014 and 2020. They found urban plant growth begins earlier and ends later than in nearby rural areas.

City lights affect plant growth 

Urban heat islands form when buildings and roads trap warmth. Cities also get brighter each year.

Artificial light at night (ALAN) has grown nearly 10% annually over the last decade. Both heat and light alter plant growing seasons, but this study reveals light matters more.

“ALAN is a critical driver of vegetation dynamics in cities,” noted the researchers. The artificial light changes how plants respond to seasonal cues, especially during autumn when natural light typically declines.

Urban plants grow earlier and stay green

Across cities, the growing season starts 12.6 days earlier and ends 11.2 days later than rural zones. That is over three weeks of extended green time.

Spring ALAN levels increased exponentially from rural to urban centers. In contrast, temperature rose more gradually. Spring light intensity climbed from 3.1 to 53.3 nW cm² sr⁻¹ across city gradients. Autumn changes followed similar trends.

The study revealed a strong correlation between higher ALAN and delayed leaf drop. EOS (end of season) was more strongly influenced by ALAN than by temperature in nearly half the cities examined.

Continents and climates respond differently

Europe experienced the earliest start to spring, with Asia following and North America close behind.

Despite being the brightest overall, North American cities showed the smallest shift in plant growing season timing. This suggests that more light does not always mean bigger changes in plant behavior.

Different Köppen climate zones responded in varied ways to artificial light. ALAN had the strongest effect in cold climates without dry seasons and in temperate zones with dry summers or winters.

In these areas, artificial light advanced spring plant growth more than rising temperatures did. This shows that light can override the usual seasonal cues driven by warmth.

However, end-of-season (EOS) patterns were more consistent. In nearly every climate, artificial light delayed the end of plant activity.

Even in warmer or drier zones, nighttime light caused plants to hold onto their leaves longer. This widespread delay in senescence shows how deeply city lighting alters natural plant rhythms.

Latitude and light sensitivity

The farther north a city lies, the more its plants respond to artificial light. In higher latitudes, the natural variation in daylight is more extreme. This makes plants especially sensitive to artificial night lighting.

“ALAN acts as an artificial extension of natural daylight,” wrote the study authors. In high-latitude cities, it disrupts the signals that tell plants when to shed their leaves.

Temperature still plays a role, especially in triggering spring growth. But ALAN’s influence on leaf fall in autumn now appears stronger and more widespread.

LED light shifts plant timing in cities

The shift from sodium lamps to white LEDs has intensified ALAN effects. LEDs emit more blue light, which plants detect through specialized receptors. That may change how plants time their seasonal shifts.

“Plants sense different wavelengths through different photoreceptor pathways,” noted the researchers.

However, most satellites cannot yet detect blue light well. Future monitoring will need to include this spectral range to track ecological impacts accurately.

Looking beyond greener cities

Longer growing seasons may sound beneficial. Trees may take up more carbon and offer better shade. But there are downsides. A longer leaf season raises the risk of frost damage, mismatched plant-pollinator cycles, and more severe allergy seasons.

City planners face a complex challenge. Lighting designs must meet safety needs but avoid harming plants.

A smarter urban lighting strategy could reduce ecosystem disruptions while maintaining livable cities. Urban ecology is shifting under our feet and our lights. It is time we took notice.

The researchers are affiliated with Wuhan University, Vanderbilt University , Northern Arizona University, Leibniz Institute of Freshwater Ecology and Inland Fisheries, Freie Universität Berlin, Oak Ridge National Laboratory , and the University of California, Los Angeles .

The study is published in the journal Nature Cities.

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