The city’s lights flicker all night, severely disturbing the phenology of urban plants—the changes that occur when the stamens bloom in the spring, and the leaves change color and fall in the fall. A new study published in the journal PNAS Nexus shows how nighttime lights prolong the growing season in cities, affecting everything from allergies to local economies . The researchers analyzed trees and shrubs at about 3,000 sites in U.S. cities to see how they responded to different light conditions over five years. Plants use their natural day-night cycle as a signal that the seasons change with temperature. Artificial lighting alone moved spring budding earlier by an average of about 9 days compared to sites without night lights. The timing of fall foliage color changes is more complex, but in the contiguous 48 states, the timing of leaf color changes is delayed by an average of nearly 6 days. In general, the stronger the light, the greater the difference. This shift in the plant circadian clock has important implications for the economic, climatic, health and ecological services provided by urban plants. On the positive side, the longer growing season keeps urban crops active for longer. As global temperatures rise, plants can provide shade earlier in spring and later in fall, cooling communities. But changing growing seasons can also make plants more vulnerable to spring frost damage. It can also create a temporal mismatch between urban plants and other organisms on which they depend, such as pollinators. The longer active season for urban plants also suggests that the pollen season starts earlier and lasts longer, which can exacerbate asthma and other breathing problems. A Maryland study found a 17 percent increase in asthma hospitalizations in years when the plants bloomed very early.
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