When I moved to Utah, I was struck by how dark the nights could be. Camping deep in a canyon, I’d peek out of my tent and see juniper trees around me — quiet sentinels in deep darkness.
But our urban trees live under a very different sky. Streetlights glow through the night. Porch lights shine through branches. For many city trees, night is never truly dark.
Why should we care? Because trees use light — and darkness — to measure time. The changing length of days and nights is their seasonal clock, guiding their responses to seasonal changes.
Artificial light at night can scramble those signals. Trees growing near streetlights often open their buds earlier in spring and hold their leaves longer in fall — by days or even weeks. That sounds harmless, but early leaves are vulnerable to the late frosts that are common in Utah. Delayed dormancy can leave trees less prepared for winter cold and drought.
Night lighting also affects the insects that trees depend on. Many trees are pollinated by nocturnal moths and beetles that navigate by moonlight and stars. Bright lights disrupt those signals, reducing pollination and shrinking the number of seeds available for wildlife.
So when I walk beneath city trees at night, I think about how their long presence on our planet has relied upon dependable darkness. Caring for trees doesn’t only mean planting more of them. It can also mean letting the night be dark again.
