How streetlights keep leaves on trees longer

HostMost of us have seen it while walking the dog late in the fall. All the trees on the block are bare, but there's one maple right under a streetlight that's still covered in bright green leaves. It looks like it totally missed the memo that winter is coming. Why does that happen?

GuestIt comes down to how trees tell time. They don't have a calendar, so they use the sun as their clock. As the days get shorter in the fall, the tree senses that change and starts to shut down for the winter. But streetlights mess with that signal. To the tree, that bright glow feels like the sun is still up for hours and hours. It basically thinks it's still summer, so it keeps its leaves and keeps trying to grow.

HostSo the tree is just looking at the lamp like a tiny sun? I always figured it was the heat coming off the bulb that kept the branches warm.

GuestThat's a common guess, but it's mostly the light itself. Specifically, it's the color of the light. Trees are very sensitive to certain types of red light that are common in the sun. Many of our old streetlights give off those same red tones. When a tree catches that light, it sends a message to the rest of the plant saying the days are still long, so keep those leaves working. It stops the tree from making the tiny seal at the base of the leaf that would normally let it drop off.

HostBut is that really a problem? If they have leaves longer, they can take in more light and make more food. It seems like a bit of a bonus for them.

GuestYou would think so, but it's a huge risk. Trees need to go into a deep sleep, which people in the field call being dormant, to survive the frost. If a tree stays active, the water inside its cells doesn't have time to move out or change. When the first big freeze hits, that water turns to ice and can pop the cells like a soda can left in a freezer. It causes real damage to the wood and the buds that are supposed to bloom next spring.

HostBut surely the tree can feel the cold? Even if the light is on, the freezing air should tell the tree that the light is lying.

GuestFor many trees, the length of the day is a much stronger sign than the cold. A random cold snap can happen in the middle of May, but that doesn't mean winter is here. The sun is much more steady. The way the days shorten is the most reliable clock nature has. When we put a light right next to a branch, we're breaking that clock. The tree trusts the light more than it trusts the wind.

HostSo a single branch could be awake while the rest of the tree is sleeping?

GuestYes, and you can see that clearly on many city streets. You'll see trees that are half-bare and half-green. The side facing the street is still in summer mode, while the dark side is ready for snow. This gets even trickier with new LED lights. Some of those give off a lot of blue light, which is another sign trees use to stay awake. It can make the tree stay active even later into the year than the old orange bulbs did.

HostIt sounds like we're accidentally forcing these trees to pull an all-nighter for months on end.

GuestIt really is like that. And it's not just about the leaves falling late. Because the tree hasn't hardened its wood for winter, the branches stay soft. When we get a heavy snow, those green leaves catch the flakes like a sail. The weight of the snow on a leafy branch is way more than a bare branch can handle. That's why you see so many broken limbs near streetlights after an early storm.

HostIf we know the color of the light is the issue, can we just swap the bulbs for something the trees won't notice?

GuestWe can make it better, yeah. Some cities are starting to use lights that point straight down, so the glow doesn't hit the treetops as much. We can also use warmer lights that don't have as much of that bright blue or red that triggers the tree. But even then, light is light. If it's bright enough for us to see the sidewalk, a tree might still get confused.

HostIt's strange to think that our safety at night is making things harder for the nature around us. We just want to see where we're going, but we're accidentally keeping the oaks awake.

GuestIt shows how much we have changed the world in a very short time. Trees have had millions of years to learn that short days mean cold is coming. Then, in just a hundred years, we filled the night with fake suns. Some trees are starting to change, though. We're seeing some city trees that are a bit more tough and don't react to the light as much as their cousins in the woods, but that kind of change takes a long, long time.

HostDo all trees fall for the trick? Some of them seem to drop their leaves right on time no matter where they're standing.

GuestNot every tree is the same. Some, like oaks and maples, are very sensitive to light. Others pay more attention to the cold or have an internal timer that's harder to mess with. But for the ones that do listen to the light, that streetlight is like a song telling them to stay young and green, even when the air is turning bitter.

HostThose green branches hanging on in the middle of a snowstorm are a vivid sign of how our fake suns have rewritten the rules for the woods in our own backyards.