The Yellow Blanket of Spring

For a few weeks every spring, Chapel Hill and Carrboro are covered in a yellow blanket of pine tree pollen and everything’s a mess. Birches, oaks, pines, and more get the signal to “spread the love” and distribute their genetic material all over the place, irritating our eyes and noses. But how do plants know when it’s time to release their pollen? You may have learned in elementary school that warm weather activates the flowering gene in plants. While temperature plays a role, it is not the only trigger. Remember that random week of warm weather we had at the end of February? Why didn’t the pine trees start distributing their pollen then?

Spring ushers in longer days along with warmer weather. The amount of daylight a hemisphere receives changes as the Earth orbits the sun. In the winter, our hemisphere (the northern one) tilts away from the sun, reducing the amount of sunlit hours in the day. In the summer, our hemisphere tilts toward the sun. This means that between the winter and summer solstices, as the Northern Hemisphere transitions from tilting away from the sun to tilting towards it, the amount of daylight increases.

The changing length of day throughout the year being a consistent phenomenon, plants have developed a mechanism that uses the amount of daylight as an indicator to flower. The photoreceptors that allow sunlight to enter and initiate photosynthesis, the process by which plants transform light into energy, also activate proteins. Thus more daylight means more protein. And once enough protein builds up, the plant gets the signal to flower.

Imagine the protein as sand in an hour glass. When the sun rises, you flip the hourglass and sand begins to trickle down. After the sun sets, you flip the hourglass back over and the sand that built up during the day pours out. During the winter, all the sand that builds up during the day will empty at night because the nights last longer than the days. As you approach spring, each day the sun is up a little longer, meaning a little more sand can accumulate. At a certain point, your hourglass will build up enough sand during the day that some will still remain at the end of night. Once the protein reaches this threshold, the flowering gene in plants can activate.

But what if the days have lengthened and it is still cold out? If it’s too cold there won’t be bugs or other animals around to help spread the pollen. Thus, plants also rely on the temperature as a secondary indicator. This is where things have recently started to get messy. Global climate change has reduced the number of lingering cold days in the transition from winter to spring. As a result, scientists have noticed that plants are flowering earlier, and allergy season is starting earlier and lasting longer than in previous decades. So, if you’ve noticed your nose itching sooner and you can’t seem to shake your sniffles, you’re not crazy.