Why Cottonwood Seeds Stop Dispersing in Late Afternoon

If you've ever watched a cottonwood tree release its fluffy seeds, you may have noticed a peculiar pattern: the "snowstorm" of seeds often halts abruptly between 4 and 5 p.m., even when the wind continues to blow. This isn't a coincidence—it's a survival strategy rooted in the tree's physiology.

The Daily Rhythm of Seed Release

Cottonwoods (Populus deltoides) disperse their seeds on fine, hair-like structures that catch the wind. Research shows that seed release is strongly tied to temperature and humidity. During warm, dry afternoons, the seed capsules (catkins) open fully, allowing the seeds to fly. As evening approaches, humidity rises and temperatures drop, causing the capsules to close and seed release to stop—often like clockwork around 4–5 p.m.

Why Timing Matters

This daily cycle ensures seeds are launched when conditions favor long-distance dispersal. Warm, turbulent afternoon air creates updrafts that carry seeds farther. If seeds were released in the cool, damp evening, they'd fall closer to the parent tree, increasing competition. The tree essentially "waits" for optimal launch conditions.

What Triggers the Shutdown?

• Rising humidity makes the seed hairs clump together, reducing buoyancy.
• Falling temperatures reduce thermal updrafts.
• Circadian regulation—the tree's internal clock may also play a role, as seen in other wind-dispersed species.

So, while it may seem like the tree "decides" to stop, it's actually responding to environmental cues that maximize the next generation's chances. Next time you see the blizzard end on schedule, you're witnessing a finely tuned evolutionary adaptation.