Ragweed, the bane of summer and autumn allergy sufferers, spreads vigorously with help from a surprising source: our cars and trucks. A new study finds chaotic wakes of air currents from heavy traffic can disperse ragweed seeds tens of meters from their starting point—a huge boost from the usual 1-meter travel radius of seeds from their parent plants.
The researchers set up a field experiment to determine how far ragweed seeds traveled on a busy road with fast cars, versus less busy roads. In each trial, they placed 100 seeds painted in fluorescent color along the edge of roads, where seeds would normally drop, and let moving vehicles decide their fate. They then returned with ultraviolet torchlights to mark the new positions of seeds.
Within 48 hours, the seeds had settled into new spots. Most remained close to the starting location. But air currents from heavy traffic propelled some seeds tens of meters away, with the most distant traveling 71 meters—about two-thirds the length of a U.S. football field. Even on roads with less traffic, seeds still scattered up to 40 meters.
Then the researchers mapped roadside ragweed plants for two consecutive years to understand how much ragweed populations grew in the direction of traffic movement. In the second year, the team recorded twice as many seedlings flourishing in the direction of traffic movement versus the opposite direction, where the influence of cars was almost nil.
The study, which will be published in the Journal of Applied Ecology, is the first to link spread of an invasive species to traffic patterns. Its results suggest bad allergy seasons could be tamped down by requiring municipalities to closely mow roadside plants. But the right time to mow would be shortly before seeds are ripe—otherwise, mowers would spray them even farther afield.