How weeds hitchhike across the country

Before you head off for that Thanksgiving trip to grandma’s house, you might want to wash your car. It seems our vehicles are inadvertently transporting hundreds of millions of plant seeds from place to place, many of which will grow up to be weeds or invasive species.

That cars can move seeds around has been known for a while. Seeds have been found underneath the car’s chassis, trapped in the front and rear bumpers and tucked away in wheel wells and arches, as well as on tires, mudguards and the floor mats inside. If those seeds are native species being transferred from natural habitat to natural habitat, it’s not really a big deal. But if those seeds are weeds or invasive species, well, then that’s a problem.

To determine how many weedy and invasive species might hitchhike on cars, Michael Ansong and Catherine Pickering of Griffith University in Queensland, Australia, looked through past studies that examined the transport of seeds by cars. They found 13 studies from Europe, North America and Australia and tallied up the plants using, among other sources, the Global Compendium of Weeds to determine which ones might cause trouble. Their results were published November 12 in PLOS ONE.

Ansong and Pickering totaled up a list of 626 plants species documented to travel accidentally by car; 599 of them are considered weeds in some part of the world and hundreds are invasive. Each car only carries a handful of seeds, two to four on average, but with the millions of cars on the road around the world, that’s a lot of seeds. The team estimates that cars move 490 to 980 million seeds in the United States, 480 to 960 million seeds around the European Union, 248 to 496 million seeds around India and 240 to 496 million seeds across China. Most of those seeds probably don’t travel too far, but some could be transported hundreds of kilometers from where they were picked up.

Some trips are more likely to acquire weedy hitchhikers than others. “Cars traveling on unpaved/untarred roads and off-road, for instance, often collected more seed in mud/soil than those driven on paved roads, particularly if conditions are wet,” Ansong and Pickering note. More adventurous drivers who love to go off-roading with their vehicles, therefore, are more likely to be spreading weeds and invasive species from place to place.

Driving in dry conditions also tends to result in more seed transport on cars, the researchers found. With no rain to wash seeds away, the hitchhiking seeds can hang on for a long ride.

There are actions that can limit this type of dispersal. The grass next to roads, for instance, can be mown before those plants start to seed, eliminating the chances that seeds can be transported by the cars driving nearby. Machinery and vehicles can be cleaned before they’re taken into sensitive conservation areas, such as national parks. Car owners can wash their own cars regularly, particularly before and after driving on unpaved roads and through the mud. Doing so is an easy method to prevent a potential pest from ending up in a beautiful natural habitat or even in your own backyard.