When you take your four-wheel drive out for a spin this fall, you might be bringing home more than memories. Researchers at Montana State University have found that vehicles are routinely transporting invasive weed seeds.

Seeds can stow away on tires, bumpers, wheel wells or the underside of a vehicle and sometimes travel great distances before falling off in a new locale. As weed seeds sprout and grow, they can crowd out native plants, disrupt native ecosystems and wildlife habitats and reduce crop yields when they spread to nearby fields.

"Take a look at the many types of weeds growing along most any roadside and you’ll get a big clue about the role vehicles play,” says Lisa Rew, Ph.D., a member of the Weed Science Society of America and an assistant professor at Montana State University. “With an estimated 4 million miles of roads crisscrossing the U.S. and an estimated 256 million registered vehicles, even a few weed seeds per car can make a significant impact on the spread of weeds."

Montana State researchers measured the number of seeds picked up by a variety of vehicles and the distance traveled before the seeds fell off. Among their key findings:

Seed volume is seasonal. The study showed thousands more seeds per mile were transported by vehicles during the fall than in the spring.

Moisture matters. Wet conditions make it easier for seeds to be picked up by a vehicle – and easier for them to drop off miles down the road. Tests conducted at military installations showed Humvees picked up 14 times more seeds when conditions were wet, while tanks picked up 26 times more.

Distance is no barrier. The distances seeds can travel may be surprising. When researchers examined vehicles over several distance intervals, they found even at the 160-mile mark many seeds stayed attached. “If seeds are lodged in mud that dries on the vehicle, they can travel almost indefinitely, or at least until it rains again and the road surface is wet,” Rew says. Scientists consider that bad news. When vehicles transport seeds long distances, it increases the likelihood weeds will be spread into areas where they don’t yet occur.

Off-road travel increases the risk. Outdoor sports enthusiasts trucking to remote trailheads or riding ATVs off the beaten path are at special risk for spreading weeds. Researchers found vehicles picked up almost 20 times more seeds off-trail than on-trail.