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Humans can contribute to the long-distance dispersal of many plants, including weeds. We assessed the distance for which seed remained attached to the socks and trouser legs of walkers. The experiment in Kakadu National Park, Australia used seed of the Australian native Black Spear Grass ( Heteropogon contortus) as a surrogate for the potential dispersal of weed seed. Two models were fitted to the data, with a double exponential model fitting the data slightly better than a power exponential model. Although 19% of seed were dispersed within the first 5 m of walking, most (55%) seed remained attached to socks and trousers at 5 km. Humans may, therefore, unintentionally carry and eventually disperse this grass and other invasive grasses with similar long awns and pointed seeds over long distances. Implementing strategies to reduce the potential for seed to attach to walkers (gaiters) and reduce potential dispersal (removal and careful disposal of seed) would reduce the risks posed by this type of human-mediated seed dispersal. [ABSTRACT FROM AUTHOR]