Evaluating flotation and sieving for removing Nassella neesiana seeds from gravel harvested from infested riverbeds in New Zealand

<i>Nassella neesiana</i> (Chilean needle grass) reduces farm profitability through its sharp seeds that damage wool, skins, hides and carcasses. It poses a biosecurity risk in New Zealand’s Hawke’s Bay region due to its seed dispersal via gravel and crushed aggregate from braided rivers. This study evaluated flotation and sieving to remove both dry and imbibed panicle seeds. We measured their length, width, mass, flotation, and passage through sieves with and without the awn (a hygroscopic, bristle-like structure). The dry awned seeds were, on average, 70.12 mm long and 1.13 mm wide, increasing to 74.07 mm long and 1.21 mm wide when imbibed. With imbibition, their mass with awn, increased from 9.41 to 14.06 mg, and from 3.96 to 5.04 mg with awn detached. Flotation was highest for the dry awned seeds (50%) and lowest for imbibed awned seeds (3%). The awn and imbibition restricted the seeds’ passage through sieves while sand and a crushed aggregate (GAP 20 metal) facilitated their passage. The results indicate that flotation would be ineffective for removing <i>N. neesiana</i> seeds from river gravel but that an 8 mm aperture sieve could effectively remove them along with fine aggregate particles.