Seed survival after commercial processing of river gravels to produce aggregate

Biosecurity staff were concerned with the potential spread of invasive plants through gravel aggregate products. Weeds, including regulated or otherwise unwanted species, occasionally grow near gravel extraction sites, and therefore are believed to present some risk of contaminating extracted aggregate. River sourced gravel is processed to form various products, and it was thought that some of these processes may reduce the risk of weed seed contamination.We worked with local contractors, in a replicated experiment, to process fifty tonnes of gravel aggregate ‘spiked’ with known numbers of seeds of eight surrogate weed species of different seed size (using commercial species) at two braided river sites, one in Hawke’s Bay (Waipawa River) and one in Canterbury (Selwyn River).Report Number 12434