Crack willow is a large tree that originates from Europe and West Asia. It grows in the margins of rivers, streams, ponds and lakes. It spreads through branches falling off (hence the term 'crack') and rooting elsewhere. It displaces native vegetation from stream and wetland margins and will restrict water flows causing flooding.
Extensive crack willow infestations exist throughout New Zealand. In Marlborough, crack willow is the dominant species in numerous wetlands and riverbanks.
Crack willow is also an unwanted organism under the Biosecurity Act 1993 given it is on the National Pest Plant Accord list of species banned from sale, propagation, distribution or commercial display.