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Russell lupin (Lupinus polyphyllus) is an exotic plant that can grow up to 1.5 metres. It is a perennial species, i.e. it flowers and sets seed in the summer, dies back to the stem base over winter, to re-emerge the following summer. Russell lupins produce long, colourful flower heads. The flowers are pea-like and come in a variety of colours; blue, purple, orange, yellow, pink, white or a mixture. Flowers appear from September to February. The leaves, divided into green leaflets, are splayed out like fingers on a hand. Stout seedpods are produced that explode in the summer heat, releasing many dark brown seeds. The seeds are dropped close to the parent plant, allowing the population to spread several metres annually.
Although Russell lupins may look beautiful, they are an aggressive exotic weed that smothers braided riverbeds by growing in dense stands on bare gravel areas. Well adapted to living in the challenging environments of braided rivers, they can produce their own nutrients (nitrogen) and are very effective at dispersing their seeds. Their roots become entwined and hold the gravel together, forming stable areas. The river erodes the edges, creating steep banks. This restricts the water so that instead of braiding, it develops deep, fast-flowing channels, unsuitable for wading birds to feed in. The dense stands also take over the open spaces where braided river birds like to nest. This is deeply concerning for rare and endangered birds that nest exclusively along braided rivers, particularly the wrybill, black-fronted tern and kakī/black stilt, which normally feeds in shallow river braids. Russell lupin also provide cover to predators such as stoats and cats to sneak up on birds.