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Tree lupin (Lupinus arboreus) is a short-lived, perennial shrub with a deep taproot that grows up to 2-3m high. The stems are densely silky-hairy when young, tough, erect, branching, becoming soft-woody as it grows. The leaves are grey-green, hairless above, silky below, and divided into 5-11 leaflets that spread out finger-like from a single point. The leaflets are 15-40 x 3-10mm. It flowers between October-May and the flowers are pea-like, 15-18mm long, usually pale yellow (rarely white or bluish) and sweetly scented. The seed pod is stout, softly hairy, 40-80mm long, and firmly attached. It splits explosively to disperse dark brown, mottled seeds, 4-6mm long.
Tree lupin lowers light levels in open habitats including braided rivers. This leads to subsequent invasion by other invasive exotic plants. Like their relative, the Russell lupin, tree lupin also increases soil nitrogen, and that further increases the changes of invasion by exotic weeds, and degraded and eventually destroys the habitat of native plant communities. This in turn causes river gravel to build up, altering the character of braided rivers. 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. Tree lupin also provide cover to predators such as stoats and cats to sneak up on birds.