Tree lucerne is a common woody weed in New Zealand, but it's one of those weeds that's not really much of a problem. It was deliberately introduced (sometime before 1919) as a hedging plant and as livestock food, hence the New Zealand name, which is a reference to lucerne or alfalfa (Medicago sativa). It comes from the Canary Islands, where it's known as tagasaste, a name which is sometimes also used in New Zealand. Now tree lucerne is naturalized pretty much throughout the country. Its value as a fodder plant is also reflected in its value as a food for kererū.
|Tree lucerne, Chamecytisus palmensis. A, a cluster of flowers; B, flowers, seen from the side and from beneath; C, seed pods developing; D, leaves: upper surface on left; E, seeds.|
|Tree lucerne, Chamaecytisus palmensis, near Northland Tunnel, Wellington, New Zealand.|
Like gorse and other legumes, it's a nitrogen-fixer, or at least it harbours bacteria that are nitrogen-fixers. That's part of its value as a livestock food, and also might give it a role in eco-restoration, as a nurse crop for native forest regeneration. If kererū are attracted to the leaves, they might bring seeds of native plants to regenerate under the shelter of the tree lucerne.
This is a plant with so many virtues, it's perhaps unkind and misleading to call it a weed, and you'll note I've renamed the series "Wednesday Wildflower".