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Lupinus arboreus, yellow bush lupine (USA) or tree lupin (UK), is a species of flowering plant in the legume family Fabaceae, native to California, where it is widely distributed among coastal scrub and sand dunes. Because it has been widely introduced, there is some uncertainty about its native range; it is thought to be native from Point Reyes National Seashore south to San Luis Obispo County.
It is an evergreen shrub growing to 2 m (7 ft) tall (hence the alternative common name, tree lupine) in sheltered situations, but more typically 1–1.5 m (3–5 ft) tall. It has green to gray-green palmate leaves, with 5-12 leaflets per leaf. The leaflets are 2–6 centimetres (0.79–2.4 in) long, often sparsely covered with fine silky hairs. In spring it bears many racemes, 30 cm (12 in) long, of fragrant, soft yellow, pea-like flowers. Both yellow and lilac to purple flowering forms are known; however, the yellow form is more common, except in the north of the species' range. It is capable of tolerating temperatures down to -12°C and living for up to seven years.
It has escaped from cultivation to become an invasive species in many areas. Outside of its native range in North America, it is somewhat invasive both in southern California, and further north to southwestern Canada. It has also been introduced in western Europe, Australia (where it is considered a potential noxious weed), New Zealand, Anatolia Aegean Region of Turkey and southern South America, including the Falkland Islands.
Like many members of the Fabaceae, it is an effective fixer of nitrogen in the soil. Where it has been introduced, it changes the chemistry of the soil, and therefore allows other exotics to establish themselves, to the detriment of native vegetation adapted to low nitrogen levels. It also hybridizes with other lupine species such as Lupinus littoralis (seashore lupin) and Lupinus rivularis (riverbank lupin), further endangering the survival of those forms.
The yellow-flowering form is widely grown as an ornamental plant for its attractive flowers and also to bind drifting sand dunes. It can often be seen growing in cracks in vertical stone walls, where it appreciates the sharp drainage. It has gained the Royal Horticultural Society's Award of Garden Merit.
A number of other related species of lupine also grow as bushes. They include:
- Lupinus chamissonis Chamisso bush lupine, silver bush lupine
- Lupinus albifrons Bush lupine
- Lupinus excubitus Grape soda lupine
- Lupinus longifolius Longleaf bush lupine