About 270 species (estimates range 200–600+) of annual or perennial herbs, subshrubs, shrubs and small trees, native to the Mediterranean Basin and the Americas. Leaves palmate (very rarely unifoliate), with 3–17 entire leaflets, petiolate, with stipules fused to the petiole base. Inflorescence a raceme, usually terminal, with flowers in whorls or spiralled. Flowers with 2-lipped calyx; corolla with 5 lobes (banner, 2 wings, 2 forming the keel). Stamens 10; filaments fused, of alternating lengths (5 long with short anthers, 5 short with long anthers). Fruit a dehiscent legume, oblong to elliptic. (Sholars & Riggins 2022; OregonFlora 2023; Cullen et al. 2011).
Lupinus ranks as a major genus of the pea family, yet is peripheral to our project, with only a handful of woody species cultivated in our area, just one of them commonly. The genus belongs to Tribe Genisteae within Subfamily Faboideae of the Fabaceae (Leguminosae), along with important garden genera such as Cytisus, Genista and Laburnum. Lupins are easily recognized by their palmate rather than pinnate leaves, and conspicuous bumblebee-pollinated flowers in erect spikes or racemes. Many are showy, charismatic plants. Few who have seen yellow-flowered L. arboreus making pristine shrubs in mobile sand on the Californian coast, the herbaceous perennial L. latifolius scenting the summer air on mountain slopes in the Oregon Cascades, or expanses of the blue-flowered annual L. texensis flowering with scarlet Castilleja in Texas will quickly forget the experience.
Molecular evidence supports the idea of Lupinus as a natural, monophyletic group (Aïnouche, Bayer & Misset 2004). Lupins have very diverse life forms, including annual and perennial herbs, subshrubs, shrubs and even small trees. They are found in habitats as diverse as coastal dunes, shrublands, grasslands, open mountain forests, mountain meadows and disturbed slopes, from sea level to the alpine zone (Drummond et al. 2012); most are found where there is a degree of habitat disturbance. They are taxonomically problematic: estimates of species numbers range from 200 to over 600; around 270 is a frequent contemporary view.
The story of lupin evolution is a fascinating and complex one which is still far from fully understood. The genus emerged perhaps 12–14 MYA (Käss & Wink 1997) in the Old World; about 12 species are still found in the Mediterranean Basin, the centre of diversity of Genistae. These are all annuals, apparently the ancestral condition; none are of interest as ornamentals although some (eg. L. albus L.) are seed crops. Only these were known when Linnaeus named the genus; his epithet is derived from the Latin lupus (wolf) supposedly from a belief that the plants take over and exhaust land. However, the main centres of diversity for Lupinus are in the Americas, where there were three major radiations much more recently (Drummond et al. 2012). Two of these radiations are from the ancestral group, which must have crossed the Atlantic at least once. The first major radiation was in western North America, dating from about 4.6 MYA. Here there are about 60 annual, herbaceous perennial and woody species (arguably more) occupying a full range of more or less open habitats, mostly in the mountains where the fine scale mosaic of habitats may have encouraged speciation. It is mainly these herbaceous and woody lupins which have found their way into gardens. The unifoliate annual species of eastern North America are related to them. A later radiation in western Mexico, Central America and the Andes (perhaps 130 living species) originated from various species of the Western group migrating south, perhaps facilitated by the colonization of South America by bumblebees, their pollinators. The Andean radiation shows one of the highest rates of species diversification known in plants (Contreras-Ortiz et al. 2018). Among them are some extraordinary tree-forming species such as L. semperflorens Hartw. ex Benth., which can reach 6 m in height. None from these areas have serious prospects in the garden. The third great radiation was in subtemperate grassland and campo rupestre habitats of eastern South America (around 35 species). These are not in cultivation but a related species, the annual Texas Bluebonnet L. texensis is well known. The woody habit is clearly secondary in Lupinus. Evolution of woody species from herbaceous ancestors is often a feature of islands (cf. Echium in Macaronesia), and at least in the Andes the habitats of woody species can be seen as ‘sky islands’ (Nürk, Atchison & Hughes 2019). Polyploidy has been important in lupin evolution, and confuses molecular study of their relationships, although most species of ancient polyploid origin seem to behave today as if they were diploid species (‘chromosomal diploidization’) (Naganowska et al. 2003). Hybridization and introgression are frequent, making species delineation and identification tricky (OregonFlora 2023).
All lupins have nitrogen-fixing root nodules, suiting them to nutrient-poor soils. Lupins generally lack mycorrhizas, but some species have an unusual way of releasing phosphorus in such places. Huge amounts of phosphate-mobilizing carboxylate ions are released into the soil, sometimes (but not always) by specialized ‘cluster roots’. This may be an advantage in colonizing disturbed places and mineral soils. L. arboreus lacks cluster roots, but seems capable of colonizing phospate-poor soils (Lambers, Clements & Nelson 2013).
Despite a few species being grown as seed crops, lupins have a well-deserved reputation for toxicity. Many plant parts, but especially seeds, may contain bitter-tasting lupinine and other quinolizidine alkaloids, acutely toxic to humans. Lupin seeds may contain as much as 1% lupinine, (Hama & Strobel 2017). Lupinosis, a quite different issue, is the poisoning of livestock which have grazed lupin stems in particular, often when let onto the stubble of lupin crops. It is caused by phomopsins, macrocyclic hexapeptides produced by the ascomycete Diaporthe toxica inhabiting the plant (Western Australia Agriculture and Food 2021).
We describe just three woody species from the American West, the widely grown L. arboreus plus L. albifrons and L. chamissonis. One further shrubby species L. excubitus M.E. Jones, is a low-growing Californian desert plant closely related to L. albifrons and very like it in flower; it is most unlikely to be seen in gardens. While they are entirely herbaceous, it is worth noting that the familiar Russell Lupins, hybrids based on herbaceous L. polyphyllus Lindl., have genes from woody L. arboreus in their makeup (Huxley, Griffiths & Levy 1992).
In gardens the woody species are best tried in full sun in well drained, poor soil. However long-lived they might be in the wild, they tend to be short-lived in the garden, and best replaced from seed after a very few years. In Britain Bean (1981) noted of L. arboreus that it will ‘pass through a winter quite unharmed but then, a few years later, will succumb during another winter not any more severe’. Dead-heading – removal of developing seed pods – might help longevity, so long as some seed is allowed to develop to replace the plant in time. Seed tends to germinate quickly. Cuttings are possible in L. arboreus and probably other shrubby species; Bean (1981) short side shoots with a heel in July or August, with gentle heat.
Perennial lupins, both herbaceous and woody, have significant pest and disease issues. The Lupin Aphid Macrosiphon albifrons, an impressively large species, infests lupins in North America and is now well established in Europe too. Various viruses show themselves by colour breaks in flowers, leaf banding and yellowing; care should be taken to avoid transmission during vegetative propagation, and affected plants should be destroyed. Fungal leaf spotting and various types of rot can also be a problem (Huxley, Griffiths & Levy 1992).