Tree lupine

Family

Fabaceae

Habit

Shrub

Height

1.5 m

Spread

1.2 m

Growth

very fast

Lifespan

6 years

First yield

after 2 years

Hardiness

zone 8–11

Shade tolerance

full sun

Soil

Sand, Silt

Soil moisture

dry – very moist

pH

5 – 7.5

Synonyms

Buschlupine

Times apply to the northern hemisphere in a temperate climate and shift with altitude and cultivar.

Other uses: The main value of the tree lupin lies in its work as a pioneer. Through its root nodules it fixes atmospheric nitrogen and passes some of it to its surroundings, which is why permaculture uses it as a nurse plant: it grows quickly to about a metre and a half even on poor sand and in exposed positions, creating shelter and soil improvement in short order for slower shrubs and perennials. Land reclamation and restoration programmes use it to stabilise sandy ground and dunes — though the same property makes it a problem outside its native range. The flowers yield a bright yellow dye, and the root fibres were formerly twisted into cord and netting. The flower spikes, scented like white clover, are good bee forage. As a fodder plant it is unsuitable because of its alkaloid content.

Site: Two properties govern where it belongs. First, on ground that suits it the tree lupin seeds itself heavily and then comes up everywhere; where that is unwanted the pods must be cut off before they split, and it has no place near dunes, poor grassland or other nutrient-poor protected ground — it fertilises them through its root nodules and changes them for good. Second, it passes that nitrogen on to neighbouring plants, which makes it valuable in the vegetable garden and in young woody plantings: as a fast-growing, salt- and wind-firm pioneer it supplies shelter about a metre and a half high within two years, under which more delicate species can establish. It needs full sun and free-draining, preferably limy soil. All parts, especially seeds and pods, contain lupin alkaloids — so it is out of place beside pasture, small-livestock runs and children's play areas. Care: Care consists largely in allowing for its short life. The tree lupin rarely exceeds seven years, considerably less on rich soil, and ages visibly: growth becomes straggly and hardiness declines. Raise replacements regularly and replace over-aged specimens rather than trying to save them. Do not feed — it brings its own nitrogen, and rich soil shortens its life further. Cutting back by about a third after flowering keeps it compact and prevents self-seeding at the same time; cuts into old wood, by contrast, are taken badly. It is hardy to about minus fifteen degrees, young plants more so than old. It copes in rough grass provided this is mown once a year in autumn.

The tree lupin is easily raised from seed and gets going fast: on rich soil seedlings reach two and a half to three metres in height and spread within four years, flower and set seed abundantly. The hard-coated seeds germinate unevenly if sown untreated; scuffing with sandpaper or a night in tepid water raises the rate markedly. Sow from March in pots or from May directly in place. Because the plants form a sensitive taproot, prick out early into deep pots or sow direct — older specimens are practically impossible to move. Inoculating the seed with rhizobia is worthwhile on ground with no history of legumes. Cuttings of semi-ripe, non-flowering side shoots in late summer are possible but unreliable, and worth the trouble only for particular colour forms.

The tree lupin is emphatically a pollen plant. Like lupins generally it produces virtually no nectar; what bumblebees and larger solitary bees collect from the flower spikes, scented like white clover, is pollen, and it supplies that freely from May into high summer. The flowers are of the tripping type, opening only under the weight of a strong visitor — short-tongued small insects are shut out, and buff-tailed and red-tailed bumblebees are the typical users. It is not a larval food plant for Central European moths and butterflies; the species tied to lupins sit in its North American home range. The dense subshrubby growth gives cover to small mammals and ground-nesting birds. The other side must be set against this: through its root nodules it fertilises nutrient-poor ground and displaces the specialised flora there — on dry sandy sites that outweighs its value to pollinators.

Special features: short-lived (max. ~7 years, considerably less on rich soil) - plan for replacements; nitrogen fixer, feeds neighbouring plants - unwanted on nutrient-poor sites; self-seeds heavily (cut pods before they split); hardy to approx. -15°C, hardiness declines with age; cuts into old wood are taken badly; all parts contain alkaloids (not fodder);

Western North America, originally with a very small range: the tree lupin is thought to be native only to a narrow strip of the central Californian coast between Point Reyes and San Luis Obispo County. Everything beyond that is human-assisted spread — and it has been successful. Northwards it has extended through Oregon and Washington to British Columbia, southwards to southern California and Baja California, and in several of these areas it is now regarded as invasive. It has further been introduced to western Europe, where it is naturalised on British and Dutch coasts, to Australia, where it is listed as a potential noxious weed, to New Zealand, to the Turkish Aegean and to southern South America including the Falkland Islands. In Germany it occurs only in cultivation.

The tree lupin is a species of open, sandy coastal country. In its native range it occupies stabilised dunes, Californian coastal sage scrub and open pine stands near the sea — sites with sharp drainage, abundant light, salt air and few nutrients. Its root nodules fix atmospheric nitrogen, which allows it to grow on almost raw sand; it will even manage in the cracks of vertical stone walls, where water drains away at once. That ability is also its ecological problem: where it invades outside its range it enriches the soil with nitrogen and so permanently alters conditions in favour of further non-native species, while the dune flora adapted to poor ground is displaced.