Castor bean

Family

Euphorbiaceae

Habit

annual

Height

3 m

Spread

1.5 m

Growth

very fast

Lifespan

10 years

First yield

after 1 year

Hardiness

zone 9–12

Shade tolerance

full sun

Soil

Sand, Silt, Loam

Soil moisture

dry – very moist

pH

5 – 8

Synonyms

Wunderbaum; Christuspalme; Castor bean

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

Other uses: Castor oil is the only industrially available plant oil made up of roughly 90 per cent of a single hydroxy fatty acid — ricinoleic acid. That OH group makes it chemically unusual: it stays viscous across a very wide temperature range, which is why it classically served as a lubricant for aero and racing engines, and it is the feedstock for nylon-11, polyurethanes, paints, printing inks, soaps and cosmetics. Cold-pressed and ricin-free it is the best-known plant laxative. Ricin stays entirely in the press cake; in India that cake goes onto the fields as a nitrogen-rich fertiliser (castor cake, about 4 per cent N) but is unusable as feed. In the garden the plant delivers several cubic metres of leaf mass in a single season, useful as mulch or a compost activator.

Full sun, heat and shelter — the large leaves shred otherwise, and tall specimens blow over. Soil should be deep, free-draining and fertile, pH 5 to 8; lime is no problem. Castor is a heavy feeder and repays compost with double the growth. Water regularly while establishing; later the taproot makes it surprisingly drought-tolerant. No frost: even light ground frost ends the crop, which is why it is grown as an annual north of the Alps. In Portugal, southern Spain and other frost-free areas it persists, becomes woody and turns into a small tree — there, cut it back hard in late winter or it goes bare at the base. Allow generous space, 1 to 1.5 m per plant. If you do not want self-seeding, cut the fruiting spikes before the capsules burst — which also removes the risk of seeds lying around.

Seed only, and it is straightforward. Soak the hard, mottled seeds for 24 hours in lukewarm water, then start them in pots from mid-April at 20 to 25 °C — germination in 10 to 20 days. Plant out from mid-May after the last frosts; direct sowing outdoors works from May in warm areas. The seedling drives a taproot immediately and resents transplanting, so use deep pots from the start and plant out promptly. In frost-free climates none of this is needed: the plant self-seeds freely. Store harvested seed securely, out of reach of children, and label it clearly.

The monoecious inflorescences carry male flowers below and female above and are wind-pollinated. There is no nectar but plenty of pollen, which honeybees will collect when pollen is scarce. Extrafloral nectaries on the leaf stalks and leaf base attract ants and predatory wasps. Beyond that the species means little to Central European fauna; birds do not take the seeds.

Few plants combine so much use with so much danger. Castor grows faster than almost anything else: from seed to three metres in one season, and in frost-free climates into a woody small tree up to ten metres in a few years. As a pioneer it covers open ground in record time, shades it and produces biomass. At the same time its seeds contain ricin, one of the most potent plant toxins known — two to eight thoroughly chewed seeds can kill an adult, one to three a child. The toxin sits in the seed, not the oil; pressed castor oil is harmless. For garden planning that means: a magnificent, fast structural and ornamental plant for areas without children and without free-ranging poultry, and a serious industrial crop. It has no place in family or school gardens. The widespread claim that castor repels voles and moles is unsupported.

Originally East Africa, probably Ethiopia and north-east Africa. Now naturalised and cultivated worldwide across the tropics, subtropics and warm-temperate zones. India supplies around 85 per cent of world castor oil production, followed by Mozambique and Brazil. Naturalised along roadsides, wasteland and watercourses in the Mediterranean and Portugal; in Central Europe grown only as an annual.

Pioneer of open, disturbed ground: rubble sites, river gravel, roadsides, abandoned fields, often on warm, nutrient-rich soil. Needs full sun and summer heat, tolerates drought once established, but no frost.