Common Castor Bean: Planting, Sowing, Cultivation

Description and botany

The Castor bean or Ricinus communis is a herbaceous perennial with a woody base from the Euphorbiaceae family. Native to fallow lands and rocky slopes of Northeast Africa and Southwest Asia, it is a perennial of tropical and subtropical climates that is mainly cultivated as an annual in our temperate climates. Frost-tender, it dies at -3°C and can only potentially survive outdoors in regions where it does not freeze. The Ricinus genus includes only one species: Ricinus communis which comes in about ten varieties such as ‘Carmencita’ with dark red foliage or ‘Zanzibar’ with fruits and very large green leaves.

Ricinus communis: botanical plate circa 1896

While in its native habitat it can form a bush nearly 10 metres tall, in our climates it remains more modest in size. Fast-growing, this large perennial reaches between 0.90 and 2 m in the ground within a single season. It has a bushy, upright habit. The ‘Zanzibar’ variety is one of the tallest, able to climb 3 to 4m high in frost-free regions.

The castor bean is characterised by the beauty of its foliage. Its dark branches are spectacular! It develops large palmate leaves that are green or purple depending on the variety. Alternate, deeply palmatilobate (5 to 7 pointed lobes), dentate, they can measure up to 20 cm in diameter, sometimes more. They resemble an open palm, the leaf of a plane tree or that of a maple, revealing purple or white veins. These lush leaves are connected by long petioles to sturdy, knotted stems that are also coloured.

The foliage is magnificent from spring to autumn. This ample foliage constantly changes colour, in red tones when juvenile, then bright green to purple in the type species. It takes on more or less purple, wine-red, fiery orange-red or bronze hues depending on the cultivars, and appears green in ‘Zanzibar’. The glossy foliage has a shiny texture.

The flowering is not particularly remarkable. Discreet, it occurs in summer, from July to October. Red and yellow in colour, the rather insignificant flowers are grouped in pyramidal racemes at the ends of the stems. The male flowers form yellowish tufts at the base, the female flowers are grouped at the top. The fruiting, however, is highly decorative. The flowers are followed by clusters of ovoid fruits in capsules composed of three protruding shells, bristling with soft thorns. They sport a scarlet red colour, or are sometimes pink in ‘Carmencita pink’, contrasting with the foliage.

At maturity, these fruits explode, releasing seeds that are projected several metres away. They will be dispersed by wind or birds and induce self-seeding. They are highly toxic if ingested.

The ricin contained in most parts of the plant is very toxic. The seeds, resembling blood-engorged ticks, contain a powerful alkaloid once used to make a deadly poison. Castor oil is extracted from them, a non-edible oil, now used in the cosmetic, pharmaceutical and even automotive industries. It is among the most widely used industrial vegetable oils in the world. It is notably used in cosmetic care products. Once its toxic enzyme is removed, it can be used for its benefits for skin and hair; it is reputed to make eyelashes grow!

Foliage and seeds of the castor bean

Main varieties

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Planting castor bean plants

Where to Plant It?

Despite its robust appearance, the castor bean plant is not hardy. This is why, in most of our gardens, it is grown as an annual, unable to survive even the slightest frost. In the south of the country, in the most sheltered regions, it can be cultivated as a perennial. The stump can be protected with a thick mulch of leaves applied in autumn. Choose a sunny location, as this tropical plant thrives in full sun and warmth. Select a spot sheltered from strong, cold winds that could cause its stems to bend. In exposed areas, staking may be necessary. Provide it with deep, humus-rich garden soil that is well-drained and remains relatively moist throughout the growth period.

It will thrive in many sheltered gardens. It is ideal for creating a temporary display in a vibrant exotic summer border. In the garden, the castor bean plant quickly fills empty spaces. Given its tall stature, it is best placed in full sun, at the back of borders with large perennials, away from draughts. It adds an exotic touch while enhancing neighbouring plants. It also works well in large containers on patios, creating an exotic setting, and can be moved to a conservatory in winter for protection.

Castor bean plant in a flower bed and in a container on a patio.

When to Plant the Castor Bean Plant?

Planting takes place in spring, during May or June. Our plug plants can be placed outdoors once the risk of frost has completely passed.

How to Plant It?

In the Ground

Space plants about 1 m apart. We recommend adding well-rotted compost, as it prefers soil rich in organic matter.

• Dig a hole 2 to 3 times wider than the root ball
• In heavy soil, add clay pebbles or pumice to the bottom of the hole to improve drainage
• Add manure or well-rotted compost if the soil is poor
• Place the plant in the hole
• Fill with a mix of good potting compost, compost, and garden soil
• Insert stakes if needed and firm the soil gently
• Water thoroughly

In a Pot

Choose a large container (at least 50 cm in diameter) with drainage holes, as this tall perennial catches the wind easily. Good drainage is essential.

• Place a layer of gravel or clay pebbles at the bottom
• Plant in a mix of good potting compost, compost, and garden soil

Castor bean plant in a red-themed border (© Gwenaelle David)

Sowing castor bean seeds

Sowing castor bean seeds is done in March or April in a warm, sheltered spot. Soak the seeds in lukewarm water for 24 hours before sowing.

• Sow one seed per bucket (or in a heated mini propagator) filled with good sowing compost and sand
• Cover the seeds with 1 cm of compost
• Lightly firm down
• Water thoroughly with a fine spray and keep moist throughout germination
• Maintain a minimum temperature of 20°C
• Pot on into larger containers once the young plants have grown sufficiently
• Transplant into the garden once all risk of frost has passed

Caring for Castor Oil Plant

In the ground

This annual requires little maintenance. Water twice a week to keep the soil moist throughout summer, especially during heatwaves. Avoid overwatering, however. The soil can be kept cool with mulch in summer. Stake if planted in a windy spot.

Pull up the plants in autumn. If you live in a warm climate, you may leave the plant in place but protect it from cold snaps with a thick layer of mulch. Ensure annual organic matter additions to maintain good soil humus content. Clear dead foliage in late winter.

In pots

Water generously in summer, never letting the growing medium dry out completely between waterings. This plant is very hungry: feed every fortnight with a liquid fertiliser during the growth period.

At the first frost, move the plant to a frost-free conservatory or greenhouse where temperatures won’t drop below 0°C. In winter, reduce watering, allowing the soil to dry almost completely, and stop fertilising.

Top-dress with compost and potting mix annually in spring. Once frost risks have passed, return it outdoors to enjoy the growing season.

Diseases and potential pests

The castor plant is rarely affected by diseases and pests. When grown indoors, it may occasionally be attacked by red spider mites, which thrive in dry conditions. As a preventive measure, mist the foliage regularly.

In very humid and warm conditions, it can be susceptible to Botrytis and powdery mildew, fungi that thrive in confined spaces with warm, humid air.

Propagation

As we’ve seen, it is possible to sow castor bean seeds to propagate them (refer to our section above titled “Sowing castor bean seeds”). You can harvest the seeds when they’re ripe (the capsules turn brown and open) then store them in a paper bag, ready for sowing in spring. But remember that the toxic ricin is mainly contained in the seeds, keep them well out of children’s reach.

What are the benefits of castor seeds? (Note: The botanical name for "ricin" is *Ricinus communis*, which remains unchanged. The common name "graines" is translated as "seeds" per the glossary.)

Castor seeds are contained within bristly shells; they are toxic and deadly if ingested. Cold or hot pressing produces a non-edible oil that is safe, with medicinal virtues and numerous benefits. Castor oil is ancient, its use dates back millennia, to 4000 BC! Highly nourishing and used externally, it is ideal for strengthening and nourishing hair, eyelashes, eyebrows, and nails. It also has emollient, hydrating, antimicrobial, and antifungal properties, very useful for the skin. In Chinese medicine, castor oil is renowned for attenuating rheumatic and muscular pains. Rest assured, commercial oils are free of ricin!

Castor seeds and oil

Pair with

With its striking scarlet spiny fruits and large, lush palmate foliage, the Castor Oil plant is perfect for enhancing an exotic border in summer. Its broad lobed leaves blend seamlessly into a tropical-inspired display. It creates magnificent ephemeral backdrops for borders or in large pots on the terrace.

For a truly exotic scene, pair it with other plants boasting ample foliage, such as banana trees (Musa basjoo), Chinese windmill palms (Trachycarpus fortunei), or Colocasia, another stunning tropical plant. To achieve harmony, add bushes and perennials with dark foliage, such as Colocasia ‘Black Magic’, Albizia ‘Summer Chocolate’ in mild climates, Black Elder ‘Black Lace’, and Astilbe ‘Chocolate Shogun’. Introduce floral accents in warm tones with crocosmias, Cannas, kniphofias, gladioli, and daylilies—these will help conceal its tendency to become leggy at the base.

For striking contrast, pair the Castor Oil plant’s deep purple foliage with tall blue agapanthus. In pots, surround it with shorter perennials like begonias and small annuals such as petunias, pansies, and Coleus.

Ricinus communis, Colocasia ‘Black Magic’, Canna ‘Red King Humbert’, and Albizia julibrissin ‘Summer Chocolate’