Common Castor Bean: Planting, Sowing, Cultivation

Description and botany

The Common 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’s 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 doesn’t 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 maintains more modest dimensions. Fast-growing, this large perennial reaches between 0.90 and 2 m in open ground within a single season. It has a bushy, upright habit. The ‘Zanzibar’ variety is one of the tallest, potentially climbing to 3-4m 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 maple, revealing purple or white veins. These lush leaves are connected by long petioles to robust, 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, while 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 ripeness, these fruits explode, releasing seeds that are projected several metres away. They will be dispersed by wind or birds and lead to self-sowing. 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’s reputed to make eyelashes grow!

Foliage and seeds of the castor bean

Main varieties

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Planting castor beans

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 spot, as this tropical plant thrives in full sun and warmth. Select a location sheltered from strong, cold winds that could cause its stems to bend. In exposed areas, staking will be necessary. Provide deep, humus-rich, well-drained garden soil that 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 sunny spots, at the back of borders with large perennials, away from draughts. It adds an exotic touch while highlighting neighbouring plants. It also works well in large containers on patios, creating an exotic atmosphere, 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?

Planting takes place in spring, during May or June. Our plug plants can be placed outdoors once the risk of frost has 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 rotted compost if the soil is poor
• Place the plant in the hole
• Fill with a mix of good compost, garden soil, and potting mix
• Insert stakes if needed and firm the soil gently
• Water thoroughly

In Pots

Use a large container (at least 50 cm in diameter) with drainage holes, as this tall perennial is prone to wind damage. Good drainage is essential.

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

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

Sowing castor oil plant 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 generously 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 though. The soil can be kept moist with summer mulching. Stake if planted in windy spots.

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

In pots

Water generously in summer, never letting the compost dry out between waterings. This is a hungry plant: feed every fortnight with watering during growth periods.

At the first frost, move the plant to a frost-free greenhouse or conservatory where temperatures stay above 0°C. In winter, reduce watering, allowing the compost to almost dry out, and stop feeding.

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

Castor plants are rarely affected by diseases and pests. When grown indoors, they 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, they can be susceptible to Botrytis and powdery mildew, fungal diseases that develop in confined spaces with warm, humid air.

Propagation

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

What are the benefits of castor seed?

The seeds of the castor oil plant 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 properties and numerous benefits. Castor oil has ancient origins, with its use dating 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, moisturising, antimicrobial, and antifungal properties, making it very useful for the skin. In Chinese medicine, castor oil is renowned for attenuating rheumatic and muscular pain. Rest assured, commercial oils are free from ricin!

Castor seeds and oil

Pairing

With its beautiful scarlet thorny fruits and its large, lush palmate foliage, the Castor Oil plant is perfect for filling out an exotic border in summer. Its large lobed leaves will blend perfectly into a tropical-style display. It allows you to create magnificent ephemeral backdrops at the rear of borders, or in large pots on the terrace.

As a backdrop, create a scene with an exotic feel by pairing it with other plants with ample foliage, such as banana trees (Musa basjoo), a Chinese windmill palm (Trachycarpus fortunei), or a Colocasia, another superb tropical plant. To create harmony, add a few bushes and perennials with dark foliage, such as Colocasia ‘Black Magic’, the Albizia ‘Summer Chocolate’ in mild climates, the Black Elder ‘Black Lace’ and the Astilbe ‘Chocolate Shogun’. Add floral touches in warm tones with crocosmias, some Cannas, kniphofias, gladioli, and daylilies; these plants will help conceal its base, which tends to become bare easily.

You can also create striking contrasting combinations by pairing the burgundy foliage of the castor oil plant with some tall blue agapanthus. In pots, you can surround it with shorter perennials such as begonias, and small annuals like petunias, pansies, and a few Coleus.

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