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 is a perennial of tropical and subtropical climates that is primarily cultivated as an annual in temperate climates. Frost tender, it dies at -3°C and can only survive outdoors in regions where frost does not occur. The genus Ricinus includes only one species: Ricinus communis, which comes in around ten varieties, such as ‘Carmencita’ with dark red foliage or ‘Zanzibar’ with green fruits and very large leaves.

Ricinus communis: botanical plate circa 1896

In its native habitat, it can form a bush nearly 10 metres tall, but in temperate climates, it remains more modest in size. With rapid growth, this large perennial reaches between 0.90 and 2 metres in the ground within a single season. It has a bushy, upright habit. The ‘Zanzibar’ variety is one of the tallest, climbing to 3–4 metres in frost-free regions.

The castor bean is distinguished by the beauty of its foliage. Its dark branches are spectacular! It develops large palmate leaves, 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 even that of a maple, revealing purple or white veins. These lush leaves are attached 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, starting red when juvenile, then bright green to purple in the typical species. It takes on more or less purple, wine-red, fiery orange-red, or bronze hues depending on the cultivar, and remains green in ‘Zanzibar’. The glossy foliage has a lustrous 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 tips of the stems. Male flowers form yellowish tufts at the base, while female flowers cluster 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 spines. They display a scarlet red, or sometimes pink in ‘Carmencita pink’, contrasting with the foliage.

At maturity, these fruits explode, releasing seeds that are projected several metres away. They are dispersed by wind or birds and lead to spontaneous seedlings. 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 one of 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 the skin and hair; it is reputed to promote eyelash growth!

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 location, as this tropical plant thrives in full sun and warmth. Reserve it 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, well-drained garden soil that remains relatively cool throughout the growth period.

It will thrive in many sheltered gardens. It is ideal for creating a fleeting display in a vibrant exotic summer border. In the garden, the common castor bean plant quickly fills empty spaces. Given its tall stature, it will find its place in the sun, at the back of large perennial borders sheltered from drafts. It will add a striking exotic touch while highlighting neighbouring plants. It also makes a bold statement in large containers on the terrace, creating an exotic atmosphere, and can be moved to a conservatory in winter for protection.

Castor bean plant planted in a flower bed and in a container on a terrace.

When to Plant the Castor Bean Plant?

The common castor bean plant is planted 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 planting hole to improve drainage
• Add manure or well-rotted compost if the soil is poor
• Place the plant in the hole
• Fill with a mixture of good potting soil, compost, and garden soil
• Insert stakes if needed and firm the soil gently
• Water generously

In a Pot

Plan to place it in a large pot with drainage holes, at least 50 cm in diameter, as this tall perennial catches the wind easily. Good drainage is essential.

• Spread a thick layer of gravel or clay pebbles at the bottom of the container
• Plant in a mixture of good potting soil, compost, and garden soil

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

Sowing castor bean seeds

The sowing of castor seeds is done in March or April in a warm, sheltered environment. 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
• Press lightly
• Water generously with a fine spray and keep moist throughout germination
• Maintain a minimum temperature of 20°C
• Repot into larger pots when the young plants have sufficiently sprouted
• Transplant into the garden when all risk of frost has passed

Maintenance of the Castor Oil Plant

In the Ground

This annual requires no special maintenance. Water twice a week to keep the soil cool throughout the summer, especially during periods of intense heat. However, avoid excess moisture. The soil can be kept cool with mulching in summer. Stake the plant if it is exposed to windy conditions.

Pull up the plants in autumn. If you live in a warm climate, you can leave the plant in place but protect it from cold snaps with a thick layer of mulch. Ensure you add organic matter each year to maintain good humus levels in the soil. Clean up the clump at the end of winter by trimming away any dry parts.

In Pots

Water generously in summer, never allowing the substrate to dry out between waterings. This plant is very demanding: feed with fertiliser every two weeks in the watering water during the growth period.

At the first sign of frost, bring the plant indoors to a frost-free conservatory or greenhouse where temperatures will not drop below 0°C. In winter, reduce watering, allowing the soil to dry out almost completely, and stop fertilising.

Top-dress with compost and potting soil once a year in spring. Once the risk of frost has passed, you can move it back outdoors to enjoy the growing season.

Diseases and potential pests

The castor plant is rarely affected by diseases or 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.

Multiplication

As we have 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 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 seeds?

The seeds of the castor plant are contained within bristly shells; they are toxic and lethal if ingested. Cold or hot pressing yields a non-edible oil that is safe and boasts medicinal properties with numerous benefits. Castor oil is ancient, with its use dating back millennia, to 4000 BC! Highly nourishing and used externally, it is ideal for strengthening and nourishing hair, cilia, eyebrows, and nails. It also possesses emollient, hydrating, 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 of ricin!

Castor seeds and oil

Associate

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

As a backdrop, create a scene that transports you 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 achieve 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 hues with crocosmias, a few Cannas, some kniphofias, gladioli, and daylilies—these plants will hide its base, which tends to become bare easily.

You can also achieve striking contrasting combinations by pairing the Castor plant’s purple foliage with tall blue agapanthus. In pots, you can surround it with shorter perennials like begonias and small annuals such as petunias, pansies, and a few Coleus.

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