Castor oil plants are best-known for the oil extracted from their seeds, but they can also be cultivated as attractive ornamental plants. Their beauty is deceptive, however, as they are one of the most poisonous plants in the world.
The remarkable leaves and flowerheads of the castor oil plant are eye-catching but dangerous, because its cells form ricin, one of the most potent plant poisons. How poisonous is the castor bean, and how is it cultivated? Read on to discover this hazardous beauty and get tips for growing it yourself.
Castor bean: origins and characteristics
Castor oil plants (Ricinus communis), also known as castor beans, are now common in various parts of the world. They are mostly found in temperate zones as well as tropical regions. These plants originate from Northeast Africa and the Middle East, but are now also grown as ornamentals around the globe. Castor beans belong to the spurge family known as Euphorbiaceae. The genus name Ricinus is Latin for ‘tick’, which hints at the fact that the plant’s mature seeds resemble certain tick species when they are full.
In areas prone to frost, castor oil plants typically grow as annuals. With ideal growing conditions at temperate latitudes, these shrubs can reach a height of 6m after only 3 to 4 months. In the tropics and subtropics, on the other hand, they grow as perennials that become woody and can even reach a height of 13m after several years. Castor bean leaves are alternate, have long stalks and grow anywhere from 30 to 70cm long. The leaves are usually dark green, but some varieties have red or purple colouration. In most varieties, the petiole and veins are red. Castor leaves are also characteristically palmate with five to 11 lobes and a serrated leaf margin. Castor oil plants also form small stipules surrounding each stem.
Castor oil plants bloom between August and October. If started inside and with proper care, they can even bloom as early as June. They produce flower stalks 30 to 40cm long, topped with bracteate or paniculate inflorescences. Castor oil plant flowers are monoecious, meaning that each plant produces both female and male flowers. The red female flowers that grow at the top of the inflorescence have ornamental value, but fall off quickly. The yellow male flowers that emerge from the lower half of the inflorescence remain on the plant for longer. Pollinated and fertilised flowers produce brown fruit capsules about 1.5 to 2.5cm large. These fruit capsules are covered with conspicuous, soft spines about 5mm long and resemble chestnuts. Castor bean fruit capsules ripen in September and each contain three bean-like seeds.
Are castor oil plants poisonous?
Yes, castor oil plants are poisonous to humans. In fact, castor beans are one of the most toxic plants in the world. They produce the toxic protein ricin, which inhibits protein biosynthesis within human cells. The ricin is mainly contained in the endosperm of the seeds, but the seed coat is also toxic, as are all other parts of the plant. It only takes 1mg to 20mg of ingested ricin per kg of body weight to be lethal, which is as little as one to 10 seeds. Within 48 hours of ingesting castor seeds, the body’s circulatory system fails, resulting in death. As of yet, there is no known antidote against ricin poisoning. In fact, ricin is so toxic that it has even been used as a biological weapon and is subject to the United Nations’ Chemical Weapons Convention. Ricin is not only dangerous when taken orally — even coming into contact with broken seed coats can cause localised dermal poisoning symptoms.
Castor oil, on the other hand, is no cause for concern and is often used in the beauty industry, for instance. The ricin in the seeds does not pass through into the oil but remains in the pressed castor cake. The castor cakes can then be used as fertiliser, or treated with heat to deactivate the ricin, making the cakes safe to use as feed for livestock.
Castor bean look-alikes
As castor oil plants have distinctive identifying characteristics, there are generally only a few other plants they are confused with. One such plant is the rice-paper plant (Tetrapanax papyrifer), whose young leaves resemble those of the castor bean. However, older rice-paper trees have significantly larger leaves than those of castor oil plants. Rice-paper tree leaves can reach a diameter of up to 1m. It is easy to mistake castor bean seeds for edible nuts, but eating just one castor bean seed can have fatal consequences.
Are castor oil plants banned?
Despite being so poisonous, castor oil plants are not currently banned in the UK. However, because of occasional poisonings of children and adults, there are intermittent calls to make it mandatory to report any purchases and sales of castor bean plant parts, especially the seeds.
Types and varieties
Some castor bean varieties have become popular in recent years due to their distinct growth or ornamental value. Here is an overview of some of the better-known castor varieties:
- Ricinus communis ‘Carmencita Bright Red’: this variety grows up to 2.5m tall and produces red-brown, glossy leaves. It blooms between August and October and has showy, bright red fruits.
- Ricinus communis ‘Impala’: this is a smaller variety that tops out at 1.2m tall. These shrubs flower between August and November and form red leaves and fruits.
- Ricinus communis ‘Zanzibarensis’: this green-leaved variety with a growth height of 2 – 2.5m blooms between August and October in striking yellow.
- Ricinus communis ‘Apache’: with red shimmering flowers and green and red leaves, this small 1m-tall variety is great for less spacious gardens.
Planting castor oil plants
When planting castor, there are a few things to keep in mind. Choosing the right location and sowing castor oil plant seeds correctly are crucial to growing a healthy castor oil plant.
Where to grow castor oil plants
Castor oil plants prefer warm and windless locations with full sun. These large colourful shrubs work wonderfully as accent plants, as a background to other plants or planted in containers. Ensure your plant’s soil is well-drained and rich in humus and nutrients. Young castor oil plants need plenty of water, but once a plant is established, it does well on its own. These shrubs can tolerate short periods of drought, but longer periods of drought stunt their growth.
Important: remember, when handling the plant, it is extremely important to wear gloves. Even touching a broken castor bean seed can lead to localised skin poisoning!
Propagating with castor bean seeds
The best way to propagate castor oil plants is by sowing castor bean seeds, even though this can be a bit tricky due to uneven germination. It is not possible to propagate castor bean shrubs by cuttings or layering. Start castor bean seeds indoors between March and April. Direct sowing outdoors is possible in late spring.
We suggest using a nutrient-rich potting soil to start castor bean seeds. As opposed to a nutrient-poor sowing soil, a fertile soil will give the seedlings a valuable nutrient boost. Simply fill the growing container with the appropriate soil, tap it a few times on the table to get rid of any large air pockets, and press one seed into the soil of each pot. Then, water the soil, and place the container on a windowsill in partial shade at about 20 °C. Castor bean seeds germinate irregularly. If you start to lose hope, try putting your seed pots in the refrigerator for 2 weeks. Keeping moisture levels consistent is also important for castor seeds to germinate. When the castor seedlings become too big for their container, repot them.
Our Plantura Organic Enriched Compost is a nutrient-rich compost that is ideal for castor oil plants. And by swapping out peat for a more sustainable wood fibre and compost composition, our compost is the perfect foundation for your beautiful, eco-friendly garden.
Whether started inside or purchased, plant your castor oil plant out from mid-May or after the last frost. Before planting the shrub out, place the plant’s pot in a container with water to allow the root ball to soak up as much water as possible. Then dig a planting hole and mix in some compost, manure or an organic slow-release fertiliser. Take the castor oil plant out of its growing container, and put it in the planting hole. Take care to set the plant to the same depth it was in its pot, not any deeper. Lightly press down the soil around the base of the castor bean and water it again.
Tip: organic fertilisers have microorganisms that break down and convert the nutrients in the soil into a form that plants can absorb. The advantage here is that the plants are supplied with nutrients over a longer period of time. This also prevents overfertilisation and nutrients leaching from the soil. Our Plantura Flower Food, for instance, is a great organic fertiliser with suitable nutrients for castor beans.
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Castor oil plant care
During the growing season, castor beans have high nutrient and water requirements. Regular watering is especially important during prolonged dry periods. Castor beans do not tolerate waterlogging, so take care to wait until the soil has dried before watering again. It is best to water early in the morning or in the evening. This allows the castor bean shrub to absorb the water better, and less water evaporates from the soil in the heat of the day.
If you use a slow-release granular fertiliser or an organic fertiliser for planting, you will not need additional fertiliser at the beginning of the growing period. Otherwise, we recommend feeding weekly with a high-quality liquid fertiliser, such as our Plantura Liquid Flower Food. This fertiliser is easy to apply by diluting into the water you use for watering. When applied weekly, our fertiliser gives your castor oil plant all the nutrients it needs to grow strong and healthy.
When the gardening season comes to an end, it is time to prune your castor bean. To do this, cut the plant to a hand’s width above the ground. Dispose of the cuttings in your organic waste bin or your bin for non-recyclable waste. Do not compost castor bean cuttings in your own garden compost. Remember to wear gloves with pruning castor oil plants — all plant parts are toxic and even just touching them can irritate the skin!
Overwintering castor oil plants
Castor oil plants are not winter hardy or frost tolerant. However, pruning them in autumn prepares them for overwintering. If your castor bean is planted out in the garden, layer some soil, leaves, straw, coarse compost or brushwood over the rootstock to protect the roots from freezing. Move potted castor beans to bright winter quarters with temperatures around 8 to 10 °C. Water the shrubs in the garden when the ground is frost-free, and water potted shrubs when the topsoil is dry. After the last frost in spring, take potted plants back outside and remove the layers of frost protection from outdoor castor oil plants.