Introduction

Castor Bean, or Castor Oil Plant, Ricinus communis, is a member of the monotypic genus Ricinus of the Euphorbiaceae family. It is an aggressive invasive perennial around the world, and the subtropical and tropical Americas are no exception. Castor Bean is an extremely widespread weed throughout Mexico and is also quite common throughout the southern USA. It also grows into the temperate northeastern corner of the USA, showing that it is more than capable of one day invading the Pacific Northwest and southern Canada, even though so far, it has not been reported there. The seeds of the Castor Oil plant are extremely toxic, so much so that the ingestion of even 3 or 4 seeds can cause severe gastrointestinal distress and, in some cases, even death.

Description of Ricinus communis

Ricinus communis Leaves & Stems

Castor Oil Plant grows from 1 – 5 m tall from a large taproot with several lateral roots.

It is a somewhat woody perennial shrub in tropical and subtropical climates but can behave as a herbaceous annual in temperate climates. It has a large woody hollow stem that is often purplish and covered with white powdery wax, along with the petioles and often the leaves. The stems and branches have conspicuous nodes and ring-like scars.

Leaves are arranged alternately on the stems, and at their bases are stipules 1 – 3 cm long that unite into a sheathing bud. The round leaf stalks (petioles) are 3 – 50 cm long and support very large palmate leaves with 5 – 9 lobes that are united at the bottom, with the petiole being placed just off-center (peltate) on the backside of the leaf. Leaf blades are irregularly toothed (serrated) and are very large, from 10 – 70 cm across.

Ricinus communis Flowers & Fruits

It is a monoecious plant with separate male and female flowers on the same plant.

Flowers appear in erect terminal panicles up to 40 cm long. The male flowers are located on the base of the panicle, with the female flowers located at the top of the panicle. The male flowers lack petals and sepals and are made of clusters of many stamens in branched bundles.

The female flowers lack petals but possess sepals, but they fall off early (caducous). They have a superior soft spiny ovary with 3 styles that are red or green and are 2 cleft (divided).

Fruits are ellipsoid or sub-globose and 15 – 25 mm long. They are green when young and turn brown when mature. They are usually covered in conspicuous spines but occasionally are smooth.

The fruits hold ellipsoid seeds 9 – 17 mm long that are brittle and mottled brown with a caruncle located at the base. The seeds are extremely toxic and can be lethal, even if ingested in small quantities.

Ricinus communis Toxicity

The seeds of Ricinus communis are extremely toxic. They are one of the most potent natural biological toxins known to man. This is due to the presence of an albumin called ricin. The ingestion of just a few seeds can cause severe gastroenteritis, dehydration, liver or kidney damage, and even death.

Similar Species Frequently Confused With

There are no other species in the Ricinus genus, but there are several unrelated plants that, at first glance, are often confused with Ricinus communis. However, as none are related, they can easily be differentiated by their flowers and fruits but also as follows:

• Kalopanax septemlobus is a member of the Araliaceae family. While it is native to Asia, it has been introduced to the northeastern USA. It has similar-looking leaves but its stems are covered with very long spines, and the plant reaches heights of 30 m. Its flowers have 4 or 5 petals, and they grow in umbels.
• Tetrapanax papyrifer is native to Taiwan but has been introduced along the east and west coasts of North America and central Mexico. It grows to similar heights and has large palmate leaves, but its palmate leaves’ lobes are also divided, unlike Ricinus communis. Its flowers are produced in large umbels at the top of the plant, and the flowers have 4 or 5 petals.
• Carica papaya Papaya is native to Mesoamerica but has been widely introduced into northern Mexico and the southern USA for its edible fruit. When not in fruit, it can be differentiated by its more succulent than woody and usually unbranched stem. Its large palmate leaves are also clustered near the top of the plant, and the base is deeply cordate, not united with a peltate attachment as in Castor.
• Fatsia japonica Japanese Aralia has been introduced along the east and west coasts of North America and possibly into Mexico. It grows to similar heights with palmate leaves, but its bases are not united and do not have a peltate attachment to its petioles. They produce umbels of bisexual flowers that possess both petals and sepals.
• Cnidoscolus aconitifolius or Mala Mujer (Bad Woman) is native to Mesoamerica but has also been introduced into northern Mexico, Florida, and sporadically throughout the southern USA. It can be distinguished by its large palmate leaves that only have 3-5 lobes. It is also monecious, but its flowers are small and white.
• Jatropha gossypifolia is native from Mexico to South America but has been introduced to the southern USA. It is a smaller plant with smaller leaves 7-15 cm wide with a cordate base. It also produces flowers with purple to reddish petals.
• Manihot esculenta Cassava is native to South America but has been widely introduced throughout the sub and tropical Americas as a food plant. It usually grows to only 2 m, and its palmate leaves have only 3-5 lobes that usually have smooth edges or minutely toothed at most. Its fruit is a six-angled globose capsule.

Native Distribution of Ricinus communis

The castor plant is native to northeastern Africa but has naturalized throughout Africa, some of which may have been naturally spread, and others were aided by humans as far back as the stone age.

Habitat Types Where Ricinus communis is Found

In its native environment, Ricinus communis grows naturally in any disturbed habitat. It has a wide ecological tolerance allowing it to adapt to a multitude of conditions.

Castor Bean tolerates a wide range of soil conditions from acidic to alkaline, heavy to light, including infertile soils and shallow soils.

Its moisture requirements are moderate, and it can easily tolerate extended drought and even some inundation as long as the soil drains readily and is not submerged for extended periods of time.

Ricinus communis also tolerates a wide range of temperatures. It will survive -15 C in the winter and tolerates temperatures of 35 C in the summer.

Human Uses of Castor Bean or Castor Oil Plant

Ricinus communis has a long history of use by people. It has been cultivated for its oil for at least 6000 years, with the earliest recorded use in ancient Egypt.

The plant is used as an ornamental and botanical curiosity due to its enormous size. The oil was used both medicinally and for illumination before the use of other fuels, candles, and electricity became widespread.

Medicinally, Castor Oil is used as a purge internally as well as a cure-all in smaller doses for numerous internal ailments. Externally it is used for a variety of sores and skin conditions.

Castor Oil is also used in cosmetics, soap making, foods, as a lubricant, in paints, plastic, and linoleum. The press cake is poisonous and cannot be fed to animals, but it is often used as a fertilizer or fuel.

Distribution of Ricinus communis in North America

The species was first brought to North America in the 1700s. By 1760 it was documented as naturalized in Florida.

In Canada, Ricinus communis has not yet been recorded.

In the USA, Castor Bean has been recorded throughout many southern states, including California, Arizona, Utah, Kansas, Texas, Missouri, Louisiana, Alabama, Mississippi, Georgia, Florida, and North and South Carolina. It is also found in the northeastern states of Illinois, Michigan, Ohio, Virginia, Delaware, Maryland, New York, New Jersey, Connecticut, Massachusetts, and New Hampshire. It is also found in Hawaii.

In Mexico, Ricinus communis has been reported in every state. It is particularly abundant on the east and west coasts and in south-central Mexico. It is less common in the northern desert states but has still been reported in those locations.

Castor Bean has been introduced on every continent except Antarctica and is even widespread throughout the Pacific Islands. It is a global problem.

How Castor Bean Spreads

It is primarily spread through long distances by deliberate human introductions as an ornamental or for its industrial uses.

Short-distance dispersal occurs through the explosive release of mature seeds from the fruits.

Humans also aid in short-distance dispersal by spreading seeds in garden waste, soils, and vehicles.

Habitats at Risk of Invasion in North America

Due to its wide range of ecological tolerances, many habitats are at risk of invasion, particularly any habitat with disturbance. This means that anywhere humans develop, the land is at risk of invasion.

In addition to developed areas, it also invades grasslands, heathlands, riparian communities, and farmland. It is also commonly found on roadsides and in waste areas.

It is often a primary invader of recently burned lands.

In Spain, it has even invaded sand dunes, whereas it was previously thought unable to survive in permanently arid areas.

The only habitats not thought to currently be at risk are permanent wetlands, dense old-growth forests, and alpine habitats.

Potential Benefits of Invasion

Since Ricinus communis is extremely toxic, it provides no real wildlife value.

It also reduces biodiversity where it invades.

There are no known benefits of its invasion other than the occasional native bee that may visit its flowers.

Methods to Remove Ricinus communis

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in some local garden stores. Do not buy or transport any, and do not plant them in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Castor Bean

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

The best time to remove Ricinus communis is while the plants are still small enough to pull out by hand. Due to the plant’s toxicity, gloves should be worn even when the plant is young. Pulling young plants when the soil is moist, and the roots cling less tightly to the soil is ideal. Sometimes even medium-sized plants can be pulled by hand from moist soil.

If the soil is dry or the plants are larger, weed pullers can be useful in removing more stubborn plants. Try to get as much of the root and crown as possible to help prevent resprouting. This is especially important when the soil is dry, as the root system will often break rather than give to the pulling.

With larger plants or colonies, you will need to cut the above-ground growth down, then dig out the individual crowns or cultivate the soil repeatedly with a machine for large colonies. This will help prevent any regeneration.

Burning is not recommended as a means of control as Castor Bean is a quick regenerator on burned sites, and burning may encourage rather than discourage its growth.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. Solarizing is usually best with Ricinus communis.

However, sometimes with large patches burning is more efficient for disposing of large amounts of plant matter rather than solarizing. If burning, be aware of their ability to regenerate rapidly from seed after a fire. If the fire does not attain sufficient heat, there will be a surge in seedlings not long after the fire. The area will need ongoing monitoring to deal with seedlings as they emerge.

Otherwise, to solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 – 10 weeks at least to be sure that all seeds are no longer viable. Many sources recommend shorter solarization periods. However, success depends on latitude, sun exposure, daytime high temperatures, and other factors that make shorter time periods prone to failure. Leaving invasive species to solarize as long as possible is always the best. Then they can be disposed of in a landfill, but be sure to inform them of their invasive status so they can be dealt with accordingly.

Chemical Control of Castor Oil Plant

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Ricinus communis.

Chemical control is not recommended.

Biological Control of Ricinus communis

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also usually not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach.

Because Ricinus communis is cultivated as a crop, no biological control methods are being developed for it. It is, however, prone to several pests and diseases, including mung moth, pink bollworm, seedling blight, rust spot, leaf spot, gray mold, stem canker, leaf blight, and bacterial wilt. However, these all affect other crops as well, so none are being developed to control Castor Bean.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings. Otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In many cases of removal, the site will need to be replanted immediately. This is because the bare soil will allow the seed bank to germinate and reinvade the patch they were removed from. Removal of single isolated individuals does not require replanting but the removal of a patch will. A replanting program should already be planned and ready to implement immediately upon the removal of the Castor Bean.

Ongoing Monitoring is Essential

In all cases of invasive Ricinus communis removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not.

If seedlings are allowed to emerge unchecked, the invasive Castor Bean will simply re-invade and take over before the native species are able to grow back in their place. Removal of young plants by physical means is always the easiest and most effective means of control.

References and Resources

CABI on Ricinus communis https://www.cabi.org/isc/datasheet/47618

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

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