By Larry Hodgson
I have been growing Zanzibar castor beans (Ricinus communis ‘Zanzibarensis’) ever since I was a child. Not every year, but often enough that it seems like an old friend. I’ve tried many other castor beans, including those with more colorful foliage, but ‘Zanzibarensis’ remains my favorite.
It’s a heritage annual, a very old cultivar that has been known to gardeners since at least the 1870s. As the name suggests, this plus size variety was originally selected from the Zanzibar Archipelago, just a small part of the castor bean’s wide Asian and African range in the wild.
Of course, the ordinary castor bean plant (Ricinus communis) is already a tall plant, but the Zanzibar selection is far taller. In its native Africa, it becomes a small tree up to 12 m (40 ft) high. In temperate regions, where it is treated as an annual, it can still easily reach at least 2.5 m (8 ft), even 4 m (13 ft) if the summer turns out to be long and hot. (This is not a plant that fears heat waves!)
And its huge size is precisely its main drawing card, because this plant bears titanic leaves 60 to 90 cm (2 to 3 ft) in diameter … sometimes even more! They are peltate, that is to say in the form of a shield, with the petiole joining the leaf in the middle. Combine that with the leaf’s 5 to 13 lobes and it really looks like a huge green star, giving your garden a guaranteed tropical effect. The green leaf is highlighted by whitish veins, while the thick stem can be green or reddish.
Zanzibar castor bean is mainly grown for its foliage. The foamy pale yellow flowers are not unattractive, but are carried at the top of the stem, generally well above eye level, and are more or less hidden by the foliage when seen from below. So are the very prickly green seed capsules that turn brown as seeds mature. They’re more noticeable when frost has killed the plant in the autumn.
The castor bean plant has a well-earned reputation for being poisonous, as it contains ricin, a strongly toxic compound.
Should we therefore banish it from our gardens? I’m sure some readers will say we should. However, if we take that attitude, we should also ban cherry trees, potatoes and rhubarb, all of which have poisonous parts, as do over a quarter of the ornamental plants that grow in our flower borders. Plus, we’d need to empty forests of almost half of its native plants!
The truth is that castor bean leaves, stems, roots, seeds, etc. are only toxic if you eat them. So just do as we all do with the other toxic plants in our lives: not put any part in our mouth!
Poisonous or not, the castor bean plant is the source of a medicine that was once very popular, although less so today: castor oil. The oil extracted from the seeds contains no toxic substances: not only is ricin destroyed by heating during the extraction process, but it isn’t even soluble in oil. Castor oil is therefore neither toxic nor dangerous … but it sure doesn’t taste good!
Start Off on the Right Foot
The secret in growing a beautiful, huge Zanzibar castor bean plant is to make sure its growth is never inhibited. Anything that can slow down its development—and that would include a pot that is too small and prevents its roots from expanding or temperatures that are too cool—will leave the plant stunted. Once that happens, the plant will never reach its full size. It may look nice enough, but won’t produce the large leaves it’s famous for.
That’s why store-bought castor bean seedlings often fail to live up to their full potential. If they stay just a few days too long in the nursery, their roots become cramped and then they never resume normal growth after they’re planted.
The ideal situation is therefore to sow castor bean seeds yourself, because then you can make sure that doesn’t happen.
In a warm climate, you can simply sow the seeds outdoors and just stand back and watch them grow. (This plant grows like a weed in many tropical climates and is not a plant that needs a lot of coddling when so grown.) However, in temperate climates, it’s already summer or nearly so before the soil has warmed up enough to allow germination. That doesn’t give the plant enough time to reach its full potential. You have to start it indoors … but not too early!
When to Sow
Knowing that this plant hates the cold, it should not be planted out until the night air temperatures regularly exceed 10 °C (50 °F). If you’re the type of gardener who keeps notes, you probably have a good idea of when that will be, but if not, calculate you’ll be planting it out about 2 weeks after the last frost date.
Since you’ll want a vigorous young seedling to plant out, still bearing its cotelydons (seed leaves), start the seed indoors about 4 to 6 weeks before then.
You can soak the seeds for 24 hours in lukewarm water before sowing them, but that isn’t absolutely necessary. Fill a 15-cm (6-in) peat pot with moist potting mix and insert the large seed to a depth of 2.5 to 5 cm (1 to 2 in). The peat pot, with its permeable sides, allows you to transplant the seedling pot and all, resulting in minimal root damage. You could also sow it in a plastic pot… but then you’ll have to remove the root ball from the pot at planting time. If so, be extra careful to not disturb the roots. If they’re damaged, the plant will fail to thrive.
Place the pot in a warm location (around 21–24 °C/72–75 °F). Since windowsills can be a bit on the cool side, a heat mat could well be helpful. Germination is rather slow (2–3 weeks), but growth thereafter is rapid … even very rapid! And that’s why it isn’t worthwhile starting the plant indoors too early.
Acclimatize seedlings gradually to outdoor conditions before transplanting them to a deep, rich soil in a very sunny spot. They can be grown in large containers too.
Summer maintenance? Just water castor bean plants in case of drought. Otherwise, they take care of themselves.
Where to Find Seeds
Zanzibar castor bean seeds are quite widely available in seed catalogs and often even garden centers. You should have no problem finding them.
Try growing a Zanzibar castor bean plant and you’ll see: it is truly a spectacular plant!