Castor Bean: the Fast Way to a Tropical Look

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Ricinus communis ‘Zanzibarensis’ is the largest cultivar, with leaves up to 2 feet/60 in diameter on a plant reaching 10 feet (3 m) tall.

With its enormous palmate leaves and huge height (some cultivars can reach 10 feet/3 m!), castor bean (Ricinus communis) is an annual that will give any landscape a tropical look. And it is fast and easy to grow… once you understand its peculiarities.

The main secret to growing a stunning castor bean plant is that its growth must never be restricted. Anything that slows it down, especially a pot that is too small, cramping its roots, will leave the plant stunted and it will never recover, even when you try giving it more room. That’s why store-bought plants are often such a disappointment: unless you buy them early in the season, just as they arrive on the market, they end up staying too long in their tiny pots and then barely grow at all once you get them home.

That’s why I suggest sowing your own: they are certainly easy enough from seed… and all you have to do is to make sure your plants are planted out at the right time.

Sowing Indoors

Castor bean can only be sown outdoors with any success in climates with long, warm summers, such as the Southern US or in the Tropics. Most readers of this blog will need to start theirs indoors. And that’s simple enough.

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Castor bean should be planted out when it is still a seedling bearing cotyledons (seed leaves).

But don’t start your castor bean plant too early! That’s the main error of beginning gardeners. It seems logical that, if you want a big plant (and who doesn’t want a their castor bean to be a giant?), you should start it as early as possible, but that will only lead to a weak, wimpy plant that won’t even stand up on its own without staking. Instead, castor beans do best when planted out as very young seedlings. That may not seem logical, but it works. Your goal is to produce a young seedling still bearing its cotyledons (seed leaves) and no more than 4 adult leaves: it’s at that early stage that the castor bean should be planted out.

 

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Ricinus is Latin for tick, because the mottle seeds of castor bean are said to resemble ticks.

Start the huge seeds indoors only about 4 to 6 weeks before your last frost date. The seeds are beautifully marbled, but don’t eat them (more about that later). If you want, you can soak the seeds in warm water for 24 hours before you sow them, but that isn’t really necessary. Fill a 4-inch (10 cm) peat pot with moist soil and insert the seed to a depth of about 1 to 2 inches (2.5 to 5 cm). You can also sow the seeds in plastic pots if you prefer, but then you’ll have to be careful not to disturb the roots when you plant the seedling out. Peat pots give you the advantage of being able to pop the whole container into the ground at planting time, ensuring the roots won’t be damaged.

Place the seedling containers in a warm spot (70 to 74˚F/21 to 24˚C). No light is needed until the cotyledons (seed leaves) appear and that can take 7 to 21 days. Subsequently, give the plants as much sun as possible. Also keep the soil moist at all times. There is no need to fertilize castor bean seedlings while they are in their seedling pots, because they really won’t spend enough time there to make it worthwhile.

When the nights outdoors consistently remain above 50˚C (10˚C) and days are above 70˚F (21˚C), you can begin to acclimate the seedlings to outdoor conditions. Place the plants in shade for 2 or 3 days, then in partial shade for another 2 or 3 days and finally in full sun. If nights suddenly become cold, move the plants to a shelter, such as a garage or a tool shed, for the night.

Planting Out

Castor beans grow best when planted in the garden in rich, moist, well-drained soil. They’ll do well in pots too… big pots! (Anything too small will restrict their growth.) You can add compost or a slow release fertilizer to the soil before planting. And do plant them in full sun.

Planting peat pots is a snap: just insert the plant, pot and all, in the ground and barely cover the root ball with soil. If your seedlings are growing in a plastic pot, extract the root ball carefully, keeping it intact, and plant it, just barely covering it with soil. In both cases, water well… and stand back: your little seedlings will soon be plants of monstrous dimensions!

Summer Maintenance

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Some varieties have reddish to purple leaves and brightly colored red or pink seed capsules.

Although castor beans are very drought tolerant when grown in the tropics and even develop well in poor soil, in your cold-climate garden you’ll want to lavish them with moisture and the best possible soil. Water as needed so they are evenly moist, neither soaking wet nor bone-dry. Despite their enormous size, no staking is usually required when you grow them in full sun.

Harvesting Seed

In areas with a very long growing season, the castor bean’s spiny seed capsules will split open forcibly, sending their seeds flying in all directions. If you want to collect seeds for sowing, cover the capsules with a net bag to catch them.

In short-season climates, frost will hit before the pods have time to explode. Even so, you can still harvest the seeds. Clip off a seed head or two in mid-fall, then set them to dry in a paper bag (out of reach of children). The capsules will open on their own during the winter and next spring you can extract the seeds for the upcoming growing season.

Castor Bean Toxicity

You’d have to be living under a rock not to know that castor bean is the source of the deadly poison ricin: it has been highly mediatized ever since Bulgarian dissident Georgi Markov died in London, England after being pricked in the leg by secret police using an umbrella contaminated with ricin powder. Ever since, ricin powder has been widely used in all sorts of detective and spy novels, movies and tv shows.

All parts of the castor bean plant are poisonous, but its seeds are especially dangerous, so keep them out of the reach of children. This is especially important in that the seeds are quite attractive and could be mistaken for candy. Note that castor bean poisoning remains exceedingly rare, though, notably because the huge seeds are too large for children to swallow.

The plant’s toxicity means it is rarely attacked by insects, mammals or other pests. However Japanese beetles sometimes try to munch on its leaves, but these tentative will prove fatal to them. In fact, some gardeners plant castor beans as trap plants for Japanese beetles.

And Medicinal Use

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Castor oil was once a popular laxative. Photo: Pete Markham, Wikimedia Commons.

You’ve probably guessed by now that castor oil is extracted from castor beans. It was once a very popular laxative, one dear to our grandmothers, who seemed to think it useful for treating all childhood ailments. Some readers certainly still recall its soapy taste. Castor oil is still available for medicinal purposes, although is no longer as popular as it once was. It is however employed in many beauty products and also has many industrial uses.

Castor can be such a spectacular plant! You just have to know how to grow it!

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