The fast-growing castor bean (Ricinus communis) plant is a small evergreen tree with a woody trunk and eye-catching gigantic five- to 11-lobe, star-shaped leaves, up to 3 feet long. The many varieties of castor bean work well in group or individual plantings. The warmth requirements of the castor bean may leave you wondering if your castor bean seed can survive the freezing temperatures in your area. Though frost-intolerant, with a tendency to wither quickly when the temperature drops below freezing, castor beans have been known to regrow in spring warmth in some areas, although the temperature conditions can affect the plant's growth rate.
How Low Can You Go?
Castor beans are hardy in U.S. Department of Agriculture plant hardiness zones 8 to 11, where winter temperatures can fall as low as 10 degrees Fahrenheit in the coldest parts of the range. Although it does not usually overwinter well through freezing temperatures, castor beans in zones with milder winters such as 8B and 9 do reseed themselves, reports Floridata. Sometimes the roots will resprout after a winter freeze.
Bring the Heat
Castor beans flourish in warm to hot temperatures with a 140- to 180-day growing season, according to University of Wisconsin Cooperative Extension's Alternative Field Crops Manual. The seeds will tolerate a light frost but require spring warming to germinate and grow. Extended periods of weather of 100 degrees F or more can prevent the seed from setting, however. If there is reason to expect a cold spring in your area or high soil moisture, the manual recommends treating the seeds with a thiram fungicide before planting.
Grow Towards the Light
Like cats, castor beans' favorite place to lounge is in full sun and the presence or absence of frost has a dramatic effect on plant growth. In a tropical climate without danger of frost or freeze, it grows as a perennial up to 40 feet tall and 6 inches in circumference. In temperate climates, it grows 3 to 8 feet tall as an annual. Floridata reports that sub-freezing temperatures cause the castor bean to behave like an annual.
Reap What You Sow
Castor beans can survive a light frost but the first hard freeze spells the end of the growing season. The University of Wisconsin Cooperative Extension recommends harvesting approximately two weeks "after the first killing frost." If you leave the castor bean seeds in the soil after harvest, some of them can survive the winter cold to produce a volunteer crop next spring.