Castor bean plants have broad, tropical-like foliage and red flowers followed by large seed clusters in the fall. They are considered an annual plant in most areas of the U.S. but can be perennial as long as temperatures remain above freezing. Castor bean plants are considered one of the fastest growing plants in the world. They are adaptable to most soils. However, they need a steady supply of moisture and full sun to grow large and produce a good crop of seeds, or beans. Expect each plant to grow up to 15 feet tall and wide. For best bean production, the castor bean plant needs a period of 140 to 180 frost-free days.
Clear an area of the garden with full sun in the spring after all danger of frost has passed. Remove all weeds and other garden debris. Loosen the soil to a depth of 8 inches. Cover the area with a 1-inch layer of compost and work the compost into the top 2 inches of soil. Rake the area smooth. Plant castor bean seeds 2 inches deep and 18 inches to 3 feet apart. Castor bean plants look nice planted in a group, but they can also serve as a fast-growing screen. Keep the area moist to encourage the seeds to germinate quickly. They should sprout in 10 to 20 days, as castor bean seed germination is relatively slow.
Water the castor bean plants generously as they grow. Do not fertilize, as fertilizer can make them produce too much heavy top growth the shallow roots cannot support and the plants may fall over in high winds. Also, fertilizer can decrease the amount of beans the plant produces. Keep a generous layer of compost around the base of the plants during the growing season, as castor bean plants thrive on organic matter in the soil.
Continue to water the plants as needed during the summer. If the plants wilt during the first half of the day, they are in need of water. It is natural for the plants to wilt slightly in the heat of the afternoon. In late summer, you will notice blooms on the plant. They are red spikes that grow from the ends of each branch. After the blooms fade, they will be replaced by large seed pods. Do not gather the seeds until they are ready to harvest. If using a water hose to water the plants, do not get the seed heads or pods wet or they may mildew.
Harvest the seeds when the seeds pods dry and begin to crack and most of the leaves have fallen from the plants. This usually occurs after the first frost. You can see the smooth brown seed in the cracked seed pods when they are ready to harvest. Simply crack open the seed pod and pull out the seed or beans.