One of the fastest-growing plants on the planet, the castor (Ricinus communis) will become nearly tree size in about three to four months after sprouting. It’s useful for filling in a large, sparse area quickly. Castor bean plants thrive in USDA planting zones 8 through 11. Even if they don’t live through the winter, these plants are known to sprout back from their roots in zones 8B and 9, even after freezing. Castor will also generously reseed itself in zones 8 and 9. Gardeners located in cooler climates successfully grow castor plants as annuals. Start them indoors about six to eight weeks before the last predicted frost for your area.
Soak the seeds in warm water for about 24 hours. Change the water when it cools to room temperature. This will soften the hard shells and promote successful germination.
Create a seed-starting mix of equal parts perlite and peat moss. Fill a plastic seed-starting 6-pack with the medium to within 1/4 inch of the tops of the cells. Set the pack in a shallow container of warm water until the surface soil feels moist to your touch. Take it out of the water and allow it to drain for about two hours.
Plant a castor bean seed about 1/2 inch deep in each cell. Firm the soil gently into place and water enough to evenly moisten the surface. Seal the pack in a clear plastic bag to retain humidity. Set it in a brightly lit, warm spot with a temperature between 60 and 85 degrees F. The top of your refrigerator or above a hot water heater are good choices. Your seeds will germinate in about two or three weeks.
Check the planting medium every two or three days. Don’t allow it to dry out. Keep the surface evenly moist.
Remove the plastic from the pack when seedlings have sprouted. Set them in a bright, warm spot. Keep the soil surface evenly moist. Move the castor bean seedlings to a warm, sunny window when they’re about 1 to 2 inches tall.
Plant the castor bean seedlings in individual 4-inch pots when they‘re about 3 or 4 inches tall. Use a good commercial potting mix. Keep them warm and give them lots of bright light. Water enough to keep the soil evenly moist.
Cultivate a 2- or 3-foot square spot in full sun for each plant after all danger of frost has passed for your area. Castor loves rich, loamy, well-draining soil with a pH of about 6.0 to 7.0. Work about one or two shovels of organic compost into the top 1 to 2 feet of the soil.
Plant your castor bean seedlings 1 to 2 feet apart. Water thoroughly so the soil is evenly moist, but not soggy or wet. Mulch with an inch or two of compost. Feed an all-purpose fertilizer throughout the season--follow the packaging instructions carefully. Water enough to keep the soil surface evenly moist until frost.