To grow Kaffir lime trees from seed, soak the seeds in warm water for 24 hours and plant them 1" deep in a small pot filled with well-draining soil. Place the pot in a warm and sunny location, and keep the soil moist to provide the right growing conditions. Kaffir lime trees prefer warm temperatures above 60°F and humidity levels above 50%. After germination, continue to water and provide ample sunlight to the seedling. Once the seedling grows to about 6 inches tall, it can be transplanted into a larger pot for continued growth and used as a fragrant houseplant. With proper care, a Kaffir lime tree can thrive indoors as a decorative plant and provide aromatic leaves for culinary use. Or alternatively, you can grow them on outside.