Lilies: How to Plant, Grow, and Care for Lily Flowers. In active growth, water freely and keep mulched. Apply a high-potassium liquid fertilizer every 2 weeks from early spring until 6 weeks after flowering. Keep moist in winter. Leave the foliage until it turns brown in the fall. This is important so that the plant stores energy for next year's flowering. Cut down the dead stalks in the late fall or early spring.