Agapanthus africanus, a tender bulbous plant also known as African lily, grows from thick rhizomes and produces clusters of funnel-shaped blue flowers on leafless upright stalks. These flower stalks can reach up to 2 feet in height, rising above a mound of strap-shaped, narrow leaves. Agapanthus africanus bloom during the summer and rest during the winter. Although not technically considered a tropical plant, Agapanthus africanus cannot tolerate cold temperatures. In most areas of the United States, gardeners grow Agapanthus africanus in containers indoors. However, it will thrive outdoors in the southernmost areas of the country.
Plant Agapanthus africanus in a container that has drainage holes in the bottom and is just large enough to hold the plant's roots. Fill the container with a fast-draining soilless potting mix. Plant rhizomes about 1/2 inch below the soil's surface or container-grown plants with the crown at soil level. The crown is the area where the plant's stem meets its roots.
Place your potted Agapanthus africanus in an area that receives direct sunlight such as a brightly lit window with a southern or western exposure. The plant can tolerate medium light exposure but performs best with several hours of full sun each day. Avoid lowlight areas.
Water the plant about once a week during the spring and summer months, or whenever the top 1/2 inch of soil feels dry to the touch. Do not allow the soil to dry out completely during active growth. Reduce watering to once every two weeks during fall and winter.
Apply an equal analysis fertiliser once a month during the spring and summer before the plant blooms. Follow the manufacturer's directions for the best results. Stop feeding after blooming occurs to prepare the plant for its winter resting period.
Maintain a consistent temperature of 18.3 to 21.1 degrees C during the day and 10 to 12.8 degrees Celsius during the night for the best results. If temperatures rise above 21.1 degrees C, the plant may grow more slowly. Do not allow temperatures to drop below 4.44 degrees Celsius or the plant may sustain permanent damage.
Cut back flower stalks after blooming ends to increase the plant's health and appearance. Remove damaged foliage throughout the year as necessary. Agapanthus enters dormancy in fall after flowering and resumes active growth in the spring.
Plant Agapanthus africanus in the ground in USDA plant hardiness zones 8 through 10 only, as it cannot tolerate extremely cold winters. Provide outdoor Agapanthus africanus with full sun to partial shade and well-drained soil. Water once a week during active growth, feed once a month with an equal analysis fertiliser during spring and summer, and allow the plant to rest during the fall and winter months. Agapanthus performs best when allowed to become rootbound, a term referring to a plant's roots when they become crowded and tangled together. The plant requires repotting only when the roots begin to grow out of the top of the container or cause the container to split open, about once every two to four years.