Calla lilies, or Zantedeschia spp., are not true lilies but related to the Jack-in-the-pulpit and caladium plants. They come from Africa where they are considered a weed. Calla lilies are hardy perennials in zones 7 to 10 and tender perennials in areas further north. They must be protected in temperatures under 40 degrees F. How often calla lilies bloom depends on the climate conditions where they grow.
When planted outdoors in cold regions, calla lilies bloom between May and September. They produce continuous blooms during this period if they have sufficient light, water and nutrition. Calla lilies cannot withstand freezing temperatures, so you must dig up the rhizomes and bring them indoors. They should be stored in a place with temperatures about 40 to 50 degrees F, according to White Flower Farm.
When left in the ground in warm climates, calla lilies produce blooms continuously as long as conditions are right. Eventually, the plant goes into a dormant period to rest for two or three months. After this resting period, calla lilies grow shoots that eventually bloom again. Avoid fertilizing with too much nitrogen, which encourages leaf growth at the expense of flower blooming.
Calla lilies are also grown in containers and brought indoors when temperatures fall to the freezing mark. Indoors, keep the plant in full sun. Blooms appear between eight and 16 weeks, and continue until the plant goes into a rest period. Calla lilies thrive in moist, almost soggy soil. Place a saucer of water under the container to provide continuous watering. Calla lilies will not bloom without sufficient water. Increase the amount of potassium in fertilizer to assist blooming.
Calla lilies are often grown indoors as houseplants. They need placement in a location just short of full sun, with not-too-hot conditions. Drying out the plants will prevent blooming, so keep the soil moist at all times. If the plant is growing but not blooming, provide a balanced liquid fertilizer monthly. If the plant is in a blooming period, feed every two weeks. When the plant stops growing, reduce fertilizing to every two to three months. Blooming will continue indoors as long as you provide the right conditions.