To ensure you get the best blooms on your calla lilies (Zantedeschia spp.), fertilize the plants while they are growing and blooming. You can use the same kind of fertilizer throughout the growing season, but calla lilies planted in the ground outdoors have nutritional needs different from callas in containers. When ingested, all portions of calla lilies are poisonous and can cause death; wear waterproof gloves when you handle the plants, and keep the gloves away from your face.
Begin fertilizing calla lilies in spring when you plant or replant their bulbs in the ground or in containers. Callas are hardy year-round outdoors in U.S. Department of Agriculture plant hardiness zones 8 through 11. Start fertilizing established calla lilies in those USDA zones when their new growth begins.
Whether they grow in the ground or in containers, callas have a dormant season, during which they don't require fertilizer. After blooming, the plants start to go dormant in late summer or early fall. Stop fertilizing when their foliage begins wilting. In the USDA zones where callas are not hardy, move container callas indoors; dig up the bulbs of in-ground callas, and store them indoors over winter.
You can keep houseplant calla lilies growing year-round, but they require a dormant period to bloom again. Stop fertilizing and reduce watering for houseplant callas after they finish blooming. After all their foliage dies, let the plants stay dormant for two to three months before watering and fertilizing them again.
To have good blooms, calla lilies need a fertilizer that contains more phosphorous than nitrogen. The amount of nitrogen a fertilizer contains is represented by the first number in the fertilizer's nitrogen-phosphorus-potassium, or N-P-K, ratio, and phosphorous is represented by the second number in that ratio. For callas, use a fertilizer with an N-P-K ratio such as 5-10-10 or 15-30-15. Those ratios are often available in fertilizers designed for flowering plants and vegetables.
If you use a dry, 5-10-10 fertilizer, then scatter 2 3/4 cups of it per 50 square feet of soil surface in the garden at planting time, or put 1/4 cup of it on the soil of each calla lily's container at planting time. Mix the fertilizer into the top 3 to 6 inches of soil, plant the calla lily bulbs and water the soil. One month later, sprinkle 2 tablespoons of the dry, 5-10-10 fertilizer around each plant, placing it at least 3 inches from each plant; mix the fertilizer lightly into the top of the soil, and then water the soil. Perform that fertilizer application at the same time as a regular watering. Repeat that smaller fertilizer application every month until the plants are done flowering and start to go dormant.
If you use a water-soluble, 15-30-15 fertilizer, then mix 1 tablespoon of the fertilizer with 1 gallon of water for use on in-ground calla lilies, or mix 1/2 teaspoon of the fertilizer with 1 gallon of water for indoor or outdoor container plants. Apply the fertilizer-water solution in place of a regular watering, starting in spring when the callas' new growth starts. Reapply the solution every two to four weeks until the plants finish blooming and begin going dormant.