I treat my callas like bulbs. They are tubers, so I used bulb fertilizer when I planted mine, which is high in phosphorus (middle number). I will apply the bulb fertilizer again in the late summer/early fall. They are hardy in the ground here.

However, if you didn't use any bulb fertilizer at the time of planting, and since they are not hardy in Quebec in the ground, it would be okay to use the high phosphorus (10-50-10) until after they quit blooming. Then I would stop fertilizing, let the foliage die back, dig up the bulbs, wash off the dirt, let the tubers dry out, and store them in peat moss or newspaper over winter.

Susan