Calla Lilies (Zanthedeschia) are one of my favorite flowers and I have several in my garden at home. Do I assume correctly that your calla lily is growing in a pot? I usually plant my calla lily bulbs in the early spring but I have planted potted calla lilies in my garden in the late summer. Normally callas are perennial only in Zones 8-12 and those who are in colder regions will dig the bulbs and store them for the winter. Although I live in a zone 6 area my calla lilies have made it through several winters. I keep the area well mulched with southern exposure that is some what shaded in the summer but gets full sun in the winter.

Depending on where you live you can plant them in the ground now and let nature take care of the rest. However, if you live in a cold area you can do one of two things. You can keep your calla lily in the pot keeping the soil moist but not soggy and slight decreasing the water as winter approaches or you can let the plant endure a slight frost and then remove the bulb from the soil cutting off the foliage. Then storage the bulb in a cool dry environment with good air circulation until spring when you can replant it.