Question
Hi Tom!  And thank-you.  I am not a bad gardener, I must say, but until I cam across your 'don't fertilize with high N2 fert."  I was pretty bad with callas.  I live in Edmonton, Alberta, Canada (our zone is 3a but I know we do it with different numbers than you do.)  So I will keep the rhizomes in for the winter (which feels like today, but really isn't!).  Do I have to treat the roots very carefully like Sprekelia Formossisima?  THose darlings don't want to bloom if you TOUCH their roots.  I keep pot after pot of them since I love 'em.
Thanks again Tom.
Lee Anna

Answer
Hi Lee Anna,
Thanx for your question.  USDA does classify Edmonton as Hardiness Zone 3A.  There is a map which has been classified for the whole of North America.

Calla rhizomes aren't as tender as those of S. Formosissima.  You are right to keep them in pots.  They don't mind being a bit crowded and every couple of years you can carefully separate them and plant the extras in other pots.  You can also treat the calla as a potted plant and just move the pot indoors like you do your Spreklia which is also known as Jacobean or Aztec Lily.  Once the calla goes dormant water only enough to keep the soil from going bone dry.  I hope this helps.
Tom