Thanks for the sharing your experiences Janet. Hopefully the price is right to grow them as annuals in the UK. I too have trouble blooming Z. aethiopica although I do occasionally get a winter bloom on the ones I don't force dormant. Our relatively hot, humid summers allow the tuberous Callas to store plenty of energy to overwinter in the basemant and they usually put on a nice show in the summer. I was surprised again this year by a few tubers I didn't lift surviving -23-26C temps without much snow cover or mulch. The Z. aethiopica 'White Giant' I tested fared less well. With 60 cm of leaves and straw mulch the main rhizome rotted, although numerous offsets have been pushing out leaves.
