Marcus, that is strange because I thought that these could virtually be grown in water! At the very least, they are considered marginals. I just bought Z. aethiopica 'Crowborough', and planted it in a small bog area in my yard. Hope it does well. Are these fast growing plants?

Susan

The yellow variety is most likely a hybrid. Z. aethiopica are always white. Z. elliottiana are yellow. The yellow and orange varieties are prone to a soft rot and a disease called Erwinia (you can google it).
Z.aethiopica is a marginal bog plant, likes it a bit wetter, but the hybrids like it much drier. I would cut back on your watering and see if it helps. Are the leaves turning color before they rot? If so it is probably a watering issue. If you have a tall flower that suddenly falls over one day and the stem literally turns to mush in front of your eyes, you have a rot/disease problem.
Good luck!

Thanks for your replies, I think it must be the disease as it was just as you say: it just fell over. Actually, I dug it up and found that despite a good root system the bulb was 3/4 mush. I cleared the rotten bit and stuck it back in a new soil mix which I hoped would drain better but all in vain as the remnants of the once beautiful flower are now clearly dead!! I'll pop back to the nursery for another and try again....Ta, Marcus