My daughter has a sunflower growing in a large pot, which has been a healthy plant up until the last week. The leaves are all turning black. How do I prevent this and what would be the cause? The plant is starting to flower, so I would love to save it as my 3-yr old girl has put her all into growing it. I wonder if where we live isn't good for sunflowers. . . we are in Christchurch, New Zealand. Cheers!