A simple case of greenfly proved something of a problem to the planters but my remedy of diluted washing up liquid made much worse damage. I know I'm not particularly green-fingered, but how did this happen?

One of my pupil's parents made these planters up for me - they looked so beautiful a couple of days ago. What did I do wrong? Did I make the solution too strong? Did I spray at the wrong time of day? Did I miss the correct phase of the moon for optimum spraying? I suspect that this gardening malarkey is more art than science. Taking the long view of this disaster I suspect that my mistake was making the concoction too strong and then spraying in the middle of a hot day and singeing the leaves. I hope that they recover!