I spent the summer of 1976 working as a trainee gardener at the Arboretum Kalmthout in Belgium. My employer was charming and kind, but I could not suppress a prickle of shame-faced irritation every time she mentioned a former student called Susan Dickinson. Whenever I leant on my hoe for a moment in the pelting heat, I was reminded how accomplished and hardworking this horticultural superheroine had been.