Step 1 : Introduction to the question "1. This is one of the few plants on which you can change the colour of the flowers. Cottage gardeners used to use rusty nails to do this. What is it?"
...1. Hydrangea 2. Lilac 3. Lupin 4. Sage Hydrangeas can be changed from blue to pink or pink to blue by addition of lime or aluminium sulphate to the soil. Folklore says that rusty nails buried near the roots will turn a hydrangea blue. The real decider on colour is the acidity of the soil, and many will change colour of their own accord as their roots go deeper and pick up different elements. The only ones that cannot be changed are white hydrangeas.