Should I divide bearded irises?

Q We would like to rescue some bearded iris growing in a neglected and overgrown border. They look quite healthy, but haven’t been flowering. Will they need to be divided, when is the best time, and how is it done? Fran Dickinson, King’s Lynn, Norfolk

- Anne Swithin Banks

A We all know it doesn’t take long for weeds to take over an untended border, but luckily, bearded iris are adept at staying alive until someone comes along to rescue them. They are known as ‘flag’ iris and their sword-like leaves will wave like flags above a sea of weeds to let you know where they are. What they won’t do is a flower, because weeds and debris will have prevented the sun baking their rhizomes.