Posted: 17/07/2012 at 21:27
Here's Millais Nurseries' advice (rhodo specialists) -
'Dead heading is the process of snapping off the spent flower truss after flowering. It really does make a difference, and gives the plant that well groomed look for the rest of the season. However in reality, it is not always possible due to the size of the plants or the size of the garden. Therefore prioritize the operation, and concentrate on your younger plants, and those which seem weak or sick.
If you do not deadhead, then the plant will waste loads of energy making big fat seedpods, and this will inhibit growth and flowering the following season.
Simply snap the flower truss off at the base, above the whorl of leaves, using your thumb and forefinger. Be careful not to break off the new growth coming through underneath. You will soon be an expert and it will certainly pay off next season!'