The first thing to consider when planting a hedge is to ask: is this really necessary? What is the function of this hedge? Surprisingly often, these questions are not asked and unnecessary hedges are planted with considerable attendant maintenance than required.
The hedge has one or other of two roles. It can have a practical value as a barrier and screening or it can have ornamental value, sometimes both at the same time. If an existing boundary is a wall or fence, there is no need to plant a hedge.