Planting a lavender hedge does present certain challenges, especially if your soil is wet. 
Waterlogged soil when planting lavender hedge plants means that most of your small lavender plants will die.
If you have well drained soil your newly planted hedge will take off. This three year old lavender plant above is in a one foot deep soil pocket on top of two feet of limestone lumps, so it is pretty well-drained.
You can buy lavender plants or take cuttings from an established plant. The cuttings will establish themselves in a few weeks and in a year will be about six inches across. You will not get many flowers this first year as the plant puts its energy into growing.
In its second year a lavender cutting will easily grow to a one foot diameter plant and flower extensively.
There are two types of lavender. The more common, taller type is the one you need to plant for a lavender hedge. It will grow to a height of nearly three feet eventually. Lavender plants tend to spread more than they grow upwards. If you are planting a lavender hedge space the plants about 18 inches apart so the individual plants can grow together to give a continuous hedge without any gaps in it.
If your soil is clay-based then build a ridge about a foot high to plant your hedge on. Mix grave with soil/compost to give you the drainage benefits of a raised bed.
Lavender does not need a particularly fertile soil, so just throw a handful of organic fertiliser such as bone meal around each plant once a year.
