Lavender planting advice
Lavenders prefer a sand or chalk soil to clay, however so long as there is plenty of sunlight and aeration then there is no reason why your lavender can't flourish with a less than ideal soil type.
It is best to plant lavender between May and September when the soil temperature is warmer. They will need watering in for the first couple of weeks but once established will never need to be watered again (unless in a pot where regular moisture is needed in Summer) Even with the hardiest of lavenders it is best to avoid planting ahead of a heavy frost. Once settled though both the English varieties and the Lavandins can comfortably withstand temperatures as low as -20c.