Pruning Lavender Plants and Hedges
Unless you are going to treat your Lavender plants as a short-term project to be replaced in a few years, you will need to prune them. Unpruned Lavender plants can quickly become straggly and leggy, liable to sprawl and open up to reveal unsightly gnarled looking branches that bear few flowers. Once this stage is reached they can be very difficult to rejuvenate as they seldom respond to very hard pruning.
Avoiding this sorry state is fortunately simple, once the flowers on the plants or hedge start to fade, cut them back by at least one third, preferably more (so removing far more than just the flower spikes) and do this as early as you can bear to. You can be as hard as you like provided that there are signs of young new shoots underneath where you are cutting. (You will only kill the plant if there is nothing but bare wood underneath)
While this can appear quite harsh to begin with the plants will quickly produce a healthy regrowth and by mid-Autumn they should be clothed with healthy new growth that will set them nicely for the Winter while ensuring masses of flowers the following year.
For the later flowering intermedia type of Lavender plants, it is often worth sacrificing some of the flowers to ensure the pruning is carried out in late summer giving them time to re grow. If you really cannot bear to lose this late colour and it is very late in the season, then it is better just to dead head them. Harder pruning in late Autumn with no protective regrowth to insulate from the Winter cold will cause damage. Instead give them the rest of their annual haircut in late spring when the weather has warmed up and they are starting to make growth. New shoots that will bear the current seasons flowers will appear shortly after.