What an odd year this is - a couple of years ago I wrote an article about cutting back Lavender: that was in October of the year. Here we are, still in July, and already I am cutting them back!!
Once the flowers have gone over - ie turned a dull greyish-brown - then there is no point leaving them, you might as well be brave and cut them back straight away. Why? Well, apart from aesthetics, Lavender has a habit of growing straggly and woody, then flopping open. By cutting it back, you can keep the plant compact and sturdy.
The trick is to take off all the stem as well as the dead flowers: there's nothing less stylish than a badly-cut Lavender bristling like a hedgehog!
People are always asking me to show them where to cut, when pruning Lavender: they have all read the phrase “lavender will not grow from old wood” but have no idea which bit is old wood, and which bit is safe to cut.
The answer is to get down on hands and knees, and look at just one stem - take hold of one at the outside of the plant, and gently pull it away from the bunch so you can see it clearly. At the very top or tip is the flower. Under that is a length of long, squarish green stem, with no leaves on it. This is the flower stem. Next comes a section that may be greenish or may be brown, with little leaves sprouting out from it. This is “new wood”. Next comes a section of dark brown stem, with no leaves on it at all - this is the dreaded “old wood” and if you cut it here, it won't grow back.
My usual technique is to grasp a handful at a time, and cut across, going about half-way down the section with leaves on it. I find that this is a good compromise between cutting each stem individually (life is too short, plus it often leaves a ragged shape), and cutting the whole lot at once, which usually means that some stems are cut too short, into old wood (ruined!) and some stems are not cut short enough, leaving bare flower stems (unsightly!).
The photo shows the Before and After: Before has nasty brown dead flowers and looks very messy, so it was time for The Chop. After shows the same row, neatly cut back.
You might have spotted that some of the ones further down the row were still flowering: unfortunately, as I am paid to garden, I am also paid to Do What The Client Says, and some Clients don't like it if I only cut back the plants which have clearly finished - they prefer me to do the whole row. This is a decision which you have to make: sometimes it is just better to cut the whole lot at the same time, to keep them all the same size. But it is perfectly possible to cut just those plants which need it, and come back to the others later in the year.
The only good thing about having to do such an early chop is that they might well flower again, later in the summer!