Why do you want to cut them back? I had a lavender plant for many years in my zone 3, until the frost finally got it. I never cut it all back. In the spring after it leafed out, I cut out all the dead branches and pruned to shape and bush out the plant.

I guess what I am asking is about pruning. As the center of each plant spreads flat...and I am not sure when to cut the dead branches...spring or fall? Now I see you say in the spring...so I will try. The weather is so odd this year it
finished flowering earlier than usual and now has new growth..
All I ever do to these plants is pull out the grass that grows
around them. I have sandy soil and I love growing this Lavender and just want them to last as long as they can..
thank you oilpainter.. I have seen how they shape the lavender in France, all the beautiful rows one next to the
other like spraying fans.. ;- > Martha Zucchini

At this point in the season, wait until spring to trim your lavender. When it starts to come back in the spring, see what branches the winter has killed - and there is always some. Trim those first and then shape your plant as needed but don't cut more than 1/3 of the living material off (general pruning rule of thumb).

Lavender is a woody plant and general tips about trimming and pruning trees/shrubs would apply if you want to do some looking up more info.

FataMorgana

Although lavender is a woody plant, as Fata says, it does not like being cut back into old wood (unlike, for example, rosemary which you can cut to the ground more or less.) Oilpainter asks why cut them back? It rather depends on the cultivar but English lavender can get very splayed out and sprawling. What looks dead is probably not if it has growth at the ends. If you have it as a hedge or want a more dense and shaped bush you really ought to have been trimming it lightly after flowering. But don't cut into the old wood, just trim the green. You will be rejuvenating the bush and it will get denser rather than gradually becoming more and more bare with growth just at the ends of the branches.

Here is a link that might be useful: lavender

Thank you oilpainter, fata and flora...I feel more confident now and will trim in the spring. I had much dried last year and it made lovely Christmas gifts, and also tried to do some
oils. This year I don't have enough... ; - > Martha/Zucchini

thank you Flora. What a wonderful link..
Martha/zucchini

When it gets too woody,it may be best too replace it.Lavender can get old and over-grown.I am having that problem now.I intend to cut my huge shrub back and dig it out this spring.
Right now it has become home to some wasps and I have to wait til it freezes to remove the plant.

Pruning a shrub from time to time helps to encourage growth in the centre, and less woodiness. It's not really necessary, just could be more aesthetically-pleasing. While some lavenders can live for many years, the average one is past its best after 5-7 years, so, come spring, take a heap of cuttings and start new plants.

I've cut back lavender to bare, old wood with not a leaf in sight and had it bounce back. I've also have my failures! It's the luck of the draw, I guess.