I have always found, that it is best to prune Lavender, in spring, awhile after new growth has well started on its way. Removing only the portions that died back permanently over the winter. Otherwise, it seemed for some odd reason, to kill the entire plant. I found this out the hard way, years a go, when my father thought my 'Hidcote' plants appeared to need 'cleaning up', before they had resumed growth in spring..
Protection from soggy winter soil is just as important, as is sited to avoid exposure to north winds, as well as good, fast draining soil.
It has also been my experience, if purchased & potted in decomposed ground up pine bark, &/or too much peat moss, it can be a problem. As it may or may not rain in May or June, while in full bloom & if you supplement water one day & then we get an unexpected down pour the next day,, they can completely rot, seemingly overnight.
So, to help with this, I sometimes start from seeds, using poor, sandy soil & grow them in pots. Amending with perlite, small gravel chips, or small decorative red lava etc.. may also help drainage, as does avoiding planting too deeply.

I, too, have killed my lavender plants by fall pruning. I was hoping to learn what I did wrong - perhaps cutting back too severely - but I still am unsure how best to approach it.

My plants flower and these days I just leave it all till spring.

Prune in spring.

I'm a zone milder, but always deadheaded in Fall, pruned after growth began again in Spring. Grosso was very long lived for me and maintained a good shape without becoming woody and gnarly.

I second what Morz said. Wait until you see growth next spring before doing any pruning. I had 3 Provence last year and I foolishly cut back long stems in early spring. Two of the plants never came back and the third is just barely alive. The two plants that died showed some signs of life before I cut them back, and the stems that I cut were alive at the time.

Cheryl

Basically, you don't prune ANY woody plant in fall, in fact best not to do so after June. It encourages new growth that will be winter killed. And lavender does best and lives longest in very lean soil--that is, gravelly, poor soil.

I'm remembering reading somewhere the time to prune lavender is right after blooming.anyway thats what I've always done and my plants live a long time. As a matter of fact I've lived here 47 years now and I've only planted lavender twice.

Here in the mild winter west my lavender is always cut back after the bloom has been spent. Al