Poor drainage, leading to wet feet. That'll do it every time for lavender and other Mediterranean plants.

Next time, prepare your soil by adding lots of organic material to it so that it becomes more friable. Compost is an excellent soil-improver, but even some leaf mulch or straw dug in as well will help. This preparation will involve lots of digging - you need to make sure that the clay isn't clay any more! It needs to be quite broken-down. Once the soil is of better friability, build up the level of your garden bed - you don't have to make a mountain, but a raised bed, higher than the surrounding path, provides another measure of drainage.

Pruning won't do much to help your plants survive - it's more of an aesthetic thing really, though it can encourage the plant to grow more lushly (if you don't overdo it). It's that rain - all that water and nowhere for it to go except in puddles and ponds around the roots of your precious lavender. They got drown-dead. (And if I'm correct, Zone 6 has cold winters, right? That wouldn't have helped.)

Were the lavenders new last year? I'm guessing so. Usually making it through the first winter seems to be the toughest. A few suggestions to increase your chances of over wintering lavender (which I do):

1.) Pick a variety hardy for your zone. The english varieties are the hardiest and I've had good results with "munstead."

2.) As Daisy says, drainage! Drainage is key. You need to amend the soil or have them located such that there will be no pooling of water in the winter. Water pooling will kill many plants that are not native to your location - especially the Mediterranean plants. I locate my lavenders in the best drainage locations I have. One is located in a sand pocket I have - my normal soil is heavy clay. And another is in an amended garden bed. Having them located where water would slope away from them may also be useful.

3.) Buy your plants locally. With plants that may be on the bubble of what you can realistically grow for your zone, my best successes have been with plants I've gotten from local nurseries. If that isn't possible, try mail-order/internet ordering from nurseries at your zone or colder. These plants tend to be better suited, as a rule, to colder temperatures than the plants you may get from places in warmer climates.

On the subject of where to buy....

While it is my opinion and you may or may not share it, don't waste your money on plants from the lawn-and-garden departments in your big-box-marts. These plants can be of inferior quality and/or could be grown and managed for shelf-life and appeal not garden viability. Plants there are often mislabled, search through back posts for evidence of that, and the staff as a rule as not as knowledgeable as your dedicated small, local nurseries. Your local nursery staff are plant people and will help you to know what you need to do for success with your plants.

I hope this helps!

FataMorgana

My Munstead lavender did quite well in zone 6, even with a bitter cold snap last Winter (though the cold was always preceded by a heavy snowfall, so that probably helped), and my other lavender (I forget the cultivar) did OK, but not quite as well.

It will depend on what variety you have, of course, but I suspect that what others have said is correct -- it's about the drainage, not the zone. I've seen lavender thrive in zone 5 in the high desert...and here in New England, I've got mine in a raised bed, filled with a mix of topsoil, peat moss, and composted manure (with a bit of lime thrown in to help with the acidity).

Good luck!

I have never had luck with lavender.

In addition to my clay soil, all I have to offer is partial sunlight.

Lavender REALLY needs full sun to survive.

I kept a few plants alive in a large pot one summer, but that is the extent of my success.

:o(

I planted my bright pink lavender at the end of May I live in zone 6 I prepped the bed first raised it. Added compost, new soil and a bit of fertilizer to the soil for 3 seasons I've added extra soil and a bit of compost. Gets sun for 6-7 hours a day and I noticed it has lost its flowers. Is this normal???