Entirely too much water. They don't need it - I never water my lavender.

It sounds as though you dug little port-holes in the clay. While the soil in the holes was drained well, where was it to drain to? The surrounding clay? Clay does not drain well. That is one of clay's greatest advantages & disadvantages.

In the future, water in well when you first plant it and after that only if there are periods of extreme heat or extended dry spells in your lavender's location.

FataMorgana

You may want to create raised mounds where these lavender go. When amending a clay soil with cactus mix the surrounding clay soil hold more water and this water migrates to the cactus mix and fills the open air spaces and floods it. Raising the area will allow more positive drainage and more likely keep your lavender happy or at least happier. Hope this makes sense for you and helps.
Even in Provence France many of the commercial fields of lavenders are on raised rows for this reason.
Happy Growing David

Hmmmm...I lost a lavender I had planted in a container this fall...to what I knew was too much water. I have bought a replacement but haven't put it outside yet because we keep having wet weather. I've had it about 2 weeks and I noticed the stems on it are turning brown too...It's in its original store container still as I plan on planting outside when it dries out.

I have been watering it when the pot feels lights? Is this too much? I thought lavender needed water to get established and then once established needed very little. I'm afraid I may have already started to kill this one... So when do you water lavender?