Where are you? When did you plant? What kind of care/watering did you provide?

Looking at the picture and considering the kind of plant likely to be root rot

Located in NJ. Plants are located in a garden that has a sprinkler system set up. All other plants did well. Is it possible that the water required for lavender is less than required for other plants in the garden which include Shasta daisys, hydrangeas, knock out roses, asters and other perennials.

Thanks. They were beautiful this summer. Will transplant those that survived to a better local

Any that are in soil that is right next to the rotted out ones should probably not be moved to another part of the property because the rot causing organisms may come with them, continue causing damage.

If you bring them inside, make sure to never wet the leaves and let them dry out very well in a porous draining mix. Spra with copper spray if you see anything just beginning to rot.